diff --git a/.build/docker/_set_java.sh b/.build/docker/_set_java.sh
index ded68e1a167f..05b8445bdcff 100755
--- a/.build/docker/_set_java.sh
+++ b/.build/docker/_set_java.sh
@@ -56,7 +56,8 @@ fi
 ################################
 
 if grep "^ID=" /etc/os-release | grep -q 'debian\|ubuntu' ; then
-    sudo update-java-alternatives --set java-1.${java_version}.0-openjdk-$(dpkg --print-architecture)
+    sudo update-alternatives --set java /usr/lib/jvm/java-${java_version}-openjdk-$(dpkg --print-architecture)/bin/java
+    sudo update-alternatives --set javac /usr/lib/jvm/java-${java_version}-openjdk-$(dpkg --print-architecture)/bin/javac
 else
     sudo alternatives --set java $(alternatives --display java | grep "family java-${java_version}-openjdk" | cut -d' ' -f1)
     sudo alternatives --set javac $(alternatives --display javac | grep "family java-${java_version}-openjdk" | cut -d' ' -f1)
diff --git a/.build/docker/almalinux-build.docker b/.build/docker/almalinux-build.docker
index 89832b67b346..43faed157866 100644
--- a/.build/docker/almalinux-build.docker
+++ b/.build/docker/almalinux-build.docker
@@ -42,6 +42,7 @@ RUN yum -y install \
    git \
    java-11-openjdk-devel \
    java-17-openjdk-devel \
+   java-21-openjdk-devel \
    make \
    rpm-build \
    sudo \
diff --git a/.build/docker/build-artifacts.sh b/.build/docker/build-artifacts.sh
index 85eec8300546..89b260fde444 100755
--- a/.build/docker/build-artifacts.sh
+++ b/.build/docker/build-artifacts.sh
@@ -18,5 +18,5 @@
 #
 # Creates the tarball artifact
 
-$(dirname "$0")/_docker_run.sh bullseye-build.docker build-artifacts.sh $1
+$(dirname "$0")/_docker_run.sh debian-build.docker build-artifacts.sh $1
 exit $?
diff --git a/.build/docker/build-debian.sh b/.build/docker/build-debian.sh
index e265fa5e8d20..e26d983dadee 100755
--- a/.build/docker/build-debian.sh
+++ b/.build/docker/build-debian.sh
@@ -32,5 +32,5 @@ echo
 #
 # Creates the debian package
 
-$(dirname "$0")/_docker_run.sh bullseye-build.docker docker/_build-debian.sh $1
+$(dirname "$0")/_docker_run.sh debian-build.docker docker/_build-debian.sh $1
 exit $?
diff --git a/.build/docker/build-jars.sh b/.build/docker/build-jars.sh
index b8039cb0a013..bf21b4564d45 100755
--- a/.build/docker/build-jars.sh
+++ b/.build/docker/build-jars.sh
@@ -18,5 +18,5 @@
 #
 # Build the jars
 
-$(dirname "$0")/_docker_run.sh bullseye-build.docker build-jars.sh $1
+$(dirname "$0")/_docker_run.sh debian-build.docker build-jars.sh $1
 exit $?
diff --git a/.build/docker/check-code.sh b/.build/docker/check-code.sh
index 4afccaea98eb..38b56f9d0bab 100755
--- a/.build/docker/check-code.sh
+++ b/.build/docker/check-code.sh
@@ -20,4 +20,4 @@
 
 export CASSANDRA_DOCKER_ANT_OPTS="-Ddependency-check.home.base=/tmp"
 
-$(dirname "$0")/_docker_run.sh bullseye-build.docker check-code.sh $1
+$(dirname "$0")/_docker_run.sh debian-build.docker check-code.sh $1
diff --git a/.build/docker/bullseye-build.docker b/.build/docker/debian-build.docker
similarity index 55%
rename from .build/docker/bullseye-build.docker
rename to .build/docker/debian-build.docker
index b31bf03b3a75..db38a0b9abbf 100644
--- a/.build/docker/bullseye-build.docker
+++ b/.build/docker/debian-build.docker
@@ -36,16 +36,31 @@ RUN echo 'Acquire::http::Timeout "60";' > /etc/apt/apt.conf.d/80proxy.conf
 RUN echo 'Acquire::ftp::Timeout "60";' >> /etc/apt/apt.conf.d/80proxy.conf
 
 # install deps
-RUN until apt-get update \
-        && apt-get -y install ant build-essential curl devscripts ed git sudo \
+RUN until apt-get update && apt-get -y install ant build-essential curl devscripts ed git sudo \
                 python3-pip rsync procps dh-python quilt bash-completion ; \
     do echo "apt failed… trying again in 10s… " ; sleep 10 ; done
 
-RUN until apt-get update \
-        && apt-get install -y --no-install-recommends openjdk-11-jdk openjdk-17-jdk ; \
+RUN until apt-get install -y --no-install-recommends openjdk-11-jdk openjdk-17-jdk; \
     do echo "apt failed… trying again in 10s… " ; sleep 10 ; done
 
-RUN update-java-alternatives --set java-1.11.0-openjdk-$(dpkg --print-architecture)
+# Download and install OpenJDK 21.0.2 for the current architecture
+RUN sh -c '\
+    JDK_OS=linux ;\
+    JDK_TAR="openjdk-21.0.2_${JDK_OS}-$(uname -m | sed "s/x86_64/x64/")_bin.tar.gz" ;\
+    JDK_VERSION_SHAS="08db1392a48d4eb5ea5315cf8f18b89dbaf36cda663ba882cf03c704c9257ec2 a2def047a73941e01a73739f92755f86b895811afb1f91243db214cff5bdac3f 8fd09e15dc406387a0aba70bf5d99692874e999bf9cd9208b452b5d76ac922d3 b3d588e16ec1e0ef9805d8a696591bd518a5cea62567da8f53b5ce32d11d22e4" ;\
+    curl -L --fail --silent --retry 2 --retry-delay 5 --max-time 3600 "https://download.java.net/java/GA/jdk21.0.2/f2283984656d49d69e91c558476027ac/13/GPL/${JDK_TAR}" -o $JDK_TAR ;\
+    JDK_SHA="$(sha256sum $JDK_TAR | cut -d" " -f2)" ;\
+    echo "$JDK_VERSION_SHAS" | sed "s/ /\n/g" | grep -q "$JDK_SHA" || { echo "SHA256 mismatch for $JDK_TAR $JDK_SHA"; exit 1; } ;\
+    mkdir -p /usr/lib/jvm/java-21-openjdk-$(dpkg --print-architecture) ;\
+    tar -C /usr/lib/jvm/java-21-openjdk-$(dpkg --print-architecture) --strip-components=1 -xzf $JDK_TAR ;\
+    rm $JDK_TAR'
+
+# Add OpenJDK 21 to update-alternatives
+RUN update-alternatives --install /usr/bin/java java /usr/lib/jvm/java-21-openjdk-$(dpkg --print-architecture)/bin/java 1
+RUN update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/java-21-openjdk-$(dpkg --print-architecture)/bin/javac 1
+
+RUN update-alternatives --set java /usr/lib/jvm/java-11-openjdk-$(dpkg --print-architecture)/bin/java
+RUN update-alternatives --set javac /usr/lib/jvm/java-11-openjdk-$(dpkg --print-architecture)/bin/javac
 
 # python3 is needed for the gen-doc target
 RUN pip install --upgrade pip
@@ -58,11 +73,8 @@ RUN sh -c '\
   GO_VERSION="1.24.3" ;\
   GO_VERSION_SHAS="3333f6ea53afa971e9078895eaa4ac7204a8c6b5c68c10e6bc9a33e8e391bdd8 a463cb59382bd7ae7d8f4c68846e73c4d589f223c589ac76871b66811ded7836 13e6fe3fcf65689d77d40e633de1e31c6febbdbcb846eb05fc2434ed2213e92b 64a3fa22142f627e78fac3018ce3d4aeace68b743eff0afda8aae0411df5e4fb" ;\
   GO_OS=linux ;\
-  [ $(uname) = "Darwin" ] && GO_OS=darwin ;\
-  GO_PLATFORM=amd64 ;\
-  [ $(uname -m) = "aarch64" ] && GO_PLATFORM=arm64 ;\
-  GO_TAR="go${GO_VERSION}.${GO_OS}-${GO_PLATFORM}.tar.gz" ;\
-  curl -L --fail --silent --retry 2 --retry-delay 5 --max-time 30 https://go.dev/dl/$GO_TAR -o $GO_TAR ;\
+  GO_TAR="go${GO_VERSION}.${GO_OS}-$(dpkg --print-architecture).tar.gz" ;\
+  curl -L --fail --silent --retry 2 --retry-delay 5 --max-time 3600 https://go.dev/dl/$GO_TAR -o $GO_TAR ;\
   GO_SHA="$(sha256sum $GO_TAR | cut -d" " -f2)" ;\
   echo "$GO_VERSION_SHAS" | sed "s/ /\n/g" | grep -q "$GO_SHA" || { echo "SHA256 mismatch for $GO_TAR $GO_SHA"; exit 1; } ;\
   tar -C /usr/local -xzf $GO_TAR ;\
@@ -70,4 +82,11 @@ RUN sh -c '\
 
 ENV GOROOT="/usr/local/go"
 ENV GOPATH="$BUILD_HOME/go"
-ENV PATH="$PATH:/usr/local/go/bin"
\ No newline at end of file
+ENV PATH="$PATH:/usr/local/go/bin"
+
+# allow lower UIDs and GIDs
+RUN sed -i 's/UID_MIN			 1000/UID_MIN			 100/' /etc/login.defs
+RUN sed -i 's/UID_MIN			 1000/UID_MIN			 10/' /etc/login.defs
+
+# suppress warnings about mismatching ownership
+RUN git config --global --add safe.directory ${CASSANDRA_DIR}
\ No newline at end of file
diff --git a/.build/docker/run-tests.sh b/.build/docker/run-tests.sh
index 0133ae3522ac..2a3af69602aa 100755
--- a/.build/docker/run-tests.sh
+++ b/.build/docker/run-tests.sh
@@ -128,7 +128,7 @@ docker --version
 pushd ${cassandra_dir}/.build >/dev/null
 
 # build test image
-dockerfile="ubuntu2004_test.docker"
+dockerfile="ubuntu-test.docker"
 image_tag="$(md5sum docker/${dockerfile} | cut -d' ' -f1)"
 image_name="apache/cassandra-${dockerfile/.docker/}:${image_tag}"
 docker_mounts="-v ${cassandra_dir}:/home/cassandra/cassandra -v "${build_dir}":/home/cassandra/cassandra/build -v ${m2_dir}:/home/cassandra/.m2/repository"
diff --git a/.build/docker/ubuntu2004_test.docker b/.build/docker/ubuntu-test.docker
similarity index 94%
rename from .build/docker/ubuntu2004_test.docker
rename to .build/docker/ubuntu-test.docker
index 9d19baef18b6..aefb59624c4b 100644
--- a/.build/docker/ubuntu2004_test.docker
+++ b/.build/docker/ubuntu-test.docker
@@ -11,7 +11,7 @@
 # limitations under the License.
 
 FROM ubuntu:20.04
-MAINTAINER Apache Cassandra <dev@cassandra.apache.org>
+LABEL org.opencontainers.image.authors="Apache Cassandra <dev@cassandra.apache.org>"
 
 # CONTEXT is expected to be cassandra/.build
 
@@ -52,7 +52,7 @@ RUN export DEBIAN_FRONTEND=noninteractive && \
         python3.11 python3.11-venv python3.11-dev \
         virtualenv net-tools libev4 libev-dev wget gcc libxml2 libxslt1-dev \
         vim lsof sudo libjemalloc2 dumb-init locales rsync \
-        openjdk-8-jdk openjdk-11-jdk openjdk-17-jdk ant ant-optional
+        openjdk-8-jdk openjdk-11-jdk openjdk-17-jdk openjdk-21-jdk ant ant-optional
 
 
 RUN update-alternatives --install /usr/bin/python python /usr/bin/python3.8 2
@@ -71,8 +71,9 @@ RUN chmod 0644 /opt/requirements.txt
 RUN pip3 install virtualenv virtualenv-clone
 RUN pip3 install --upgrade wheel
 
-# make Java 8 the default executable (we use to run all tests against Java 8)
-RUN update-java-alternatives --set java-1.8.0-openjdk-$(dpkg --print-architecture)
+# make Java 11 the default executable
+RUN update-alternatives --set java /usr/lib/jvm/java-11-openjdk-$(dpkg --print-architecture)/bin/java
+RUN update-alternatives --set javac /usr/lib/jvm/java-11-openjdk-$(dpkg --print-architecture)/bin/javac
 
 # enable legacy TLSv1 and TLSv1.1 (CASSANDRA-16848)
 RUN find /etc -type f -name java.security -exec sed -i 's/TLSv1, TLSv1.1//' {} \;
diff --git a/.build/parent-maven-pom.xml b/.build/parent-maven-pom.xml
index c75e2545337b..26d24f47cb15 100644
--- a/.build/parent-maven-pom.xml
+++ b/.build/parent-maven-pom.xml
@@ -36,8 +36,8 @@
     </license>
   </licenses>
   <properties>
-    <bytebuddy.version>1.12.13</bytebuddy.version>
-    <byteman.version>4.0.20</byteman.version>
+    <bytebuddy.version>1.17.8</bytebuddy.version>
+    <byteman.version>4.0.26</byteman.version>
     <netty.version>4.1.130.Final</netty.version>
     <ohc.version>0.5.1</ohc.version>
     <async-profiler.version>4.2</async-profiler.version>
@@ -494,13 +494,13 @@
       <dependency>
         <groupId>org.mockito</groupId>
         <artifactId>mockito-core</artifactId>
-        <version>4.7.0</version>
+        <version>5.12.0</version>
         <scope>test</scope>
       </dependency>
       <dependency>
         <groupId>org.mockito</groupId>
         <artifactId>mockito-inline</artifactId>
-        <version>4.7.0</version>
+        <version>5.2.0</version>
         <scope>test</scope>
       </dependency>
       <dependency>
@@ -891,7 +891,8 @@
       <dependency>
         <groupId>net.openhft</groupId>
         <artifactId>chronicle-queue</artifactId>
-        <version>5.23.37</version>
+        <!-- chronicle ea builds before the corresponding Final are code identical; Final builds are not made available on Maven -->
+        <version>5.25ea16</version>
         <exclusions>
           <exclusion>
             <artifactId>tools</artifactId>
@@ -907,7 +908,12 @@
       <dependency>
         <groupId>net.openhft</groupId>
         <artifactId>chronicle-core</artifactId>
-        <version>2.23.36</version>
+        <!-- While we broadly don't want to use ea libraries in the project, the Chronicle ecosystem has moved
+             to an approach of only allowing paid usage for non-ea builds. The ea builds are identical to the .0 release
+             of the next major version, so "ea" in this context signifies "stable GA build" for this major release line.
+             We plan to move away from chronicle for this and other reasons; the details of the conversation on the mailing
+             list can be found here: https://issues.apache.org/jira/browse/CASSANDRA-18831?focusedCommentId=17902034&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-17902034 -->
+        <version>2.25ea14</version>
         <exclusions>
           <exclusion>
             <artifactId>chronicle-analytics</artifactId>
@@ -922,7 +928,8 @@
       <dependency>
         <groupId>net.openhft</groupId>
         <artifactId>chronicle-bytes</artifactId>
-        <version>2.23.33</version>
+        <!-- See above for details about exception re: inclusion of ea lib -->
+        <version>2.25ea10</version>
         <exclusions>
           <exclusion>
             <artifactId>annotations</artifactId>
@@ -933,7 +940,8 @@
       <dependency>
         <groupId>net.openhft</groupId>
         <artifactId>chronicle-wire</artifactId>
-        <version>2.23.39</version>
+        <!-- See above for details about exception re: inclusion of ea lib -->
+        <version>2.25ea15</version>
         <exclusions>
           <exclusion>
             <artifactId>compiler</artifactId>
@@ -949,7 +957,8 @@
       <dependency>
         <groupId>net.openhft</groupId>
         <artifactId>chronicle-threads</artifactId>
-        <version>2.23.25</version>
+        <!-- See above for details about exception re: inclusion of ea lib -->
+        <version>2.25ea7</version>
         <exclusions>
           <exclusion>
               <!-- pulls in affinity-3.23ea1 which pulls in third-party-bom-3.22.4-SNAPSHOT -->
diff --git a/.build/run-python-dtests.sh b/.build/run-python-dtests.sh
index 86652db0b83d..337870caa4fb 100755
--- a/.build/run-python-dtests.sh
+++ b/.build/run-python-dtests.sh
@@ -124,7 +124,7 @@ set -e # enable immediate exit if venv setup fails
 # fresh virtualenv and test logs results everytime
 [[ "/" == "${DIST_DIR}" ]] || rm -rf "${DIST_DIR}/venv" "${DIST_DIR}/test/{html,output,logs}"
 
-# re-use when possible the pre-installed virtualenv found in the cassandra-ubuntu2004_test docker image
+# re-use when possible the pre-installed virtualenv found in the cassandra2004_test docker image
 virtualenv-clone ${BUILD_HOME}/env${python_version} ${DIST_DIR}/venv || virtualenv --python=python${python_version} ${DIST_DIR}/venv
 source ${DIST_DIR}/venv/bin/activate
 pip3 install --exists-action w -r ${CASSANDRA_DTEST_DIR}/requirements.txt
diff --git a/.build/run-tests.sh b/.build/run-tests.sh
index 9270922898ec..11980473646d 100755
--- a/.build/run-tests.sh
+++ b/.build/run-tests.sh
@@ -251,7 +251,11 @@ _run_testlist() {
     for ((i=0; i < _test_iterations; i++)); do
       [ "${_test_iterations}" -eq 1 ] || printf "–––– run ${i}\n"
       set +o errexit
-      ant "$_testlist_target" -Dtest.classlistprefix="${_target_prefix}" -Dtest.classlistfile=<(echo "${testlist}") -Dtest.timeout="${_test_timeout}" ${ANT_TEST_OPTS}
+      ant "$_testlist_target" \
+        -Dtest.classlistprefix="${_target_prefix}" \
+        -Dtest.classlistfile=<(echo "${testlist}") \
+        -Dtest.timeout="${_test_timeout}" \
+        ${ANT_TEST_OPTS}
       ant_status=$?
       set -o errexit
       if [[ $ant_status -ne 0 ]]; then
diff --git a/.jenkins/Jenkinsfile b/.jenkins/Jenkinsfile
index 877cdc285056..7999739bc242 100644
--- a/.jenkins/Jenkinsfile
+++ b/.jenkins/Jenkinsfile
@@ -382,7 +382,7 @@ def test(command, cell) {
       withEnv(cell.collect { k, v -> "${k}=${v}" }) {
         ws("workspace/${JOB_NAME}/${BUILD_NUMBER}/${cell.step}/${cell.arch}/jdk-${cell.jdk}/python-${cell.python}") {
           fetchSource(cell.step, cell.arch, cell.jdk)
-          fetchDockerImages(['ubuntu2004_test'])
+          fetchDockerImages(['ubuntu-test'])
           def cell_suffix = "_jdk${cell.jdk}_python_${cell.python}_${cell.cython}_${cell.arch}_${cell.split}_${splits}"
           def logfile = "stage-logs/${JOB_NAME}_${BUILD_NUMBER}_${cell.step}${cell_suffix}_attempt${attempt}.log.xz"
           def script_vars = "#!/bin/bash \n set -o pipefail ; " // pipe to tee needs pipefail
diff --git a/NEWS.txt b/NEWS.txt
index 724d860ea9bb..921a716e5b18 100644
--- a/NEWS.txt
+++ b/NEWS.txt
@@ -80,6 +80,9 @@ using the provided 'sstableupgrade' tool.
 
 New features
 ------------
+    - JDK21 and Generational ZGC are now officially supported and generational ZGC is now the default Garbage Collector
+    when using JDK21. See jvm21-server.options for more details and configuration parameters. We do not recommend using
+    non-generational ZGC with Apache Cassandra.
     [The following is a placeholder, to be revised asap]
     - CEP-37 Auto Repair is a fully automated scheduler that provides repair orchestration within Apache Cassandra. This
     significantly reduces operational overhead by eliminating the need for operators to deploy external tools to submit
@@ -204,6 +207,11 @@ Upgrading
 Deprecation
 -----------
     - `use_deterministic_table_id` is no longer supported and should be removed from cassandra.yaml. Table IDs may still be supplied explicitly on CREATE.
+    - Updating dependencies for JDK21 necessitated updating Chronicle Queue, which has changed the enums used for log
+    rolling (cassandra.yaml -> full_query_logging_options:roll_cycle). Older legacy options will still work for the
+    foreseeable future but you will see warnings in logs and future dependency upgrades may break your log rolling param.
+    The default log rolling param has been changed from HOURLY to FAST_HOURLY, primarily different on how frequently
+    indexes are built (256 in FAST_HOURLY vs. 16 in HOURLY).
     - IEndpointSnitch has been deprecated as ClusterMetadata is now the source of truth regarding topology information.
       The responsibilities of the snitch have been broken out to a handful of new classes:
       * o.a.c.locator.Locator provides datacenter and rack info for endpoints. This is not configurable as topology is
diff --git a/bin/cassandra.in.sh b/bin/cassandra.in.sh
index 881e425de1f8..19f9b7dd42ec 100644
--- a/bin/cassandra.in.sh
+++ b/bin/cassandra.in.sh
@@ -114,56 +114,36 @@ if [ -z $JAVA ] ; then
 fi
 
 # Matches variable 'java.supported' in build.xml
-java_versions_supported=11,17
+java_versions_supported="11 17 21"
+java_version_string=$(IFS=" "; echo "${java_versions_supported}")
 
 # Determine the sort of JVM we'll be running on.
-java_ver_output=`"${JAVA:-java}" -version 2>&1`
-jvmver=`echo "$java_ver_output" | grep '[openjdk|java] version' | awk -F'"' 'NR==1 {print $2}' | cut -d\- -f1`
-JVM_VERSION=${jvmver%_*}
-short=$(echo "${jvmver}" | cut -c1-2)
-
-# Unsupported JDKs below the upper supported version are not allowed
-if [ "$short" != "$(echo "$java_versions_supported" | cut -d, -f1)" ] && [ "$JVM_VERSION" \< "$(echo "$java_versions_supported" | cut -d, -f2)" ] ; then
-    echo "Unsupported Java $JVM_VERSION. Supported are $java_versions_supported"
-    exit 1;
-fi
-# Allow execution of supported Java versions, and newer if CASSANDRA_JDK_UNSUPPORTED is set
-is_supported_version=$(echo "$java_versions_supported" | tr "," '\n' | grep -F -x "$short")
-if [ -z "$is_supported_version" ] ; then
-    if [ -z "$CASSANDRA_JDK_UNSUPPORTED" ] ; then
-        echo "Unsupported Java $JVM_VERSION. Supported are $java_versions_supported"
-        echo "If you would like to test with newer Java versions set CASSANDRA_JDK_UNSUPPORTED to any value (for example, CASSANDRA_JDK_UNSUPPORTED=true). Unset the parameter for default behavior"
-        exit 1;
-    else
-        echo "######################################################################"
-        echo "Warning! You are using JDK$short. This Cassandra version only supports $java_versions_supported."
-        echo "######################################################################"
-    fi
+JAVA_VERSION=$(java -version 2>&1 | grep '[openjdk|java] version' | cut -d '"' -f2 | cut -d '.' -f1)
+
+supported=0
+for version in ${java_versions_supported}; do
+  if [ "$version" -eq "$JAVA_VERSION" ]; then
+      supported=1
+  fi
+done
+
+if [ "$supported" -eq 0 ]; then
+  if [ -z "$CASSANDRA_JDK_UNSUPPORTED" ]; then
+    echo "Unsupported Java $JAVA_VERSION. Supported are $java_version_string"
+    echo "If you would like to test with newer Java versions set CASSANDRA_JDK_UNSUPPORTED to any value (for example, CASSANDRA_JDK_UNSUPPORTED=true). Unset the parameter for default behavior"
+    exit 1
+  else
+    echo "######################################################################"
+    echo "Warning! You are using JDK$JAVA_VERSION. This Cassandra version only supports $java_version_string"
+    echo "######################################################################"
+  fi
 fi
-JAVA_VERSION=$short
-
-jvm=`echo "$java_ver_output" | grep -A 1 '[openjdk|java] version' | awk 'NR==2 {print $1}'`
-case "$jvm" in
-    OpenJDK)
-        JVM_VENDOR=OpenJDK
-        # this will be "64-Bit" or "32-Bit"
-        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $2}'`
-        ;;
-    "Java(TM)")
-        JVM_VENDOR=Oracle
-        # this will be "64-Bit" or "32-Bit"
-        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $3}'`
-        ;;
-    *)
-        # Help fill in other JVM values
-        JVM_VENDOR=other
-        JVM_ARCH=unknown
-        ;;
-esac
 
 # Read user-defined JVM options from jvm-server.options file
 JVM_OPTS_FILE=$CASSANDRA_CONF/jvm${jvmoptions_variant:--clients}.options
-if [ $JAVA_VERSION -ge 17 ] ; then
+if [ $JAVA_VERSION -ge 21 ] ; then
+    JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm21${jvmoptions_variant:--clients}.options
+elif [ $JAVA_VERSION -ge 17 ] ; then
     JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm17${jvmoptions_variant:--clients}.options
 elif [ $JAVA_VERSION -ge 11 ] ; then
     JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm11${jvmoptions_variant:--clients}.options
diff --git a/build.xml b/build.xml
index b970240df4ec..3044c4d6cd8e 100644
--- a/build.xml
+++ b/build.xml
@@ -45,7 +45,7 @@
         The use of both CASSANDRA_USE_JDK11 and use-jdk11 is deprecated.
     -->
     <property name="java.default" value="11" />
-    <property name="java.supported" value="11,17" />
+    <property name="java.supported" value="11,17,21" />
 
     <!-- directory details -->
     <property name="basedir" value="."/>
@@ -173,7 +173,7 @@
     <!-- When updating ASM, please, do consider whether you might need to update also FBUtilities#ASM_BYTECODE_VERSION
       and the simulator InterceptClasses#BYTECODE_VERSION, in particular if we are looking to provide Cassandra support
       for newer JDKs (CASSANDRA-17873). -->
-    <property name="asm.version" value="9.4"/>
+    <property name="asm.version" value="9.5"/>
     <property name="allocation-instrumenter.version" value="3.1.0"/>
 
     <condition property="is.source.artifact">
@@ -323,14 +323,82 @@
         <string>--add-opens java.base/java.lang=ALL-UNNAMED</string>
         <string>--add-opens java.base/java.util=ALL-UNNAMED</string>
         <string>--add-opens java.base/java.nio=ALL-UNNAMED</string>
-
         <string>--add-opens java.rmi/sun.rmi.transport.tcp=ALL-UNNAMED</string>
+
+        <!-- Needed for in-jvm dtests straddling 6.0+ -->
+        <string>--add-opens java.base/java.util.concurrent=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.util.concurrent.atomic=ALL-UNNAMED</string>
     </resources>
     <pathconvert property="_jvm17_args_concat" refid="_jvm17_arg_items" pathsep=" "/>
     <condition property="java-jvmargs" value="${_jvm17_args_concat}">
         <equals arg1="${ant.java.version}" arg2="17"/>
     </condition>
 
+    <resources id="_jvm21_arg_items">
+        <string>-Djdk.attach.allowAttachSelf=true</string>
+
+        <string>-XX:+UseZGC</string>
+        <string>-XX:+ZGenerational</string>
+
+        <!-- Temporary workaround for jamm having incorrect default CompressedOops for JDK21 -->
+        <!-- Without this, memory layout and sizing calculations will be incorrect as jamm assumes compressedOops -->
+        <string>-XX:-UseCompressedOops</string>
+
+        <!-- <string>-XX:+UseLargePages</string> -->
+
+        <!-- Max size ZGC will _attempt_ to stay below -->
+        <!-- <string>-XX:+SoftMaxHeapSize=12G</string> -->
+
+        <!-- Disable returning memory to the OS -->
+        <!-- <string>-XX:-ZUncommit</string> -->
+
+        <!-- Time memory should have been unused before eligible for uncommit in seconds -->
+        <!-- <string>-XX:ZUncommitDelay=300</string> -->
+
+        <!-- Pay the tax up front to pretouch memory to smooth out latency later -->
+        <!-- <string>-XX:+AlwaysPreTouch</string> -->
+
+        <!-- Need to explicitly allow security manager on JDK21; deprecated for removal -->
+        <string>-Djava.security.manager=allow</string>
+
+        <string>--add-exports java.base/java.lang.ref=ALL-UNNAMED</string>
+        <string>--add-exports java.base/java.lang.reflect=ALL-UNNAMED</string>
+        <string>--add-exports java.base/jdk.internal.misc=ALL-UNNAMED</string>
+        <string>--add-exports java.base/jdk.internal.ref=ALL-UNNAMED</string>
+        <string>--add-exports java.base/sun.nio.ch=ALL-UNNAMED</string>
+        <string>--add-exports java.management.rmi/com.sun.jmx.remote.internal.rmi=ALL-UNNAMED</string>
+        <string>--add-exports java.rmi/sun.rmi.registry=ALL-UNNAMED</string>
+        <string>--add-exports java.rmi/sun.rmi.server=ALL-UNNAMED</string>
+        <string>--add-exports java.rmi/sun.rmi.transport.tcp=ALL-UNNAMED</string>
+        <string>--add-exports java.sql/java.sql=ALL-UNNAMED</string>
+        <string>--add-exports jdk.unsupported/sun.misc=ALL-UNNAMED</string>
+        <string>--add-exports jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED</string>
+        <string>--add-exports jdk.attach/sun.tools.attach=ALL-UNNAMED</string>
+
+        <string>--add-opens java.base/java.io=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.lang=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.lang.module=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.lang.reflect=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.math=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.net=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.nio=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.util=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.util.concurrent=ALL-UNNAMED</string>
+        <string>--add-opens java.base/java.util.concurrent.atomic=ALL-UNNAMED</string>
+        <string>--add-opens java.base/jdk.internal.loader=ALL-UNNAMED</string>
+        <string>--add-opens java.base/jdk.internal.math=ALL-UNNAMED</string>
+        <string>--add-opens java.base/jdk.internal.module=ALL-UNNAMED</string>
+        <string>--add-opens java.base/jdk.internal.ref=ALL-UNNAMED</string>
+        <string>--add-opens java.base/jdk.internal.reflect=ALL-UNNAMED</string>
+        <string>--add-opens java.base/sun.nio.ch=ALL-UNNAMED</string>
+        <string>--add-opens java.rmi/sun.rmi.transport.tcp=ALL-UNNAMED</string>
+        <string>--add-opens jdk.management/com.sun.management=ALL-UNNAMED</string>
+    </resources>
+    <pathconvert property="_jvm21_args_concat" refid="_jvm21_arg_items" pathsep=" "/>
+    <condition property="java-jvmargs" value="${_jvm21_args_concat}" else="">
+        <equals arg1="${ant.java.version}" arg2="21"/>
+    </condition>
+
     <!--
       JVM arguments for tests.
 
@@ -344,21 +412,48 @@
       buffer construction (see CASSANDRA-16493)
     -->
     <resources id="_jvm11_test_arg_items">
+        <string>-XX:+HeapDumpOnOutOfMemoryError</string>
         <string>-XX:-CMSClassUnloadingEnabled</string>
         <string>-Dio.netty.tryReflectionSetAccessible=true</string>
         <string>-XX:MaxMetaspaceSize=2G</string>
     </resources>
     <pathconvert property="_jvm11_test_arg_items_concat" refid="_jvm11_test_arg_items" pathsep=" "/>
+
     <resources id="_jvm17_test_arg_items">
+        <string>-XX:+HeapDumpOnOutOfMemoryError</string>
         <string>-Dio.netty.tryReflectionSetAccessible=true</string>
+        <!-- Open things up so we don't run into any jamm rejections that'd then go unsafe and crash out on in-jvm dtests -->
+        <string>--add-opens=java.base/java.lang=ALL-UNNAMED</string>
+        <string>--add-exports=java.base/jdk.internal.vm.annotation=ALL-UNNAMED</string>
     </resources>
     <pathconvert property="_jvm17_test_arg_items_concat" refid="_jvm17_test_arg_items" pathsep=" "/>
+
+    <resources id="_jvm21_test_arg_items">
+        <string>-XX:+HeapDumpOnOutOfMemoryError</string>
+        <string>-Dnet.bytebuddy.experimental=true</string>
+        <string>-Djava.security.manager=allow</string>
+        <string>-Dio.netty.tryReflectionSetAccessible=true</string>
+
+        <!-- Open things up so we don't run into any jamm rejections that'd then go unsafe and crash out  on in-jvm dtests-->
+        <string>--add-opens=java.base/java.lang=ALL-UNNAMED</string>
+        <string>--add-exports=java.base/jdk.internal.vm.annotation=ALL-UNNAMED</string>
+
+        <!-- Revert to pre-JEP-416 Core Reflection implementation (without method handles) -->
+        <!-- without this, FieldUtils cannot overwrite static final fields, breaking some tests -->
+        <!-- removed in 22 (JDK-8309635) so will need a proper fix for 25 -->
+        <string>-Djdk.reflect.useDirectMethodHandle=false</string>
+    </resources>
+    <pathconvert property="_jvm21_test_arg_items_concat" refid="_jvm21_test_arg_items" pathsep=" "/>
+
     <condition property="_std-test-jvmargs" value="${_jvm11_test_arg_items_concat}">
-            <equals arg1="${ant.java.version}" arg2="11"/>
+        <equals arg1="${ant.java.version}" arg2="11"/>
     </condition>
     <condition property="_std-test-jvmargs" value="${_jvm17_test_arg_items_concat}">
         <equals arg1="${ant.java.version}" arg2="17"/>
     </condition>
+    <condition property="_std-test-jvmargs" value="${_jvm21_test_arg_items_concat}">
+        <equals arg1="${ant.java.version}" arg2="21"/>
+    </condition>
 
     <!-- needed to compile org.apache.cassandra.utils.JMXServerUtils -->
     <!-- needed to compile org.apache.cassandra.distributed.impl.Instance-->
@@ -484,9 +579,13 @@
 
     <target name="gen-cql3-grammar" depends="check-gen-cql3-grammar" unless="cql3current">
       <echo>Building Grammar ${build.src.antlr}/Cql.g  ...</echo>
+      <!-- antlr will crash on the first run on JDK21 and subsequently succeed if we don't fork it and execute in a separate JVM... -->
       <java classname="org.antlr.Tool"
             classpathref="cql3-grammar.classpath"
-            failonerror="true">
+            failonerror="true"
+            fork="true"
+            outputproperty="antlr.output"
+            errorproperty="antlr.error">
          <arg value="-Xconversiontimeout" />
          <arg value="10000" />
          <arg value="${build.src.antlr}/Cql.g" />
@@ -1237,6 +1336,8 @@
         <jvmarg value="-ea"/>
         <jvmarg value="-Djava.io.tmpdir=${tmp.dir}"/>
         <jvmarg value="-Dcassandra.debugrefcount=true"/>
+        <!-- Pull this up for genZGC / JDK21 -->
+        <jvmarg value="-Xms512M"/>
         <jvmarg value="${jvm_xss}"/>
         <!-- When we do classloader manipulation SoftReferences can cause memory leaks
              that can OOM our test runs. The next two settings informs our GC
@@ -1491,6 +1592,7 @@
       <test if="withMethods" name="${test.name}" methods="${test.methods}" todir="${build.test.output.dir}/" outfile="TEST-${test.name}-${test.methods}"/>
       <test if="withoutMethods" name="${test.name}" todir="${build.test.output.dir}/" outfile="TEST-${test.name}"/>
       <jvmarg value="-Dlogback.configurationFile=test/conf/logback-burntest.xml"/>
+      <jvmarg value="-Dnet.bytebuddy.experimental=true"/>
     </testmacro>
   </target>
 
diff --git a/conf/cassandra.yaml b/conf/cassandra.yaml
index b207ff289b9b..7f8a2bfd9189 100644
--- a/conf/cassandra.yaml
+++ b/conf/cassandra.yaml
@@ -2129,7 +2129,7 @@ audit_logging_options:
   # excluded_categories:
   # included_users:
   # excluded_users:
-  # roll_cycle: HOURLY
+  # roll_cycle: FAST_HOURLY
   # block: true
   # max_queue_weight: 268435456 # 256 MiB
   # max_log_size: 17179869184 # 16 GiB
@@ -2144,7 +2144,7 @@ audit_logging_options:
 # nodetool enablefullquerylog
 # full_query_logging_options:
   # log_dir:
-  # roll_cycle: HOURLY
+  # roll_cycle: FAST_HOURLY
   # block: true
   # max_queue_weight: 268435456 # 256 MiB
   # max_log_size: 17179869184 # 16 GiB
diff --git a/conf/jvm-server.options b/conf/jvm-server.options
index 3abb5505830d..12aac15b0663 100644
--- a/conf/jvm-server.options
+++ b/conf/jvm-server.options
@@ -185,6 +185,25 @@
 # For production use you may wish to adjust this for your environment.
 # If that's the case, see MAX_HEAP_SIZE (and HEAP_NEWSIZE for CMS) in cassandra-env.sh
 
+# Need experimental bytebuddy for JDK21
+-Dnet.bytebuddy.experimental
+
+# Young generation size is automatically calculated by cassandra-env
+# based on this formula: min(100 * num_cores, 1/4 * heap size)
+#
+# The main trade-off for the young generation is that the larger it
+# is, the longer GC pause times will be. The shorter it is, the more
+# expensive GC will be (usually).
+#
+# It is not recommended to set the young generation size if using the
+# G1 GC, since that will override the target pause-time goal.
+# More info: http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html
+#
+# The example below assumes a modern 8-core+ machine for decent
+# times. If in doubt, and if you do not particularly want to tweak, go
+# 100 MB per physical CPU core.
+#-Xmn800M
+
 #####################
 # OFF-HEAP SETTINGS #
 #####################
diff --git a/conf/jvm21-clients.options b/conf/jvm21-clients.options
new file mode 100644
index 000000000000..25e60a10eeb9
--- /dev/null
+++ b/conf/jvm21-clients.options
@@ -0,0 +1,53 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+###########################################################################
+#                         jvm21-clients.options                           #
+#                                                                         #
+# See jvm-clients.options. This file is specific for Java 21              #
+###########################################################################
+
+###################
+#  JPMS SETTINGS  #
+###################
+
+-Djdk.attach.allowAttachSelf=true
+
+--add-exports java.base/java.lang.ref=ALL-UNNAMED
+--add-exports java.base/jdk.internal.misc=ALL-UNNAMED
+--add-exports java.management.rmi/com.sun.jmx.remote.internal.rmi=ALL-UNNAMED
+--add-exports java.rmi/sun.rmi.registry=ALL-UNNAMED
+--add-exports java.rmi/sun.rmi.server=ALL-UNNAMED
+--add-exports java.sql/java.sql=ALL-UNNAMED
+--add-exports jdk.unsupported/sun.misc=ALL-UNNAMED
+
+--add-opens java.base/java.io=ALL-UNNAMED
+--add-opens java.base/java.lang=ALL-UNNAMED
+--add-opens java.base/java.lang.module=ALL-UNNAMED
+--add-opens java.base/java.lang.reflect=ALL-UNNAMED
+--add-opens java.base/java.nio=ALL-UNNAMED
+--add-opens java.base/java.util=ALL-UNNAMED
+--add-opens java.base/jdk.internal.loader=ALL-UNNAMED
+--add-opens java.base/jdk.internal.math=ALL-UNNAMED
+--add-opens java.base/jdk.internal.module=ALL-UNNAMED
+--add-opens java.base/jdk.internal.ref=ALL-UNNAMED
+--add-opens java.base/jdk.internal.reflect=ALL-UNNAMED
+--add-opens java.base/sun.nio.ch=ALL-UNNAMED
+--add-opens jdk.management/com.sun.management.internal=ALL-UNNAMED
+
+# The newline in the end of file is intentional
diff --git a/conf/jvm21-server.options b/conf/jvm21-server.options
new file mode 100644
index 000000000000..9427cabc37b4
--- /dev/null
+++ b/conf/jvm21-server.options
@@ -0,0 +1,149 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+###########################################################################
+#                         jvm21-server.options                            #
+#                                                                         #
+# See jvm-server.options. This file is specific for Java 21 and newer.    #
+###########################################################################
+
+#################
+#  GC SETTINGS  #
+#################
+
+
+### Generational ZGC settings
+-XX:+UseZGC
+-XX:+ZGenerational
+
+# Temporary workaround for jamm's incorrect default CompressedOops; if you're using ZGC this needs to be set
+-XX:-UseCompressedOops
+
+# Blanket enabling of all the THP / large pages available in the env
+# -XX:+UseLargePages
+
+# Max size ZGC will _attempt_ to stay below
+# -XX:+SoftMaxHeapSize=12G
+
+# Don't let ZGC return unclaimed memory to the OS by setting min and max ==
+# -Xms12G </string>
+# -Xmx12G </string>
+# Alternatively you can use this to disable returning memory to OS
+# -XX:-ZUncommit</string>
+
+# Time memory should have been unused before eligible for uncommit in seconds
+# -XX:ZUncommitDelay=300</string>
+
+# Tell GC to touch heap and allocate and fault in memory pages backing heap at startup. Can prevent later allocation pauses
+# -XX:+AlwaysPreTouch</string>
+
+### G1 Settings
+## Use the Hotspot garbage-first collector.
+#-XX:+UseG1GC
+#-XX:+ParallelRefProcEnabled
+#-XX:MaxTenuringThreshold=2
+#-XX:G1HeapRegionSize=16m
+
+# Floor the young generation size to 50% of the heap size
+#-XX:+UnlockExperimentalVMOptions
+#-XX:G1NewSizePercent=50
+
+# Have the JVM do less remembered set work during STW, instead
+# preferring concurrent GC. Reduces p99.9 latency.
+#-XX:G1RSetUpdatingPauseTimePercent=5
+
+# Main G1GC tunable: lowering the pause target will lower throughput and vise versa.
+# 200ms is the JVM default and lowest viable setting
+# 1000ms increases throughput. Keep it smaller than the timeouts in cassandra.yaml.
+#-XX:MaxGCPauseMillis=300
+
+## Optional G1 Settings
+# Save CPU time on large (>= 16GB) heaps by delaying region scanning
+# until the heap is 70% full. The default in Hotspot 8u40 is 40%.
+#-XX:InitiatingHeapOccupancyPercent=70
+
+# For systems with > 8 cores, the default ParallelGCThreads is 5/8 the number of logical cores.
+# Otherwise equal to the number of cores when 8 or less.
+# Machines with > 10 cores should try setting these to <= full cores.
+# By default, ConcGCThreads is 1/4 of ParallelGCThreads.
+# Setting both to the same value can reduce STW durations.
+# When leaving both unset then cassandra-env.sh will set them both to the number of your cores.
+#-XX:ParallelGCThreads=16
+#-XX:ConcGCThreads=16
+
+### JPMS
+
+-Djdk.attach.allowAttachSelf=true
+
+--add-exports java.base/java.lang.ref=ALL-UNNAMED
+--add-exports java.base/jdk.internal.misc=ALL-UNNAMED
+--add-exports java.management.rmi/com.sun.jmx.remote.internal.rmi=ALL-UNNAMED
+--add-exports java.rmi/sun.rmi.registry=ALL-UNNAMED
+--add-exports java.rmi/sun.rmi.server=ALL-UNNAMED
+--add-exports java.sql/java.sql=ALL-UNNAMED
+--add-exports jdk.unsupported/sun.misc=ALL-UNNAMED
+
+--add-opens java.base/java.io=ALL-UNNAMED
+--add-opens java.base/java.lang=ALL-UNNAMED
+--add-opens java.base/java.lang.module=ALL-UNNAMED
+--add-opens java.base/java.lang.reflect=ALL-UNNAMED
+--add-opens java.base/java.nio=ALL-UNNAMED
+--add-opens java.base/java.util=ALL-UNNAMED
+--add-opens java.base/jdk.internal.loader=ALL-UNNAMED
+--add-opens java.base/jdk.internal.math=ALL-UNNAMED
+--add-opens java.base/jdk.internal.module=ALL-UNNAMED
+--add-opens java.base/jdk.internal.ref=ALL-UNNAMED
+--add-opens java.base/jdk.internal.reflect=ALL-UNNAMED
+--add-opens java.base/sun.nio.ch=ALL-UNNAMED
+--add-opens jdk.management/com.sun.management.internal=ALL-UNNAMED
+
+
+### GC logging options -- uncomment to enable
+
+# Java 11 (and newer) GC logging options:
+# See description of https://bugs.openjdk.java.net/browse/JDK-8046148 for details about the syntax
+# The following is the equivalent to -XX:+PrintGCDetails -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=10M
+#-Xlog:gc=info,heap*=trace,age*=debug,safepoint=info,promotion*=trace:file=/var/log/cassandra/gc.log:time,uptime,pid,tid,level:filecount=10,filesize=10485760
+
+# Notes for Java 8 migration:
+#
+# -XX:+PrintGCDetails                   maps to -Xlog:gc*:... - i.e. add a '*' after "gc"
+# -XX:+PrintGCDateStamps                maps to decorator 'time'
+#
+# -XX:+PrintHeapAtGC                    maps to 'heap' with level 'trace'
+# -XX:+PrintTenuringDistribution        maps to 'age' with level 'debug'
+# -XX:+PrintGCApplicationStoppedTime    maps to 'safepoint' with level 'info'
+# -XX:+PrintPromotionFailure            maps to 'promotion' with level 'trace'
+# -XX:PrintFLSStatistics=1              maps to 'freelist' with level 'trace'
+
+### Netty Options
+
+# On Java >= 9 Netty requires the io.netty.tryReflectionSetAccessible system property to be set to true to enable
+# creation of direct buffers using Unsafe. Without it, this falls back to ByteBuffer.allocateDirect which has
+# inferior performance and risks exceeding MaxDirectMemory
+-Dio.netty.tryReflectionSetAccessible=true
+
+# Revert changes in defaults introduced in https://netty.io/news/2022/03/10/4-1-75-Final.html
+-Dio.netty.allocator.useCacheForAllThreads=true
+-Dio.netty.allocator.maxOrder=11
+
+
+# Need to explicitly allow security manager on JDK21; deprecated for removal
+-Djava.security.manager=allow
+
+# The newline in the end of file is intentional
diff --git a/debian/control b/debian/control
index 7e712018e47c..670d9b635b13 100644
--- a/debian/control
+++ b/debian/control
@@ -3,7 +3,7 @@ Section: misc
 Priority: extra
 Maintainer: Eric Evans <eevans@apache.org>
 Uploaders: Sylvain Lebresne <slebresne@apache.org>
-Build-Depends: debhelper (>= 11), openjdk-17-jdk-headless | openjdk-11-jdk-headless | java17-sdk | java11-sdk, ant (>= 1.10), ant-optional (>= 1.10), dh-python, python3-dev (>= 3.6), quilt, bash-completion
+Build-Depends: debhelper (>= 11), openjdk-21-jdk-headless | openjdk-17-jdk-headless | openjdk-11-jdk-headless | java21-sdk | java17-sdk | java11-sdk, ant (>= 1.10), ant-optional (>= 1.10), dh-python, python3-dev (>= 3.6), quilt, bash-completion
 Homepage: https://cassandra.apache.org
 Vcs-Git: https://gitbox.apache.org/repos/asf/cassandra.git
 Vcs-Browser: https://gitbox.apache.org/repos/asf?p=cassandra.git
@@ -11,7 +11,7 @@ Standards-Version: 3.8.3
 
 Package: cassandra
 Architecture: all
-Depends: openjdk-17-jre-headless | openjdk-11-jre-headless | java17-runtime | java11-runtime, adduser, python3 (>= 3.6), procps, ${misc:Depends}
+Depends: openjdk-21-jre-headless | openjdk-17-jre-headless | openjdk-11-jre-headless | java21-runtime | java17-runtime | java11-runtime, adduser, python3 (>= 3.6), procps, ${misc:Depends}
 Recommends: ntp | time-daemon
 Suggests: cassandra-tools
 Conflicts: apache-cassandra1
diff --git a/doc/modules/cassandra/pages/managing/configuration/cass_jvm_options_file.adoc b/doc/modules/cassandra/pages/managing/configuration/cass_jvm_options_file.adoc
index b9057a490424..132669f6438f 100644
--- a/doc/modules/cassandra/pages/managing/configuration/cass_jvm_options_file.adoc
+++ b/doc/modules/cassandra/pages/managing/configuration/cass_jvm_options_file.adoc
@@ -1,14 +1,13 @@
 = jvm-* files
 
 Several files for JVM configuration are included in Cassandra. The
-`jvm-server.options` file, and corresponding files `jvm8-server.options`
-and `jvm11-server.options` are the main file for settings that affect
+`jvm-server.options` file, and corresponding JDK specific files
+`jvmN-server.options` are the main file for settings that affect
 the operation of the Cassandra JVM on cluster nodes. The file includes
 startup parameters, general JVM settings such as garbage collection, and
-heap settings. The `jvm-clients.options` and corresponding
-`jvm8-clients.options` and `jvm11-clients.options` files can be used to
-configure JVM settings for clients like `nodetool` and the `sstable`
-tools.
+heap settings. Likewise, the `jvm-clients.options` and corresponding
+`jvmN-clients.options` files can be used to configure JVM settings for
+clients like `nodetool` and the `sstable` tools.
 
 See each file for examples of settings.
 
@@ -18,4 +17,4 @@ The `jvm-\*` files replace the `cassandra-env.sh` file used in Cassandra
 versions prior to Cassandra 3.0. The `cassandra-env.sh` bash script file
 is still useful if JVM settings must be dynamically calculated based on
 system settings. The `jvm-*` files only store static JVM settings.
-====
+====
\ No newline at end of file
diff --git a/doc/modules/cassandra/pages/managing/operating/audit_logging.adoc b/doc/modules/cassandra/pages/managing/operating/audit_logging.adoc
index a40fcc5b2e43..89387d043d04 100644
--- a/doc/modules/cassandra/pages/managing/operating/audit_logging.adoc
+++ b/doc/modules/cassandra/pages/managing/operating/audit_logging.adoc
@@ -190,9 +190,11 @@ To use `BinAuditLogger` as a logger in AuditLogging, set the logger to `BinAudit
   oldest file. Default is set to `16L * 1024L * 1024L * 1024L`
 `roll_cycle`::
   How often to roll Audit log segments so they can potentially be
-  reclaimed. Available options are: MINUTELY, HOURLY, DAILY,
-  LARGE_DAILY, XLARGE_DAILY, HUGE_DAILY.For more options, refer:
-  net.openhft.chronicle.queue.RollCycles. Default is set to `"HOURLY"`
+  reclaimed. Some available options are:
+  `FIVE_MINUTELY, FAST_HOURLY, FAST_DAILY,
+  LargeRollCycles.LARGE_DAILY, LargeRollCycles.XLARGE_DAILY, LargeRollCycles.HUGE_DAILY.`
+  For more options, refer: net.openhft.chronicle.queue.RollCycles.
+  Default is set to `"FAST_HOURLY"`
 
 == Configuring FileAuditLogger
 
diff --git a/ide/idea/workspace.xml b/ide/idea/workspace.xml
index 611a93735e59..6419f66b56d3 100644
--- a/ide/idea/workspace.xml
+++ b/ide/idea/workspace.xml
@@ -151,6 +151,7 @@
                                           -Dcassandra.storagedir=$PROJECT_DIR$/data
                                           -Dlogback.configurationFile=file://$PROJECT_DIR$/conf/logback.xml
                                           -XX:HeapDumpPath=build/test
+                                          -Dnet.bytebuddy.experimental=true
                                           -ea" />
       <option name="PROGRAM_PARAMETERS" value="" />
       <option name="WORKING_DIRECTORY" value="" />
@@ -217,6 +218,8 @@
                                           -XX:MaxMetaspaceSize=2G
                                           -XX:ReservedCodeCacheSize=256M
                                           -XX:Tier4CompileThreshold=1000
+                                          -XX:SoftRefLRUPolicyMSPerMB=0
+                                          -Dnet.bytebuddy.experimental=true
                                           -ea" />
       <option name="PARAMETERS" value="" />
       <fork_mode value="class" />
@@ -248,6 +251,7 @@
                                           -Dlogback.configurationFile=file://$PROJECT_DIR$/conf/logback.xml
                                           -XX:HeapDumpPath=build/test
                                           -Xmx1G
+                                          -Dnet.bytebuddy.experimental=true
                                           -ea" />
       <option name="PROGRAM_PARAMETERS" value="" />
       <option name="WORKING_DIRECTORY" value="file://$PROJECT_DIR$" />
diff --git a/pylib/cassandra-cqlsh-tests.sh b/pylib/cassandra-cqlsh-tests.sh
index be444f13a7b3..a0403b1696cd 100755
--- a/pylib/cassandra-cqlsh-tests.sh
+++ b/pylib/cassandra-cqlsh-tests.sh
@@ -59,7 +59,7 @@ set -e # enable immediate exit if venv setup fails
 # fresh virtualenv and test logs results everytime
 rm -fr ${DIST_DIR}/venv ${DIST_DIR}/test/{html,output,logs}
 
-# re-use when possible the pre-installed virtualenv found in the cassandra-ubuntu2004_test docker image
+# re-use when possible the pre-installed virtualenv found in the cassandra12004_test docker image
 virtualenv-clone ${BUILD_HOME}/env${python_version} ${BUILD_DIR}/venv || virtualenv --python=python3 ${BUILD_DIR}/venv
 source ${BUILD_DIR}/venv/bin/activate
 
diff --git a/redhat/cassandra.in.sh b/redhat/cassandra.in.sh
index c08a8316af21..beb82955dac1 100644
--- a/redhat/cassandra.in.sh
+++ b/redhat/cassandra.in.sh
@@ -121,57 +121,39 @@ if [ -z $JAVA ] ; then
     exit 1;
 fi
 
+# TODO: Factor this out to something we source so we don't have the duplication all over our scripts
 # Matches variable 'java.supported' in build.xml
-java_versions_supported=11,17
+java_versions_supported=(11 17 21)
+java_version_string=$(IFS=" "; echo "${java_versions_supported[*]}")
 
 # Determine the sort of JVM we'll be running on.
-java_ver_output=`"${JAVA:-java}" -version 2>&1`
-jvmver=`echo "$java_ver_output" | grep '[openjdk|java] version' | awk -F'"' 'NR==1 {print $2}' | cut -d\- -f1`
-JVM_VERSION=${jvmver%_*}
-short=$(echo "${jvmver}" | cut -c1-2)
-
-# Unsupported JDKs below the upper supported version are not allowed
-if [ "$short" != "$(echo "$java_versions_supported" | cut -d, -f1)" ] && [ "$JVM_VERSION" \< "$(echo "$java_versions_supported" | cut -d, -f2)" ] ; then
-    echo "Unsupported Java $JVM_VERSION. Supported are $java_versions_supported"
-    exit 1;
-fi
-# Allow execution of supported Java versions, and newer if CASSANDRA_JDK_UNSUPPORTED is set
-is_supported_version=$(echo "$java_versions_supported" | tr "," '\n' | grep -F -x "$short")
-if [ -z "$is_supported_version" ] ; then
-    if [ -z "$CASSANDRA_JDK_UNSUPPORTED" ] ; then
-        echo "Unsupported Java $JVM_VERSION. Supported are $java_versions_supported"
-        echo "If you would like to test with newer Java versions set CASSANDRA_JDK_UNSUPPORTED to any value (for example, CASSANDRA_JDK_UNSUPPORTED=true). Unset the parameter for default behavior"
-        exit 1;
-    else
-        echo "######################################################################"
-        echo "Warning! You are using JDK$short. This Cassandra version only supports $java_versions_supported."
-        echo "######################################################################"
-    fi
+JAVA_VERSION=$(java -version 2>&1 | grep '[openjdk|java] version' | cut -d '"' -f2 | cut -d '.' -f1)
+
+supported=0
+for version in "${java_versions_supported[@]}"; do
+  if [ "$version" -eq "$JAVA_VERSION" ]; then
+      supported=1
+  fi
+done
+
+if [[ "$supported" -eq 0 ]]; then
+  if [ -z "$CASSANDRA_JDK_UNSUPPORTED" ]; then
+    echo "Unsupported Java $JAVA_VERSION. Supported are $java_version_string"
+    echo "If you would like to test with newer Java versions set CASSANDRA_JDK_UNSUPPORTED to any value (for example, CASSANDRA_JDK_UNSUPPORTED=true). Unset the parameter for default behavior"
+    exit 1
+  else
+    echo "######################################################################"
+    echo "Warning! You are using JDK$JAVA_VERSION. This Cassandra version only supports $java_version_string"
+    echo "######################################################################"
+  fi
 fi
-JAVA_VERSION=$short
-
-jvm=`echo "$java_ver_output" | grep -A 1 '[openjdk|java] version' | awk 'NR==2 {print $1}'`
-case "$jvm" in
-    OpenJDK)
-        JVM_VENDOR=OpenJDK
-        # this will be "64-Bit" or "32-Bit"
-        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $2}'`
-        ;;
-    "Java(TM)")
-        JVM_VENDOR=Oracle
-        # this will be "64-Bit" or "32-Bit"
-        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $3}'`
-        ;;
-    *)
-        # Help fill in other JVM values
-        JVM_VENDOR=other
-        JVM_ARCH=unknown
-        ;;
-esac
+
 
 # Read user-defined JVM options from jvm-server.options file
 JVM_OPTS_FILE=$CASSANDRA_CONF/jvm${jvmoptions_variant:--clients}.options
-if [ $JAVA_VERSION -ge 17 ] ; then
+if [ $JAVA_VERSION -ge 21 ] ; then
+    JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm21${jvmoptions_variant:--clients}.options
+elif [ $JAVA_VERSION -ge 17 ] ; then
     JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm17${jvmoptions_variant:--clients}.options
 elif [ $JAVA_VERSION -ge 11 ] ; then
     JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm11${jvmoptions_variant:--clients}.options
@@ -188,4 +170,4 @@ if [ -n "$USING_JDK" ] && [ "$JAVA_VERSION" -ge 17 ]; then
   JVM_OPTS="$JVM_OPTS --add-exports jdk.attach/sun.tools.attach=ALL-UNNAMED"
   JVM_OPTS="$JVM_OPTS --add-exports jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED"
   JVM_OPTS="$JVM_OPTS --add-opens jdk.compiler/com.sun.tools.javac=ALL-UNNAMED"
-fi
+fi
\ No newline at end of file
diff --git a/redhat/cassandra.spec b/redhat/cassandra.spec
index c151ebaa12b1..13a6cfddc3eb 100644
--- a/redhat/cassandra.spec
+++ b/redhat/cassandra.spec
@@ -49,7 +49,7 @@ BuildRoot:     %{_tmppath}/%{relname}root-%(%{__id_u} -n)
 BuildRequires: ant >= 1.9
 BuildRequires: ant-junit >= 1.9
 
-Requires:      (java-11-headless or java-17-headless)
+Requires:      (java-11-headless or java-17-headless or java-21-headless)
 Requires:      python(abi) >= 3.6
 Requires:      procps-ng >= 3.3
 Requires(pre): user(cassandra)
diff --git a/src/java/org/apache/cassandra/config/Config.java b/src/java/org/apache/cassandra/config/Config.java
index 193c77c8f4cb..d272c0bc6e3c 100644
--- a/src/java/org/apache/cassandra/config/Config.java
+++ b/src/java/org/apache/cassandra/config/Config.java
@@ -600,6 +600,13 @@ public static class SSTableConfig
     public volatile DurationSpec.IntMillisecondsBound gc_concurrent_phase_log_threshold = new DurationSpec.IntMillisecondsBound("1s");
     public volatile DurationSpec.IntMillisecondsBound gc_concurrent_phase_warn_threshold = new DurationSpec.IntMillisecondsBound("2s");
 
+    public volatile DurationSpec.IntMillisecondsBound gc_log_zgc_threshold = new DurationSpec.IntMillisecondsBound("20s");
+    public volatile DurationSpec.IntMillisecondsBound gc_warn_zgc_threshold = new DurationSpec.IntMillisecondsBound("30s");
+
+    // these are only used for ZGC now - for the "ZGC Minor/Major Pauses" events, which should be < 1ms in the normal case
+    public volatile DurationSpec.IntMillisecondsBound gc_pause_log_threshold = new DurationSpec.IntMillisecondsBound("2ms");
+    public volatile DurationSpec.IntMillisecondsBound gc_pause_warn_threshold = new DurationSpec.IntMillisecondsBound("10ms");
+
     // TTL for different types of trace events.
     @Replaces(oldName = "tracetype_query_ttl", converter = Converters.SECONDS_DURATION, deprecated=true)
     public DurationSpec.IntSecondsBound trace_type_query_ttl = new DurationSpec.IntSecondsBound("1d");
diff --git a/src/java/org/apache/cassandra/config/DatabaseDescriptor.java b/src/java/org/apache/cassandra/config/DatabaseDescriptor.java
index 809a44fdcc63..98e29242320b 100644
--- a/src/java/org/apache/cassandra/config/DatabaseDescriptor.java
+++ b/src/java/org/apache/cassandra/config/DatabaseDescriptor.java
@@ -4769,15 +4769,52 @@ public static long getGCLogThreshold()
 
     public static void setGCLogThreshold(int threshold)
     {
-        if (threshold <= 0)
-            throw new IllegalArgumentException("Threshold value for gc_log_threshold must be greater than 0");
+        validateGCParams(threshold, getGCWarnThreshold());
+        conf.gc_log_threshold = new DurationSpec.IntMillisecondsBound(threshold);
+    }
 
-        long gcWarnThresholdInMs = getGCWarnThreshold();
-        if (gcWarnThresholdInMs != 0 && threshold > gcWarnThresholdInMs)
-            throw new IllegalArgumentException("Threshold value for gc_log_threshold (" + threshold + ") must be less than gc_warn_threshold which is currently "
-                                               + gcWarnThresholdInMs);
+    public static void validateGCParams(long logThreshold, long warnThreshold)
+    {
+        if (logThreshold <= 0)
+            throw new IllegalArgumentException("Threshold value for gc_log*_threshold must be greater than 0");
+        if (logThreshold > Integer.MAX_VALUE)
+            throw new IllegalArgumentException("Threshold value for gc_log*_threshold must be less than Integer.MAX_VALUE");
+
+        if (warnThreshold <= 0)
+            throw new IllegalArgumentException("Threshold value for gc_warn*_threshold must be greater than 0");
+        if (warnThreshold > Integer.MAX_VALUE)
+            throw new IllegalArgumentException("Threshold value for gc_warn*_threshold must be less than Integer.MAX_VALUE");
+
+        if (warnThreshold != 0 && logThreshold > warnThreshold)
+            throw new IllegalArgumentException("Threshold value for gc_log*_threshold (" + logThreshold + ") must be less than gc_warn*_threshold which is currently "
+                    + warnThreshold);
+    }
+
+    public static long getZGCLogThreshold()
+    {
+        return conf.gc_log_zgc_threshold.toMilliseconds();
+    }
+
+    public static void setZGCLogThreshold(long threshold)
+    {
+        validateGCParams(threshold, getZGCWarnThreshold());
+        conf.gc_log_zgc_threshold = new DurationSpec.IntMillisecondsBound(threshold);
+    }
+
+    public static long getGCPauseLogThreshold()
+    {
+        return conf.gc_pause_log_threshold.toMilliseconds();
+    }
+
+    public static void setGCPauseLogThreshold(long threshold)
+    {
+        conf.gc_pause_log_threshold = new DurationSpec.IntMillisecondsBound(threshold);
+    }
+
+    public static EncryptionContext getEncryptionContext()
+    {
+        return encryptionContext;
 
-        conf.gc_log_threshold = new DurationSpec.IntMillisecondsBound(threshold);
     }
 
     public static long getGCWarnThreshold()
@@ -4785,11 +4822,14 @@ public static long getGCWarnThreshold()
         return conf.gc_warn_threshold.toMilliseconds();
     }
 
-    public static void setGCWarnThreshold(int threshold)
+    public static void setGCWarnThreshold(long threshold)
     {
         if (threshold < 0)
             throw new IllegalArgumentException("Threshold value for gc_warn_threshold must be greater than or equal to 0");
 
+        if (threshold > Integer.MAX_VALUE)
+            throw new IllegalArgumentException("Threshold must be less than Integer.MAX_VALUE");
+
         long gcLogThresholdInMs = getGCLogThreshold();
         if (threshold != 0 && threshold <= gcLogThresholdInMs)
             throw new IllegalArgumentException("Threshold value for gc_warn_threshold (" + threshold + ") must be greater than gc_log_threshold which is currently "
@@ -4798,6 +4838,37 @@ public static void setGCWarnThreshold(int threshold)
         conf.gc_warn_threshold = new DurationSpec.IntMillisecondsBound(threshold);
     }
 
+    public static long getZGCWarnThreshold()
+    {
+        return conf.gc_warn_zgc_threshold.toMilliseconds();
+    }
+
+    public static void setZGCWarnThreshold(long threshold)
+    {
+        if (threshold < 0)
+            throw new IllegalArgumentException("Threshold value for gc_warn_zgc_threshold must be greater than or equal to 0");
+
+        if (threshold > Integer.MAX_VALUE)
+            throw new IllegalArgumentException("Threshold must be less than Integer.MAX_VALUE");
+
+        long gcLogThresholdInMs = getZGCLogThreshold();
+        if (threshold != 0 && threshold <= gcLogThresholdInMs)
+            throw new IllegalArgumentException("Threshold value for  (" + threshold + ") must be greater than gc_log_zgc_threshold which is currently "
+                    + gcLogThresholdInMs);
+
+        conf.gc_warn_zgc_threshold = new DurationSpec.IntMillisecondsBound(threshold);
+    }
+
+    public static long getGCPauseWarnThreshold()
+    {
+        return conf.gc_pause_warn_threshold.toMilliseconds();
+    }
+
+    public static void setGCPauseWarnThreshold(long threshold)
+    {
+        conf.gc_pause_warn_threshold = new DurationSpec.IntMillisecondsBound(threshold);
+    }
+
     public static int getGCConcurrentPhaseLogThreshold()
     {
         return conf.gc_concurrent_phase_log_threshold.toMilliseconds();
@@ -4834,11 +4905,6 @@ public static void setGCConcurrentPhaseWarnThreshold(int threshold)
         conf.gc_concurrent_phase_warn_threshold = new DurationSpec.IntMillisecondsBound(threshold);
     }
 
-    public static EncryptionContext getEncryptionContext()
-    {
-        return encryptionContext;
-    }
-
     public static boolean isCDCEnabled()
     {
         return conf.cdc_enabled;
diff --git a/src/java/org/apache/cassandra/db/commitlog/CommitLogSegment.java b/src/java/org/apache/cassandra/db/commitlog/CommitLogSegment.java
index ea9306c62c31..98b8f9dc0da9 100644
--- a/src/java/org/apache/cassandra/db/commitlog/CommitLogSegment.java
+++ b/src/java/org/apache/cassandra/db/commitlog/CommitLogSegment.java
@@ -640,6 +640,7 @@ public String dirtyString()
         {
             TableMetadata m = Schema.instance.getTableMetadata(tableId);
             sb.append(m == null ? "<deleted>" : m.name).append(" (").append(tableId)
+              .append(", keyspace: ").append(m.keyspace)
               .append(", dirty: ").append(tableDirty.get(tableId))
               .append(", clean: ").append(tableClean.get(tableId))
               .append("), ");
diff --git a/src/java/org/apache/cassandra/index/sasi/utils/trie/AbstractPatriciaTrie.java b/src/java/org/apache/cassandra/index/sasi/utils/trie/AbstractPatriciaTrie.java
index 8067ccc30cdc..8b89eaee4a45 100644
--- a/src/java/org/apache/cassandra/index/sasi/utils/trie/AbstractPatriciaTrie.java
+++ b/src/java/org/apache/cassandra/index/sasi/utils/trie/AbstractPatriciaTrie.java
@@ -790,7 +790,7 @@ TrieEntry<K, V> nextEntryImpl(TrieEntry<K, V> start, TrieEntry<K, V> previous, T
      * This is implemented by going always to the left until
      * we encounter a valid uplink. That uplink is the first key.
      */
-    TrieEntry<K, V> firstEntry()
+    public TrieEntry<K, V> firstEntry()
     {
         // if Trie is empty, no first node.
         return isEmpty() ? null : followLeft(root);
diff --git a/src/java/org/apache/cassandra/index/sasi/utils/trie/PatriciaTrie.java b/src/java/org/apache/cassandra/index/sasi/utils/trie/PatriciaTrie.java
index 9beeb29a61a7..606f8797b23b 100644
--- a/src/java/org/apache/cassandra/index/sasi/utils/trie/PatriciaTrie.java
+++ b/src/java/org/apache/cassandra/index/sasi/utils/trie/PatriciaTrie.java
@@ -422,7 +422,7 @@ private TrieEntry<K, V> subtree(K prefix)
      * <p>This is implemented by going always to the right until
      * we encounter a valid uplink. That uplink is the last key.
      */
-    private TrieEntry<K, V> lastEntry()
+    public TrieEntry<K, V> lastEntry()
     {
         return followRight(root.left);
     }
diff --git a/src/java/org/apache/cassandra/repair/RepairSession.java b/src/java/org/apache/cassandra/repair/RepairSession.java
index 7d5d3e550bbe..33248a09eee0 100644
--- a/src/java/org/apache/cassandra/repair/RepairSession.java
+++ b/src/java/org/apache/cassandra/repair/RepairSession.java
@@ -517,4 +517,10 @@ public void shutdown()
             delegate.shutdown();
         }
     }
+
+    @VisibleForTesting
+    public int syncingCount()
+    {
+        return syncingTasks.size();
+    }
 }
diff --git a/src/java/org/apache/cassandra/repair/autorepair/AutoRepairState.java b/src/java/org/apache/cassandra/repair/autorepair/AutoRepairState.java
index 6e7a19bc1234..7ba37afe6d9e 100644
--- a/src/java/org/apache/cassandra/repair/autorepair/AutoRepairState.java
+++ b/src/java/org/apache/cassandra/repair/autorepair/AutoRepairState.java
@@ -382,4 +382,4 @@ public RepairCoordinator getRepairRunnable(String keyspace, List<String> tables,
 
         return getRepairRunnable(keyspace, option);
     }
-}
+}
\ No newline at end of file
diff --git a/src/java/org/apache/cassandra/service/GCInspector.java b/src/java/org/apache/cassandra/service/GCInspector.java
index 2d86019d9dc0..6c9caff6a9d7 100644
--- a/src/java/org/apache/cassandra/service/GCInspector.java
+++ b/src/java/org/apache/cassandra/service/GCInspector.java
@@ -64,6 +64,8 @@ public class GCInspector implements NotificationListener, GCInspectorMXBean
     // This represents the amount of direct memory that has been reserved for future use but not necessarily allocated yet.
     final static Field BITS_RESERVED;
 
+    private final boolean hasZGC;
+
     static
     {
         Class<?> bitsClass = null;
@@ -123,15 +125,17 @@ static final class GCState
         final GarbageCollectorMXBean gcBean;
         final boolean assumeGCIsPartiallyConcurrent;
         final boolean assumeGCIsOldGen;
+        final boolean isZGC;
         private String[] keys;
         long lastGcTotalDuration = 0;
 
 
-        GCState(GarbageCollectorMXBean gcBean, boolean assumeGCIsPartiallyConcurrent, boolean assumeGCIsOldGen)
+        GCState(GarbageCollectorMXBean gcBean, boolean assumeGCIsPartiallyConcurrent, boolean assumeGCIsOldGen, boolean isZGC)
         {
             this.gcBean = gcBean;
             this.assumeGCIsPartiallyConcurrent = assumeGCIsPartiallyConcurrent;
             this.assumeGCIsOldGen = assumeGCIsOldGen;
+            this.isZGC = isZGC;
         }
 
         String[] keys(GarbageCollectionNotificationInfo info)
@@ -144,6 +148,11 @@ String[] keys(GarbageCollectionNotificationInfo info)
 
             return keys;
         }
+
+        public boolean isZGC()
+        {
+            return isZGC;
+        }
     }
 
     final AtomicReference<State> state = new AtomicReference<>(new State());
@@ -155,11 +164,15 @@ public GCInspector()
         try
         {
             ObjectName gcName = new ObjectName(ManagementFactory.GARBAGE_COLLECTOR_MXBEAN_DOMAIN_TYPE + ",*");
+            boolean sawZGC = false;
             for (ObjectName name : MBeanWrapper.instance.queryNames(gcName, null))
             {
                 GarbageCollectorMXBean gc = ManagementFactory.newPlatformMXBeanProxy(MBeanWrapper.instance.getMBeanServer(), name.getCanonicalName(), GarbageCollectorMXBean.class);
-                gcStates.put(gc.getName(), new GCState(gc, assumeGCIsPartiallyConcurrent(gc), assumeGCIsOldGen(gc)));
+                if (isZGC(gc))
+                    sawZGC = true;
+                gcStates.put(gc.getName(), new GCState(gc, assumeGCIsPartiallyConcurrent(gc), assumeGCIsOldGen(gc), isZGC(gc)));
             }
+            this.hasZGC = sawZGC;
             ObjectName me = new ObjectName(MBEAN_NAME);
             if (!MBeanWrapper.instance.isRegistered(me))
                 MBeanWrapper.instance.registerMBean(this, new ObjectName(MBEAN_NAME));
@@ -182,32 +195,45 @@ public static void register() throws Exception
         }
     }
 
+    private static boolean isZGC(GarbageCollectorMXBean gc)
+    {
+        return gc.getName().contains("ZGC");
+    }
+
     /*
      * Assume that a GC type is at least partially concurrent and so a side channel method
      * should be used to calculate application stopped time due to the GC.
      *
      * If the GC isn't recognized then assume that is concurrent and we need to do our own calculation
-     * via the the side channel.
+     * via the side channel.
      */
     private static boolean assumeGCIsPartiallyConcurrent(GarbageCollectorMXBean gc)
     {
         switch (gc.getName())
         {
-                //First two are from the serial collector
+                // First two are from the serial collector
             case "Copy":
             case "MarkSweepCompact":
-                //Parallel collector
+                // Parallel collector
             case "PS MarkSweep":
             case "PS Scavenge":
             case "G1 Young Generation":
-                //CMS young generation collector
+                // CMS young generation collector
             case "ParNew":
+                // gen zgc
+            case "ZGC Minor Pauses":
+            case "ZGC Major Pauses":
+                // zgc
+            case "ZGC Pauses":
                 return false;
             case "ConcurrentMarkSweep":
             case "G1 Old Generation":
+            case "ZGC Minor Cycles":
+            case "ZGC Major Cycles":
+            case "ZGC Cycles":
                 return true;
             default:
-                //Assume possibly concurrent if unsure
+                // Assume possibly concurrent if unsure
                 return true;
         }
     }
@@ -226,15 +252,22 @@ private static boolean assumeGCIsOldGen(GarbageCollectorMXBean gc)
             case "PS Scavenge":
             case "G1 Young Generation":
             case "ParNew":
+            case "ZGC Minor Cycles":
+            case "ZGC Minor Pauses":
                 return false;
             case "MarkSweepCompact":
             case "PS MarkSweep":
             case "ConcurrentMarkSweep":
             case "G1 Old Generation":
+            case "ZGC Major Cycles":
+            case "ZGC Major Pauses":
+                // assume non-generational zgc is old gen
+            case "ZGC Cycles":
+            case "ZGC Pauses":
                 return true;
             default:
-                //Assume not old gen otherwise, don't call
-                //TransactionLogs.rescheduleFailedTasks()
+                // Assume not old gen otherwise, don't call
+                // TransactionLogs.rescheduleFailedTasks()
                 return false;
         }
     }
@@ -312,9 +345,24 @@ else if (logger.isTraceEnabled())
             }
             else
             {
-                if (getGcWarnThresholdInMs() != 0 && duration > getGcWarnThresholdInMs())
+                long warnThreshold;
+                long logThreshold;
+
+                if (gcState.isZGC && !gcState.assumeGCIsPartiallyConcurrent)
+                {
+                    // zgc pauses are usually sub 1ms - getGcPauseXThresholds are lower by default than the old thresholds
+                    warnThreshold = getGcPauseWarnThresholdInMs();
+                    logThreshold = getGcPauseLogThresholdInMs();
+                }
+                else
+                {
+                    warnThreshold = getGcWarnThresholdInMs();
+                    logThreshold = getGcLogThresholdInMs();
+                }
+
+                if (warnThreshold != 0 && duration > warnThreshold)
                     logger.warn(sb.toString());
-                else if (duration > getGcLogThresholdInMs())
+                else if (duration > logThreshold)
                     logger.info(sb.toString());
                 else if (logger.isTraceEnabled())
                     logger.trace(sb.toString());
@@ -425,19 +473,27 @@ private static long getFieldValue(Field field, boolean isAtomicLong)
         }
     }
 
+    /**
+     * While we _technically_ could only set the threshold based on which GC we're running, there's really no
+     * perceptable value-add from doing that. Responsibility for enforcing the validity of the params is entrusted
+     * to the {@link DatabaseDescriptor}
+     */
+    @Override
     public void setGcWarnThresholdInMs(long threshold)
     {
-        DatabaseDescriptor.setGCWarnThreshold((int)threshold);
+        DatabaseDescriptor.setGCWarnThreshold(threshold);
+        DatabaseDescriptor.setZGCWarnThreshold(threshold);
     }
 
     public long getGcWarnThresholdInMs()
     {
-        return DatabaseDescriptor.getGCWarnThreshold();
+        return hasZGC ? DatabaseDescriptor.getZGCWarnThreshold() : DatabaseDescriptor.getGCWarnThreshold();
     }
 
     public void setGcLogThresholdInMs(long threshold)
     {
         DatabaseDescriptor.setGCLogThreshold((int) threshold);
+        DatabaseDescriptor.setZGCLogThreshold((int) threshold);
     }
 
     public int getGcConcurrentPhaseWarnThresholdInMs()
@@ -462,7 +518,7 @@ public void setGcConcurrentPhaseLogThresholdInMs(int threshold)
 
     public long getGcLogThresholdInMs()
     {
-        return DatabaseDescriptor.getGCLogThreshold();
+        return hasZGC ? DatabaseDescriptor.getZGCLogThreshold() : DatabaseDescriptor.getGCLogThreshold();
     }
 
     public long getStatusThresholdInMs()
@@ -474,4 +530,36 @@ public long getConcurrentStatusThresholdInMs()
     {
         return getGcConcurrentPhaseWarnThresholdInMs() != 0 ? getGcConcurrentPhaseWarnThresholdInMs() : getGcConcurrentPhaseLogThresholdInMs();
     }
+
+    @Override
+    public long getGcPauseWarnThresholdInMs()
+    {
+        return DatabaseDescriptor.getGCPauseWarnThreshold();
+    }
+
+    @Override
+    public void setGcPauseWarnThresholdInMs(long threshold)
+    {
+        DatabaseDescriptor.setGCPauseWarnThreshold(threshold);
+    }
+
+    @Override
+    public long getGcPauseLogThresholdInMs()
+    {
+        return DatabaseDescriptor.getGCPauseLogThreshold();
+    }
+
+    @Override
+    public void setGcPauseLogThresholdInMs(long threshold)
+    {
+        if (threshold <= 0)
+            throw new IllegalArgumentException("Threshold must be greater than 0");
+
+        long gcWarnThresholdInMs = getGcPauseWarnThresholdInMs();
+        if (gcWarnThresholdInMs != 0 && threshold > gcWarnThresholdInMs)
+            throw new IllegalArgumentException("Threshold must be less than gcPauseWarnThresholdInMs which is currently "
+                                               + gcWarnThresholdInMs);
+
+        DatabaseDescriptor.setGCPauseLogThreshold(threshold);
+    }
 }
diff --git a/src/java/org/apache/cassandra/service/GCInspectorMXBean.java b/src/java/org/apache/cassandra/service/GCInspectorMXBean.java
index 9765e9813c3c..285a523cd1b1 100644
--- a/src/java/org/apache/cassandra/service/GCInspectorMXBean.java
+++ b/src/java/org/apache/cassandra/service/GCInspectorMXBean.java
@@ -32,5 +32,9 @@ public interface GCInspectorMXBean
     int getGcConcurrentPhaseWarnThresholdInMs();
     void setGcConcurrentPhaseLogThresholdInMs(int threshold);
     long getGcLogThresholdInMs();
+    void setGcPauseWarnThresholdInMs(long threshold);
+    long getGcPauseWarnThresholdInMs();
+    void setGcPauseLogThresholdInMs(long threshold);
+    long getGcPauseLogThresholdInMs();
     long getStatusThresholdInMs();
 }
diff --git a/src/java/org/apache/cassandra/service/StorageServiceMBean.java b/src/java/org/apache/cassandra/service/StorageServiceMBean.java
index 9c4ae481ec11..be54b4fc76dd 100644
--- a/src/java/org/apache/cassandra/service/StorageServiceMBean.java
+++ b/src/java/org/apache/cassandra/service/StorageServiceMBean.java
@@ -1216,7 +1216,7 @@ public void enableAuditLog(String loggerName, String includedKeyspaces, String e
      * Start the fully query logger.
      *
      * @param path Path where the full query log will be stored. If null cassandra.yaml value is used.
-     * @param rollCycle How often to create a new file for query data (MINUTELY, DAILY, HOURLY)
+     * @param rollCycle How often to create a new file for query data (FIVE_MINUTELY, FAST_DAILY, FAST_HOURLY)
      * @param blocking Whether threads submitting queries to the query log should block if they can't be drained to the filesystem or alternatively drops samples and log
      * @param maxQueueWeight How many bytes of query data to queue before blocking or dropping samples
      * @param maxLogSize How many bytes of log data to store before dropping segments. Might not be respected if a log file hasn't rolled so it can be deleted.
diff --git a/src/java/org/apache/cassandra/tools/AuditLogViewer.java b/src/java/org/apache/cassandra/tools/AuditLogViewer.java
index 866afd503daf..5523f82dcf8b 100644
--- a/src/java/org/apache/cassandra/tools/AuditLogViewer.java
+++ b/src/java/org/apache/cassandra/tools/AuditLogViewer.java
@@ -176,7 +176,7 @@ private boolean isSupportedType(String type)
     private static class AuditLogViewerOptions
     {
         private final List<String> pathList;
-        private String rollCycle = "HOURLY";
+        private String rollCycle = "FAST_HOURLY";
         private boolean follow;
         private boolean ignoreUnsupported;
 
@@ -238,7 +238,7 @@ static Options getCmdLineOptions()
         {
             Options options = new Options();
 
-            options.addOption(new Option("r", ROLL_CYCLE, true, "How often to roll the log file was rolled. May be necessary for Chronicle to correctly parse file names. (MINUTELY, HOURLY, DAILY). Default HOURLY."));
+            options.addOption(new Option("r", ROLL_CYCLE, true, "How often to roll the log file was rolled. May be necessary for Chronicle to correctly parse file names. (FIVE_MINUTELY, FAST_HOURLY, FAST_DAILY). Default FAST_HOURLY."));
             options.addOption(new Option("f", FOLLOW, false, "Upon reacahing the end of the log continue indefinitely waiting for more records"));
             options.addOption(new Option("i", IGNORE, false, "Silently ignore unsupported records"));
             options.addOption(new Option("h", HELP_OPTION, false, "display this help message"));
diff --git a/src/java/org/apache/cassandra/tools/nodetool/EnableAuditLog.java b/src/java/org/apache/cassandra/tools/nodetool/EnableAuditLog.java
index 4c1676ab1da9..0fc7ee4d45a1 100644
--- a/src/java/org/apache/cassandra/tools/nodetool/EnableAuditLog.java
+++ b/src/java/org/apache/cassandra/tools/nodetool/EnableAuditLog.java
@@ -49,7 +49,7 @@ public class EnableAuditLog extends AbstractCommand
     @Option(paramLabel = "excluded_users", names = { "--excluded-users" }, description = "Comma separated list of users to be excluded for audit log. If not set the value from cassandra.yaml will be used")
     private String excluded_users = null;
 
-    @Option(paramLabel = "roll_cycle", names = { "--roll-cycle" }, description = "How often to roll the log file (MINUTELY, HOURLY, DAILY).")
+    @Option(paramLabel = "roll_cycle", names = { "--roll-cycle" }, description = "How often to roll the log file (FAST_MINUTELY, FAST_HOURLY, FAST_DAILY).")
     private String rollCycle = null;
 
     @Option(paramLabel = "blocking", names = { "--blocking" }, description = "If the queue is full whether to block producers or drop samples [true|false].")
diff --git a/src/java/org/apache/cassandra/tools/nodetool/EnableFullQueryLog.java b/src/java/org/apache/cassandra/tools/nodetool/EnableFullQueryLog.java
index b2989a7e6314..d81bb576c1dc 100644
--- a/src/java/org/apache/cassandra/tools/nodetool/EnableFullQueryLog.java
+++ b/src/java/org/apache/cassandra/tools/nodetool/EnableFullQueryLog.java
@@ -26,7 +26,7 @@
 @Command(name = "enablefullquerylog", description = "Enable full query logging, defaults for the options are configured in cassandra.yaml")
 public class EnableFullQueryLog extends AbstractCommand
 {
-    @Option(paramLabel = "roll_cycle", names = { "--roll-cycle" }, description = "How often to roll the log file (MINUTELY, HOURLY, DAILY).")
+    @Option(paramLabel = "roll_cycle", names = { "--roll-cycle" }, description = "How often to roll the log file (FAST_MINUTELY, FAST_HOURLY, FAST_DAILY).")
     private String rollCycle = null;
 
     @Option(paramLabel = "blocking", names = { "--blocking" }, description = "If the queue is full whether to block producers or drop samples [true|false].")
diff --git a/src/java/org/apache/cassandra/utils/JavaUtils.java b/src/java/org/apache/cassandra/utils/JavaUtils.java
index f33b8bec6a58..8ef265580712 100644
--- a/src/java/org/apache/cassandra/utils/JavaUtils.java
+++ b/src/java/org/apache/cassandra/utils/JavaUtils.java
@@ -63,7 +63,7 @@ public static boolean supportExitOnOutOfMemory(String jreVersion)
      * @return the java version number
      * @throws NumberFormatException if the version cannot be retrieved
      */
-    private static int parseJavaVersion(String jreVersion)
+    public static int parseJavaVersion(String jreVersion)
     {
         String version;
         if (jreVersion.startsWith("1."))
diff --git a/src/java/org/apache/cassandra/utils/MerkleTree.java b/src/java/org/apache/cassandra/utils/MerkleTree.java
index a8197b426d02..b6b0141db164 100644
--- a/src/java/org/apache/cassandra/utils/MerkleTree.java
+++ b/src/java/org/apache/cassandra/utils/MerkleTree.java
@@ -1630,6 +1630,12 @@ public TreeRange get(Token t)
         return getHelper(root, fullRange.left, fullRange.right, t);
     }
 
+    @VisibleForTesting
+    public boolean isReleased()
+    {
+        return root == null;
+    }
+
     private TreeRange getHelper(Node node, Token pleft, Token pright, Token t)
     {
         int depth = 0;
diff --git a/src/java/org/apache/cassandra/utils/MerkleTrees.java b/src/java/org/apache/cassandra/utils/MerkleTrees.java
index 310c37fe66ec..115ecf5d16d8 100644
--- a/src/java/org/apache/cassandra/utils/MerkleTrees.java
+++ b/src/java/org/apache/cassandra/utils/MerkleTrees.java
@@ -458,4 +458,19 @@ public int compare(Range<Token> rt1, Range<Token> rt2)
             return rt1.compareTo(rt2);
         }
     }
+
+    /**
+     * For purposes of our checking in test, either "nothing is or was ever here" or "what was here is now gone" suffice.
+     */
+    @VisibleForTesting
+    public boolean isReleased()
+    {
+        if (merkleTrees.isEmpty())
+            return true;
+
+        boolean result = true;
+        for (MerkleTree mt : merkleTrees.values())
+            result &= mt.isReleased();
+        return result;
+    }
 }
diff --git a/src/java/org/apache/cassandra/utils/ObjectSizes.java b/src/java/org/apache/cassandra/utils/ObjectSizes.java
index 8eff7f77bba1..2e4bd7565d7a 100644
--- a/src/java/org/apache/cassandra/utils/ObjectSizes.java
+++ b/src/java/org/apache/cassandra/utils/ObjectSizes.java
@@ -37,6 +37,11 @@
  */
 public class ObjectSizes
 {
+    /**
+     * For debugging purposes, you can construct this with .printVisitedTree() to get a printout of all the objects
+     * and their calculated size on heap to debug jamm related issues. See:
+     * <a href="https://github.com/jbellis/jamm/blob/master/README.md#visited-object-tree">README.md on github</a>
+     */
     private static final MemoryMeter meter = MemoryMeter.builder().withGuessing(Guess.INSTRUMENTATION_AND_SPECIFICATION,
                                                                                 Guess.UNSAFE)
                                                                   .build();
diff --git a/src/java/org/apache/cassandra/utils/binlog/BinLog.java b/src/java/org/apache/cassandra/utils/binlog/BinLog.java
index 195c358f24f4..7734862408c9 100644
--- a/src/java/org/apache/cassandra/utils/binlog/BinLog.java
+++ b/src/java/org/apache/cassandra/utils/binlog/BinLog.java
@@ -38,6 +38,7 @@
 import net.openhft.chronicle.queue.RollCycles;
 import net.openhft.chronicle.queue.impl.single.SingleChronicleQueue;
 import net.openhft.chronicle.queue.impl.single.SingleChronicleQueueBuilder;
+import net.openhft.chronicle.queue.rollcycles.TestRollCycles;
 import net.openhft.chronicle.wire.WireOut;
 import net.openhft.chronicle.wire.WriteMarshallable;
 import net.openhft.posix.PosixAPI;
@@ -55,6 +56,7 @@
 import org.apache.cassandra.utils.concurrent.WeightedQueue;
 
 import static java.lang.String.format;
+import static net.openhft.chronicle.queue.rollcycles.TestRollCycles.TEST_SECONDLY;
 import static org.apache.cassandra.config.CassandraRelevantProperties.CHRONICLE_ANNOUNCER_DISABLE;
 import static org.apache.cassandra.utils.LocalizeString.toUpperCaseLocalized;
 
@@ -140,7 +142,11 @@ private BinLog(Path path, BinLogOptions options, BinLogArchiver archiver)
         Preconditions.checkNotNull(options.roll_cycle, "roll_cycle was null");
         Preconditions.checkArgument(options.max_queue_weight > 0, "max_queue_weight must be > 0");
         SingleChronicleQueueBuilder builder = SingleChronicleQueueBuilder.single(path.toFile()); // checkstyle: permit this invocation
-        builder.rollCycle(RollCycles.valueOf(options.roll_cycle));
+
+        // RollCycles.TEST_SECONDLY is deprecated; avoid logging a warning here in cases where test code may be referencing the old value.
+        builder.rollCycle(options.roll_cycle.equals("TEST_SECONDLY")
+                ? TEST_SECONDLY
+                : RollCycles.valueOf(options.roll_cycle));
 
         sampleQueue = new WeightedQueue<>(options.max_queue_weight);
         this.archiver = archiver;
@@ -394,7 +400,10 @@ public Builder rollCycle(String rollCycle)
             Preconditions.checkNotNull(rollCycle, "rollCycle was null");
             rollCycle = toUpperCaseLocalized(rollCycle);
             Preconditions.checkNotNull(RollCycles.valueOf(rollCycle), "unrecognized roll cycle");
-            this.rollCycle = rollCycle;
+            // Avoid using deprecated RollCycles.TEST_SECONDLY in test cases where we pass the text blindly; logs a warning
+            this.rollCycle = rollCycle.equals("TEST_SECONDLY")
+                    ? TestRollCycles.TEST_SECONDLY.toString()
+                    : rollCycle;
             return this;
         }
 
diff --git a/src/java/org/apache/cassandra/utils/binlog/BinLogOptions.java b/src/java/org/apache/cassandra/utils/binlog/BinLogOptions.java
index 614d19031c9f..b75540a99de2 100644
--- a/src/java/org/apache/cassandra/utils/binlog/BinLogOptions.java
+++ b/src/java/org/apache/cassandra/utils/binlog/BinLogOptions.java
@@ -31,11 +31,12 @@ public class BinLogOptions
      */
     public boolean allow_nodetool_archive_command = false;
     /**
-     * How often to roll BinLog segments so they can potentially be reclaimed. Available options are:
-     * MINUTELY, HOURLY, DAILY, LARGE_DAILY, XLARGE_DAILY, HUGE_DAILY.
-     * For more options, refer: net.openhft.chronicle.queue.RollCycles
+     * How often to roll BinLog segments so they can potentially be reclaimed. Some available options are:
+     * FIVE_MINUTELY, FAST_HOURLY, FAST_DAILY, LargeRollCycles.LARGE_DAILY, LargeRollCycles.XLARGE_DAILY, LargeRollCycles.HUGE_DAILY.
+     * See FAQ: <a href="https://github.com/OpenHFT/Chronicle-Queue/blob/50af322d0022fb8d265779ed6f4c1073d32b6b54/docs/FAQ.adoc" />
+     * For more options, refer: {@link net.openhft.chronicle.queue.RollCycles}
      */
-    public String roll_cycle = "HOURLY";
+    public String roll_cycle = "FAST_HOURLY";
     /**
      * Indicates if the BinLog should block if the it falls behind or should drop bin log records.
      * Default is set to true so that BinLog records wont be lost
diff --git a/src/java/org/apache/cassandra/utils/btree/BTreeSet.java b/src/java/org/apache/cassandra/utils/btree/BTreeSet.java
index 20ee7cf0440b..ea61e7ebd163 100644
--- a/src/java/org/apache/cassandra/utils/btree/BTreeSet.java
+++ b/src/java/org/apache/cassandra/utils/btree/BTreeSet.java
@@ -328,6 +328,40 @@ public ListIterator<V> listIterator(int index)
     {
         throw new UnsupportedOperationException();
     }
+    public BTreeSet<V> reversed()
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    public V removeLast()
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    public V removeFirst()
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    public V getLast()
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    public V getFirst()
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    public void addLast(V v)
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    public void addFirst(V v)
+    {
+        throw new UnsupportedOperationException();
+    }
 
     public static class BTreeRange<V> extends BTreeSet<V>
     {
diff --git a/src/java/org/apache/cassandra/utils/concurrent/Ref.java b/src/java/org/apache/cassandra/utils/concurrent/Ref.java
index e69ce34a4e4e..5008e6e5f32a 100644
--- a/src/java/org/apache/cassandra/utils/concurrent/Ref.java
+++ b/src/java/org/apache/cassandra/utils/concurrent/Ref.java
@@ -125,6 +125,10 @@ public final class Ref<T> implements RefCounted<T>
     public static final boolean DEBUG_EVENTS_ENABLED = TEST_DEBUG_REF_EVENTS.getBoolean();
     static OnLeak ON_LEAK;
 
+    /** NOT INTENDED FOR USE OUTSIDE TESTS AND DEBUGGING */
+    @VisibleForTesting
+    private static boolean TEST_TRACE_ENABLED = false;
+
     @Shared(scope = SIMULATION)
     public interface OnLeak
     {
@@ -146,6 +150,14 @@ public Ref(T referent, Tidy tidy)
         this.referent = referent;
     }
 
+    /**
+     * Generally don't go around mutating this willy-nilly in non-test code please.
+     */
+    @VisibleForTesting
+    public static void enableTestTracing() { TEST_TRACE_ENABLED = true; }
+    @VisibleForTesting
+    public static void disableTestTracing() { TEST_TRACE_ENABLED = false; }
+
     /**
      * Must be called exactly once, when the logical operation for which this Ref was created has terminated.
      * Failure to abide by this contract will result in an error (eventually) being reported, assuming a
@@ -629,7 +641,7 @@ void traverse(final RefCounted.Tidy rootObject)
             {
                 if (Thread.currentThread().isInterrupted())
                     throw new UncheckedInterruptedException(new InterruptedException());
-                //If necessary fetch the next object to start tracing
+                // If necessary fetch the next object to start tracing
                 if (inProgress == null)
                     inProgress = path.pollLast();
 
@@ -646,6 +658,25 @@ void traverse(final RefCounted.Tidy rootObject)
                         field = p.right;
                     }
 
+                    if (TEST_TRACE_ENABLED)
+                    {
+                        logger.debug("[Ref tracing for {}, Object: {}]", rootObject, child);
+                        if (field != null && child != null)
+                        {
+                            logger.debug(" - Visiting field '{}' of object {} -> value class {}",
+                                    field.getName(),
+                                    inProgress.o.getClass().getName(),
+                                    child.getClass().getName());
+                        }
+                        else if (field != null)
+                        {
+                            // Field exists but the next child is null – still useful to know the path.
+                            logger.debug(" - Visiting field '{}' of object {} -> value is null",
+                                    field.getName(),
+                                    inProgress.o.getClass().getName());
+                        }
+                    }
+
                     if (child != null && visited.add(child))
                     {
                         path.offer(inProgress);
diff --git a/test/resources/nodetool/help/enableauditlog b/test/resources/nodetool/help/enableauditlog
index 86cb7a0f4cb7..005499eaf205 100644
--- a/test/resources/nodetool/help/enableauditlog
+++ b/test/resources/nodetool/help/enableauditlog
@@ -83,7 +83,8 @@ OPTIONS
             Path to the JMX password file
 
         --roll-cycle <roll_cycle>
-            How often to roll the log file (MINUTELY, HOURLY, DAILY).
+            How often to roll the log file (FAST_MINUTELY, FAST_HOURLY,
+            FAST_DAILY).
 
         -u <username>, --username <username>
             Remote jmx agent username
diff --git a/test/resources/nodetool/help/enablefullquerylog b/test/resources/nodetool/help/enablefullquerylog
index 08ff8bee1d7a..5335600e1350 100644
--- a/test/resources/nodetool/help/enablefullquerylog
+++ b/test/resources/nodetool/help/enablefullquerylog
@@ -55,7 +55,8 @@ OPTIONS
             Path to the JMX password file
 
         --roll-cycle <roll_cycle>
-            How often to roll the log file (MINUTELY, HOURLY, DAILY).
+            How often to roll the log file (FAST_MINUTELY, FAST_HOURLY,
+            FAST_DAILY).
 
         -u <username>, --username <username>
             Remote jmx agent username
diff --git a/test/simulator/asm/org/apache/cassandra/simulator/asm/InterceptClasses.java b/test/simulator/asm/org/apache/cassandra/simulator/asm/InterceptClasses.java
index 82f3b8012aae..cd6b007884f2 100644
--- a/test/simulator/asm/org/apache/cassandra/simulator/asm/InterceptClasses.java
+++ b/test/simulator/asm/org/apache/cassandra/simulator/asm/InterceptClasses.java
@@ -48,7 +48,7 @@
 // WARNING: does not implement IClassTransformer directly as must be accessible to bootstrap class loader
 public class InterceptClasses implements BiFunction<String, byte[], byte[]>
 {
-    public static final int BYTECODE_VERSION = Opcodes.ASM7;
+    public static final int BYTECODE_VERSION = Opcodes.ASM9;
 
     // TODO (cleanup): use annotations
     private static final Pattern MONITORS = Pattern.compile( "org[/.]apache[/.]cassandra[/.]utils[/.]concurrent[/.].*" +
diff --git a/test/simulator/asm/org/apache/cassandra/simulator/asm/ShadowingTransformer.java b/test/simulator/asm/org/apache/cassandra/simulator/asm/ShadowingTransformer.java
index ca42f245c4e1..8ed4c7a3cffb 100644
--- a/test/simulator/asm/org/apache/cassandra/simulator/asm/ShadowingTransformer.java
+++ b/test/simulator/asm/org/apache/cassandra/simulator/asm/ShadowingTransformer.java
@@ -243,6 +243,12 @@ public void visitInnerClass(String name, String outerName, String innerName, int
         super.visitInnerClass(name, toShadowType(outerName), innerName, access);
     }
 
+    @Override
+    public void visitPermittedSubclass(String permittedSubclass)
+    {
+        super.visitPermittedSubclass(toShadowType(permittedSubclass));
+    }
+
     @Override
     public FieldVisitor visitField(int access, String name, String descriptor, String signature, Object value)
     {
diff --git a/test/simulator/main/org/apache/cassandra/simulator/systems/InterceptorOfGlobalMethods.java b/test/simulator/main/org/apache/cassandra/simulator/systems/InterceptorOfGlobalMethods.java
index ccbf3568cce5..9181747fdf15 100644
--- a/test/simulator/main/org/apache/cassandra/simulator/systems/InterceptorOfGlobalMethods.java
+++ b/test/simulator/main/org/apache/cassandra/simulator/systems/InterceptorOfGlobalMethods.java
@@ -21,6 +21,7 @@
 import java.util.ArrayDeque;
 import java.util.UUID;
 import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.IntSupplier;
 import java.util.function.LongConsumer;
 import java.util.function.ToIntFunction;
@@ -487,21 +488,35 @@ public int getAsInt()
 
         private final IntSupplier nextId;
         private final WeakIdentityHashMap<Object, Integer> saved = new WeakIdentityHashMap<>();
+        private final ReentrantLock mapLock = new ReentrantLock();
 
         public IdentityHashCode(IntSupplier nextId)
         {
             this.nextId = nextId;
         }
 
-        public synchronized int applyAsInt(Object value)
+        /**
+         * As {@link #saved} is not thread safe, we want to preserve weak reference semantics, and we want to gracefully
+         * support virtual threads, we need to use another locking mechanism other than synchronized since that is a risk
+         * of deadlock in JDK21 as virtual threads block on held monitors and won't release.
+         */
+        public int applyAsInt(Object value)
         {
-            Integer id = saved.get(value);
-            if (id == null)
+            mapLock.lock();
+            try
             {
-                id = nextId.getAsInt();
-                saved.put(value, id);
+                Integer id = saved.get(value);
+                if (id == null)
+                {
+                    id = nextId.getAsInt();
+                    saved.put(value, id);
+                }
+                return id;
+            }
+            finally
+            {
+                mapLock.unlock();
             }
-            return id;
         }
     }
 
diff --git a/test/simulator/main/org/apache/cassandra/simulator/systems/SimulatedTime.java b/test/simulator/main/org/apache/cassandra/simulator/systems/SimulatedTime.java
index 55cd832ff5e7..f0cc635e2c8d 100644
--- a/test/simulator/main/org/apache/cassandra/simulator/systems/SimulatedTime.java
+++ b/test/simulator/main/org/apache/cassandra/simulator/systems/SimulatedTime.java
@@ -49,7 +49,8 @@
 // TODO (cleanup): when we encounter an exception and unwind the simulation, we should restore normal time to go with normal waits etc.
 public class SimulatedTime
 {
-    private static final Pattern PERMITTED_TIME_THREADS = Pattern.compile("(logback|SimulationLiveness|Reconcile)[-:][0-9]+|RMI Scheduler\\(\\d+\\)");
+    private static final Pattern PERMITTED_TIME_THREADS = Pattern.compile("(logback|SimulationLiveness|Reconcile|Reference-Reaper)[-:][0-9]+|RMI Scheduler\\(\\d+\\)");
+    private static final Pattern PERMITTED_TIME_CLEANER_THREADS = Pattern.compile("(LocalPool-Cleaner[-a-z]+[-:][0-9]+|Common-Cleaner)");
 
     @Shared(scope = Shared.Scope.SIMULATION)
     public interface Listener
@@ -148,7 +149,7 @@ private ClockAndMonotonicClock check()
                 if (interceptibleThread.isIntercepting())
                     return interceptibleThread.time();
             }
-            if (PERMITTED_TIME_THREADS.matcher(Thread.currentThread().getName()).matches())
+            if (PERMITTED_TIME_THREADS.matcher(Thread.currentThread().getName()).matches() || PERMITTED_TIME_CLEANER_THREADS.matcher(Thread.currentThread().getName()).matches())
                 return disabled;
             throw new IllegalStateException("Using time is not allowed during simulation");
         }
diff --git a/test/unit/org/apache/cassandra/audit/AuditLoggerTest.java b/test/unit/org/apache/cassandra/audit/AuditLoggerTest.java
index c07cdc4a2f91..a7cf62504d5d 100644
--- a/test/unit/org/apache/cassandra/audit/AuditLoggerTest.java
+++ b/test/unit/org/apache/cassandra/audit/AuditLoggerTest.java
@@ -755,7 +755,7 @@ public void testEnableDisable() throws IOException
         assertEquals(1, AuthEvents.instance.listenerCount());
 
         Path p = Files.createTempDirectory("fql");
-        StorageService.instance.enableFullQueryLogger(p.toString(), RollCycles.HOURLY.toString(), false, 1000, 1000, null, 0);
+        StorageService.instance.enableFullQueryLogger(p.toString(), RollCycles.FAST_HOURLY.toString(), false, 1000, 1000, null, 0);
         assertEquals(2, QueryEvents.instance.listenerCount());
         assertEquals(1, AuthEvents.instance.listenerCount()); // fql not listening to auth events
         StorageService.instance.resetFullQueryLogger();
@@ -778,7 +778,7 @@ public void testConflictingPaths() throws IOException
         {
             assertEquals(1, QueryEvents.instance.listenerCount());
             assertEquals(1, AuthEvents.instance.listenerCount());
-            StorageService.instance.enableFullQueryLogger(options.audit_logs_dir, RollCycles.HOURLY.toString(), false, 1000, 1000, null, 0);
+            StorageService.instance.enableFullQueryLogger(options.audit_logs_dir, RollCycles.FAST_HOURLY.toString(), false, 1000, 1000, null, 0);
             fail("Conflicting directories - should throw exception");
         }
         catch (IllegalStateException e)
@@ -796,7 +796,7 @@ public void testConflictingPathsFQLFirst() throws IOException
         disableAuditLogOptions();
         AuditLogOptions options = getBaseAuditLogOptions();
         DatabaseDescriptor.setAuditLoggingOptions(options);
-        StorageService.instance.enableFullQueryLogger(options.audit_logs_dir, RollCycles.HOURLY.toString(), false, 1000, 1000, null, 0);
+        StorageService.instance.enableFullQueryLogger(options.audit_logs_dir, RollCycles.FAST_HOURLY.toString(), false, 1000, 1000, null, 0);
         try
         {
             assertEquals(1, QueryEvents.instance.listenerCount());
diff --git a/test/unit/org/apache/cassandra/audit/BinAuditLoggerTest.java b/test/unit/org/apache/cassandra/audit/BinAuditLoggerTest.java
index d93522c88315..6dfa2661aa88 100644
--- a/test/unit/org/apache/cassandra/audit/BinAuditLoggerTest.java
+++ b/test/unit/org/apache/cassandra/audit/BinAuditLoggerTest.java
@@ -25,8 +25,8 @@
 
 import net.openhft.chronicle.queue.ChronicleQueue;
 import net.openhft.chronicle.queue.ExcerptTailer;
-import net.openhft.chronicle.queue.RollCycles;
 import net.openhft.chronicle.queue.impl.single.SingleChronicleQueueBuilder;
+import net.openhft.chronicle.queue.rollcycles.TestRollCycles;
 
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -76,7 +76,7 @@ public void testSelectRoundTripQuery() throws Throwable
         ResultSet rs = session.execute(pstmt.bind(1));
 
         assertEquals(1, rs.all().size());
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptTailer tailer = queue.createTailer();
             assertTrue(tailer.readDocument(wire -> {
diff --git a/test/unit/org/apache/cassandra/db/commitlog/CommitlogShutdownTest.java b/test/unit/org/apache/cassandra/db/commitlog/CommitlogShutdownTest.java
index 298f568ef75b..6a9104d5f0e6 100644
--- a/test/unit/org/apache/cassandra/db/commitlog/CommitlogShutdownTest.java
+++ b/test/unit/org/apache/cassandra/db/commitlog/CommitlogShutdownTest.java
@@ -62,7 +62,7 @@ public class CommitlogShutdownTest
     @BMRule(name = "Make removing commitlog segments slow",
     targetClass = "CommitLogSegment",
     targetMethod = "discard",
-    action = "Thread.sleep(50)")
+    action = "Thread.sleep(250L)") // We need to add the unit to Thread.sleep calls in ByteBuddy now
     public void testShutdownWithPendingTasks() throws Exception
     {
         new Random().nextBytes(entropy);
@@ -77,8 +77,7 @@ public void testShutdownWithPendingTasks() throws Exception
                                     KeyspaceParams.simple(1),
                                     SchemaLoader.standardCFMD(KEYSPACE1, STANDARD1, 0, AsciiType.instance, BytesType.instance));
 
-                                    CompactionManager.instance.disableAutoCompaction();
-
+        CompactionManager.instance.disableAutoCompaction();
         ColumnFamilyStore cfs1 = Keyspace.open(KEYSPACE1).getColumnFamilyStore(STANDARD1);
 
         final Mutation m = new RowUpdateBuilder(cfs1.metadata.get(), 0, "k")
diff --git a/test/unit/org/apache/cassandra/db/compaction/AntiCompactionBytemanTest.java b/test/unit/org/apache/cassandra/db/compaction/AntiCompactionBytemanTest.java
index df37cfabc951..14049537431a 100644
--- a/test/unit/org/apache/cassandra/db/compaction/AntiCompactionBytemanTest.java
+++ b/test/unit/org/apache/cassandra/db/compaction/AntiCompactionBytemanTest.java
@@ -61,7 +61,7 @@ public class AntiCompactionBytemanTest extends CQLTester
              targetMethod = "antiCompactGroup",
              condition = "not flagged(\"done\")",
              targetLocation = "AFTER INVOKE prepareToCommit",
-             action = "Thread.sleep(2000);") } )
+             action = "Thread.sleep(2000L);") } ) // We need to add the unit to Thread.sleep calls in ByteBuddy now
     public void testRedundantTransitions() throws Throwable
     {
         createTable("create table %s (id int primary key, i int)");
diff --git a/test/unit/org/apache/cassandra/db/compaction/CompactionsBytemanTest.java b/test/unit/org/apache/cassandra/db/compaction/CompactionsBytemanTest.java
index d5d94be92b81..715ecdf77b90 100644
--- a/test/unit/org/apache/cassandra/db/compaction/CompactionsBytemanTest.java
+++ b/test/unit/org/apache/cassandra/db/compaction/CompactionsBytemanTest.java
@@ -121,7 +121,7 @@ public void testRuntimeExceptionWhenNoDiskSpaceForCompaction() throws Throwable
             targetMethod = "submitBackground",
             targetLocation = "AT INVOKE java.util.concurrent.Future.isCancelled",
             condition = "!$cfs.getKeyspaceName().contains(\"system\")",
-            action = "Thread.sleep(5000)")
+            action = "Thread.sleep(5000L)") // We need to add the unit to Thread.sleep calls in ByteBuddy now
     public void testCompactingCFCounting() throws Throwable
     {
         createTable("CREATE TABLE %s (k INT, c INT, v INT, PRIMARY KEY (k, c))");
diff --git a/test/unit/org/apache/cassandra/db/memtable/MemtableQuickTest.java b/test/unit/org/apache/cassandra/db/memtable/MemtableQuickTest.java
index b06ced531c2f..1a615861417a 100644
--- a/test/unit/org/apache/cassandra/db/memtable/MemtableQuickTest.java
+++ b/test/unit/org/apache/cassandra/db/memtable/MemtableQuickTest.java
@@ -45,13 +45,13 @@ public class MemtableQuickTest extends CQLTester
 {
     static final Logger logger = LoggerFactory.getLogger(MemtableQuickTest.class);
 
-    static final int partitions = 50_000;
+    static final int partitions = 45_000;
     static final int rowsPerPartition = 4;
 
-    static final int deletedPartitionsStart = 20_000;
+    static final int deletedPartitionsStart = 15_000;
     static final int deletedPartitionsEnd = deletedPartitionsStart + 10_000;
 
-    static final int deletedRowsStart = 40_000;
+    static final int deletedRowsStart = 35_000;
     static final int deletedRowsEnd = deletedRowsStart + 5_000;
 
     @Parameterized.Parameter()
diff --git a/test/unit/org/apache/cassandra/fql/FullQueryLoggerTest.java b/test/unit/org/apache/cassandra/fql/FullQueryLoggerTest.java
index 3d4296fda117..5dbfdc41e8f6 100644
--- a/test/unit/org/apache/cassandra/fql/FullQueryLoggerTest.java
+++ b/test/unit/org/apache/cassandra/fql/FullQueryLoggerTest.java
@@ -33,8 +33,8 @@
 
 import net.openhft.chronicle.queue.ChronicleQueue;
 import net.openhft.chronicle.queue.ExcerptTailer;
-import net.openhft.chronicle.queue.RollCycles;
 import net.openhft.chronicle.queue.impl.single.SingleChronicleQueueBuilder;
+import net.openhft.chronicle.queue.rollcycles.TestRollCycles;
 import net.openhft.chronicle.wire.ValueIn;
 import net.openhft.chronicle.wire.WireOut;
 
@@ -323,7 +323,7 @@ public void release()
 
     private boolean checkForQueries(List<String> queries)
     {
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptTailer tailer = queue.createTailer();
             List<String> expectedQueries = new LinkedList<>(queries);
@@ -434,7 +434,7 @@ public void testRoundTripQueryWithKeyspace() throws Exception
 
     private void assertRoundTripQuery(@Nullable String keyspace)
     {
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptTailer tailer = queue.createTailer();
             assertTrue(tailer.readDocument(wire ->
@@ -473,7 +473,7 @@ public void testRoundTripBatchWithKeyspace() throws Exception
 
         Util.spinAssertEquals(true, () ->
         {
-            try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+            try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
             {
                 return queue.createTailer().readingDocument().isPresent();
             }
@@ -497,7 +497,7 @@ public void testRoundTripBatchWithKeyspaceNull() throws Exception
 
         Util.spinAssertEquals(true, () ->
         {
-            try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+            try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
             {
                 return queue.createTailer().readingDocument().isPresent();
             }
@@ -509,7 +509,7 @@ public void testRoundTripBatchWithKeyspaceNull() throws Exception
 
     private void assertRoundTripBatch(@Nullable String keyspace)
     {
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(tempDir.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptTailer tailer = queue.createTailer();
             assertTrue(tailer.readDocument(wire -> {
diff --git a/test/unit/org/apache/cassandra/hints/HintServiceBytemanTest.java b/test/unit/org/apache/cassandra/hints/HintServiceBytemanTest.java
index 66e4991a42ec..e296c0a58378 100644
--- a/test/unit/org/apache/cassandra/hints/HintServiceBytemanTest.java
+++ b/test/unit/org/apache/cassandra/hints/HintServiceBytemanTest.java
@@ -22,9 +22,12 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 
 import org.awaitility.Awaitility;
 import org.jboss.byteman.contrib.bmunit.BMRule;
+import org.jboss.byteman.contrib.bmunit.BMUnitConfig;
 import org.jboss.byteman.contrib.bmunit.BMUnitRunner;
 import org.junit.After;
 import org.junit.Before;
@@ -91,12 +94,14 @@ public void reinstanciateService() throws Throwable
         HintsService.instance.startDispatch();
     }
 
-    @Test
+    // When this test hangs it does it _hard_. Working now but don't want to make the worker in CI hang out for full timeout.
+    @Test (timeout = 300000)
+    @BMUnitConfig(bmunitVerbose=true, verbose=true)
     @BMRule(name = "Delay delivering hints",
     targetClass = "DispatchHintsTask",
     targetMethod = "run",
     action = "Thread.sleep(DatabaseDescriptor.getHintsFlushPeriodInMS() * 3L)")
-    public void testListPendingHints() throws InterruptedException, ExecutionException
+    public void testListPendingHints() throws InterruptedException, ExecutionException, TimeoutException
     {
         HintsService.instance.resumeDispatch();
         MockMessagingSpy spy = sendHintsAndResponses(metadata, 20000, -1);
@@ -111,7 +116,9 @@ public void testListPendingHints() throws InterruptedException, ExecutionExcepti
         assertEquals(1, info.totalFiles);
         assertEquals(info.oldestTimestamp, info.newestTimestamp); // there is 1 descriptor with only 1 timestamp
 
-        spy.interceptMessageOut(20000).get();
+        // JDK21 genZGC uncovered some flakiness / hanging here waiting on Condition
+        spy.interceptMessageOut(20000).get(60, TimeUnit.SECONDS);
+        spy.printMessageCounts();
         assertEquals(Collections.emptyList(), HintsService.instance.getPendingHints());
     }
 }
diff --git a/test/unit/org/apache/cassandra/hints/HintsServiceTest.java b/test/unit/org/apache/cassandra/hints/HintsServiceTest.java
index 3f220f120c5b..6249a5f3b7d4 100644
--- a/test/unit/org/apache/cassandra/hints/HintsServiceTest.java
+++ b/test/unit/org/apache/cassandra/hints/HintsServiceTest.java
@@ -32,7 +32,9 @@
 import org.junit.After;
 import org.junit.Before;
 import org.junit.BeforeClass;
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.Timeout;
 
 import accord.primitives.Keys;
 import accord.primitives.TxnId;
@@ -64,6 +66,7 @@
 import org.apache.cassandra.transport.Dispatcher;
 import org.apache.cassandra.utils.MockFailureDetector;
 
+import static java.util.concurrent.TimeUnit.MINUTES;
 import static org.apache.cassandra.Util.spinAssertEquals;
 import static org.apache.cassandra.config.CassandraRelevantProperties.HINT_DISPATCH_INTERVAL_MS;
 import static org.apache.cassandra.hints.HintsTestUtil.sendHintsAndResponses;
@@ -83,6 +86,14 @@ public class HintsServiceTest
     private final MockFailureDetector failureDetector = new MockFailureDetector();
     private static TableMetadata metadata;
 
+    // Had some trouble with this test OOM'ing and misbehaving; trying to tighten things up a bit. It's good now but leaving
+    // this here to defend against future long CI hangs making workers burn cycles.
+    @Rule
+    public final Timeout perTestTimeout = Timeout.builder()
+                                          .withTimeout(8, MINUTES)              // match test.timeout in build.xml
+                                          .withLookingForStuckThread(true)      // dumps stack of a likely stuck thread
+                                          .build();
+
     @BeforeClass
     public static void defineSchema()
     {
diff --git a/test/unit/org/apache/cassandra/net/MockMessagingService.java b/test/unit/org/apache/cassandra/net/MockMessagingService.java
index 54f9071ed31b..72e9a296b203 100644
--- a/test/unit/org/apache/cassandra/net/MockMessagingService.java
+++ b/test/unit/org/apache/cassandra/net/MockMessagingService.java
@@ -33,7 +33,6 @@
  */
 public class MockMessagingService
 {
-
     private MockMessagingService()
     {
     }
diff --git a/test/unit/org/apache/cassandra/net/MockMessagingSpy.java b/test/unit/org/apache/cassandra/net/MockMessagingSpy.java
index bef7d1913261..b27c9b0d7b30 100644
--- a/test/unit/org/apache/cassandra/net/MockMessagingSpy.java
+++ b/test/unit/org/apache/cassandra/net/MockMessagingSpy.java
@@ -36,12 +36,37 @@
 import static org.apache.cassandra.utils.concurrent.BlockingQueues.newBlockingQueue;
 
 /**
- * Allows inspecting the behavior of mocked messaging by observing {@link MatcherResponse}.
+ * Test utility that spies on mocked messaging interactions.
+ *
+ * <p>This class records inbound mocked messages and outbound messages that would have been
+ * sent, allowing tests to capture, assert or wait for specific communication patterns.</p>
+ *
+ * <h2>Debug logging</h2>
+ * <p>The internal {@code debugLog} method writes detailed information about the spy’s
+ * activity. By default, the logger uses TRACE level; to make the output visible at INFO
+ * level, set the environment variable {@code MOCK_MESSAGING_SPY_DEBUG=true} before starting
+ * the JVM.
+ *
+ * @see MatcherResponse
+ * @see MockMessagingService
  */
 public class MockMessagingSpy
 {
     private static final Logger logger = LoggerFactory.getLogger(MockMessagingSpy.class);
 
+    // checkstyle: suppress below 'blockSystemPropertyUsage'
+    private static boolean DEBUG_ENABLED = Boolean.parseBoolean(System.getenv("MOCK_MESSAGING_SPY_DEBUG"));
+    private void debugLog(String format, Object... args)
+    {
+        if (DEBUG_ENABLED)
+            logger.info(format, args);
+        else
+            logger.trace(format, args);
+    }
+
+    public static void enableDebug() {DEBUG_ENABLED = true; }
+    public static void disableDebug() {DEBUG_ENABLED = false; }
+
     private final AtomicInteger messagesIntercepted = new AtomicInteger();
     private final AtomicInteger mockedMessageResponses = new AtomicInteger();
 
@@ -142,17 +167,22 @@ public int mockedMessageResponses()
         return mockedMessageResponses.get();
     }
 
+    public void printMessageCounts()
+    {
+        logger.info("Messages intercepted: {}. Mocked Message responses: {}", messagesIntercepted(), mockedMessageResponses());
+    }
+
     void matchingMessage(Message<?> message)
     {
-        messagesIntercepted.incrementAndGet();
-        logger.trace("Received matching message: {}", message);
+        int count = messagesIntercepted.incrementAndGet();
+        debugLog("messagesInterceptedCount: {}. Received matching message: {}", count, message);
         interceptedMessages.add(message);
     }
 
     void matchingResponse(Message<?> response)
     {
-        mockedMessageResponses.incrementAndGet();
-        logger.trace("Responding to intercepted message: {}", response);
+        int count = mockedMessageResponses.incrementAndGet();
+        debugLog("mockedMessageResponseCount: {}. Responding to intercepted message: {}", count, response);
         deliveredResponses.add(response);
     }
 
diff --git a/test/unit/org/apache/cassandra/repair/RepairJobTest.java b/test/unit/org/apache/cassandra/repair/RepairJobTest.java
index 43df31f9af59..6fcb0416bacc 100644
--- a/test/unit/org/apache/cassandra/repair/RepairJobTest.java
+++ b/test/unit/org/apache/cassandra/repair/RepairJobTest.java
@@ -266,6 +266,10 @@ public void testNoTreesRetainedAfterDifference() throws Throwable
 
         long singleTreeSize = ObjectSizes.measureDeep(mockTrees.get(addr1));
 
+        assertEquals(0, session.syncingCount());
+        for (var mt : mockTrees.values())
+            assertThat(mt.isReleased()).isFalse();
+
         // Use addr4 instead of one of the provided trees to force everything to be remote sync tasks as
         // LocalSyncTasks try to reach over the network.
         List<SyncTask> syncTasks = RepairJob.createStandardSyncTasks(SharedContext.Global.instance, sessionJobDesc, mockTreeResponses,
@@ -275,30 +279,41 @@ public void testNoTreesRetainedAfterDifference() throws Throwable
                                                                      session.pullRepair,
                                                                      session.previewKind);
 
+        // All trees in our mockTrees should be released after SyncTask creation
+        for (var mt : mockTrees.values())
+            assertThat(mt.isReleased()).isTrue();
+        assertThat(session.syncingCount()).isEqualTo(0);
+
         // SyncTasks themselves should not contain significant memory
         SyncTaskListAssert.assertThat(syncTasks).hasSizeLessThan(0.2 * singleTreeSize);
 
-        // Remember the size of the session before we've executed any tasks
-        long sizeBeforeExecution = ObjectSizes.measureDeep(session);
+        // We can't directly check the memory size in the session post JDK21; jamm has trouble walking the internal
+        // object graph of a ConcurrentHashMap's implementation post JDK21 and has bugs with the @Contended case (which
+        // indicates we should consider moving away from it... jamm, that is. Which we've discussed as a project).
+        // Instead, we treat the presence or removal of SyncTasks as indicative of the memory usage since the reference
+        // to the MerkleTrees lives on there after creation.
 
         // block syncComplete execution until test has verified session still retains the trees
         CompletableFuture<?> future = new CompletableFuture<>();
         session.registerSyncCompleteCallback(future::get);
+
         ListenableFuture<List<SyncStat>> syncResults = job.executeTasks(syncTasks);
+        // Immediately following execution the SyncTasks should still be live and thus taking memory
+        assertThat(session.syncingCount()).isNotEqualTo(0);
 
-        // Immediately following execution the internal execution queue should still retain the trees
-        long sizeDuringExecution = ObjectSizes.measureDeep(session);
-        assertThat(sizeDuringExecution).isGreaterThan(sizeBeforeExecution + (syncTasks.size() * singleTreeSize));
         // unblock syncComplete callback, session should remove trees
         future.complete(null);
 
         // The session retains memory in the contained executor until the threads expire, so we wait for the threads
         // that ran the Tree -> SyncTask conversions to die and release the memory
         long millisUntilFreed;
+
+        // Confirm that it's the thread timeout mechanism that's causing the SyncTasks to retire
+        assertThat(session.syncingCount()).isNotEqualTo(0);
         for (millisUntilFreed = 0; millisUntilFreed < TEST_TIMEOUT_S * 1000; millisUntilFreed += THREAD_TIMEOUT_MILLIS)
         {
             TimeUnit.MILLISECONDS.sleep(THREAD_TIMEOUT_MILLIS);
-            if (ObjectSizes.measureDeep(session) < (sizeDuringExecution - (syncTasks.size() * singleTreeSize)))
+            if (session.syncingCount() == 0)
                 break;
         }
 
diff --git a/test/unit/org/apache/cassandra/service/GCInspectorTest.java b/test/unit/org/apache/cassandra/service/GCInspectorTest.java
index 089e022d2e21..852fb314b999 100644
--- a/test/unit/org/apache/cassandra/service/GCInspectorTest.java
+++ b/test/unit/org/apache/cassandra/service/GCInspectorTest.java
@@ -17,10 +17,12 @@
  */
 package org.apache.cassandra.service;
 
+import org.junit.Assert;
 import org.junit.Before;
 import org.junit.BeforeClass;
 import org.junit.Test;
 
+import org.apache.cassandra.config.CassandraRelevantProperties;
 import org.apache.cassandra.config.DatabaseDescriptor;
 
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
@@ -30,7 +32,6 @@
 
 public class GCInspectorTest
 {
-    
     GCInspector gcInspector;
     
     @BeforeClass
@@ -44,12 +45,22 @@ public void before()
     {
         gcInspector = new GCInspector();
     }
-    
+
     @Test
     public void ensureStaticFieldsHydrateFromConfig()
-    {    
-        assertEquals(DatabaseDescriptor.getGCLogThreshold(), gcInspector.getGcLogThresholdInMs());
-        assertEquals(DatabaseDescriptor.getGCWarnThreshold(), gcInspector.getGcWarnThresholdInMs());
+    {
+        // For purposes of unit testing, we're just going to assume if you're running this on JDK21 you're using ZGC.
+        if (CassandraRelevantProperties.JAVA_VERSION.getString().compareTo("21") > 0)
+        {
+            Assert.assertEquals(DatabaseDescriptor.getZGCLogThreshold(), gcInspector.getGcLogThresholdInMs());
+            Assert.assertEquals(DatabaseDescriptor.getZGCWarnThreshold(), gcInspector.getGcWarnThresholdInMs());
+        }
+        else
+        {
+            Assert.assertEquals(DatabaseDescriptor.getGCLogThreshold(), gcInspector.getGcLogThresholdInMs());
+            Assert.assertEquals(DatabaseDescriptor.getGCWarnThreshold(), gcInspector.getGcWarnThresholdInMs());
+        }
+
         assertEquals(DatabaseDescriptor.getGCConcurrentPhaseLogThreshold(), gcInspector.getGcConcurrentPhaseLogThresholdInMs());
         assertEquals(DatabaseDescriptor.getGCConcurrentPhaseWarnThreshold(), gcInspector.getGcConcurrentPhaseWarnThresholdInMs());
     }
@@ -64,12 +75,12 @@ public void ensureStatusIsCalculated()
     public void ensureWarnGreaterThanLog()
     {
         assertThatThrownBy(() -> gcInspector.setGcWarnThresholdInMs(gcInspector.getGcLogThresholdInMs()))
-        .isInstanceOf(IllegalArgumentException.class)
-        .hasMessage("Threshold value for gc_warn_threshold (200) must be greater than gc_log_threshold which is currently 200");
+            .isInstanceOf(IllegalArgumentException.class)
+            .hasMessageContaining("must be greater than");
 
         assertThatThrownBy(() -> gcInspector.setGcConcurrentPhaseWarnThresholdInMs(gcInspector.getGcConcurrentPhaseLogThresholdInMs()))
-        .isInstanceOf(IllegalArgumentException.class)
-        .hasMessage("Threshold value for gc_concurrent_phase_warn_threshold (1000) must be greater than gc_concurrent_phase_log_threshold which is currently 1000");
+            .isInstanceOf(IllegalArgumentException.class)
+            .hasMessageContaining("must be greater than");
     }
     
     @Test
@@ -79,30 +90,45 @@ public void ensureZeroIsOk()
         assertEquals(gcInspector.getStatusThresholdInMs(), gcInspector.getGcLogThresholdInMs());
         assertEquals(0, DatabaseDescriptor.getGCWarnThreshold());
         assertEquals(200, DatabaseDescriptor.getGCLogThreshold());
+        assertEquals(20000, DatabaseDescriptor.getZGCLogThreshold());
 
         gcInspector.setGcConcurrentPhaseWarnThresholdInMs(0);
         assertEquals(0, DatabaseDescriptor.getGCConcurrentPhaseWarnThreshold());
         assertEquals(1000, DatabaseDescriptor.getGCConcurrentPhaseLogThreshold());
     }
-    
+
+    /**
+     * We keep the concurrent log values the same for JDK21 since we have to infer
+     * See {@code GCInspector#assumeGCIsPartiallyConcurrent()}
+     */
     @Test
     public void ensureLogLessThanWarn()
     {
-        assertEquals(200, gcInspector.getGcLogThresholdInMs());
-        gcInspector.setGcWarnThresholdInMs(1000);
-        assertEquals(1000, gcInspector.getGcWarnThresholdInMs());
+        int gcLog = 200;
+        int gcWarn = 1000;
+        int gcConcurrentLog = 1000;
+        int gcConcurrentWarn = 2000;
+
+        if (CassandraRelevantProperties.JAVA_VERSION.getString().compareTo("21") > 0)
+        {
+            gcLog = 20000;
+            gcWarn = 30000;
+        }
+        Assert.assertEquals(gcLog, gcInspector.getGcLogThresholdInMs());
+        gcInspector.setGcWarnThresholdInMs(gcWarn);
+        assertEquals(gcWarn, gcInspector.getGcWarnThresholdInMs());
 
         assertThatThrownBy(() -> gcInspector.setGcLogThresholdInMs(gcInspector.getGcWarnThresholdInMs() + 1))
-        .isInstanceOf(IllegalArgumentException.class)
-        .hasMessage("Threshold value for gc_log_threshold (1001) must be less than gc_warn_threshold which is currently 1000");
+            .isInstanceOf(IllegalArgumentException.class)
+            .hasMessageContaining("must be less than");
 
-        assertEquals(1000, gcInspector.getGcConcurrentPhaseLogThresholdInMs());
-        gcInspector.setGcConcurrentPhaseWarnThresholdInMs(2000);
-        assertEquals(2000, gcInspector.getGcConcurrentPhaseWarnThresholdInMs());
+        assertEquals(gcConcurrentLog, gcInspector.getGcConcurrentPhaseLogThresholdInMs());
+        gcInspector.setGcConcurrentPhaseWarnThresholdInMs(gcConcurrentWarn);
+        assertEquals(gcConcurrentWarn, gcInspector.getGcConcurrentPhaseWarnThresholdInMs());
 
         assertThatThrownBy(() -> gcInspector.setGcConcurrentPhaseLogThresholdInMs(gcInspector.getGcConcurrentPhaseWarnThresholdInMs() + 1))
-        .isInstanceOf(IllegalArgumentException.class)
-        .hasMessage("Threshold value for gc_concurrent_phase_log_threshold (2001) must be less than gc_concurrent_phase_warn_threshold which is currently 2000");
+            .isInstanceOf(IllegalArgumentException.class)
+            .hasMessageContaining("must be less than");
     }
     
     @Test
@@ -120,6 +146,10 @@ public void testDefaults()
         assertEquals(1000, gcInspector.getGcConcurrentPhaseLogThresholdInMs());
         assertEquals(2000, DatabaseDescriptor.getGCConcurrentPhaseWarnThreshold());
         assertEquals(2000, gcInspector.getGcConcurrentPhaseWarnThresholdInMs());
+        Assert.assertEquals(2, DatabaseDescriptor.getGCPauseLogThreshold());
+        Assert.assertEquals(2, gcInspector.getGcPauseLogThresholdInMs());
+        Assert.assertEquals(10, DatabaseDescriptor.getGCPauseWarnThreshold());
+        Assert.assertEquals(10, gcInspector.getGcPauseWarnThresholdInMs());
     }
 
     @Test(expected=IllegalArgumentException.class)
diff --git a/test/unit/org/apache/cassandra/tools/AuditLogViewerTest.java b/test/unit/org/apache/cassandra/tools/AuditLogViewerTest.java
index 1c9868b7f514..17ed45556c40 100644
--- a/test/unit/org/apache/cassandra/tools/AuditLogViewerTest.java
+++ b/test/unit/org/apache/cassandra/tools/AuditLogViewerTest.java
@@ -31,8 +31,8 @@
 import net.openhft.chronicle.core.io.IORuntimeException;
 import net.openhft.chronicle.queue.ChronicleQueue;
 import net.openhft.chronicle.queue.ExcerptAppender;
-import net.openhft.chronicle.queue.RollCycles;
 import net.openhft.chronicle.queue.impl.single.SingleChronicleQueueBuilder;
+import net.openhft.chronicle.queue.rollcycles.TestRollCycles;
 import net.openhft.chronicle.wire.WireOut;
 
 import org.apache.commons.io.FileUtils;
@@ -98,7 +98,8 @@ public void testMaybeChangeDocs()
                        " -i,--ignore             Silently ignore unsupported records\n" + 
                        " -r,--roll_cycle <arg>   How often to roll the log file was rolled. May be\n" +
                        "                         necessary for Chronicle to correctly parse file\n" +
-                       "                         names. (MINUTELY, HOURLY, DAILY). Default HOURLY.\n";
+                       "                         names. (FIVE_MINUTELY, FAST_HOURLY, FAST_DAILY).\n" +
+                       "                         Default FAST_HOURLY.\n";
         Assertions.assertThat(tool.getStdout()).isEqualTo(help);
     }
 
@@ -162,7 +163,7 @@ public void testDisplayRecord()
         records.add("Test foo bar 1");
         records.add("Test foo bar 2");
 
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptAppender appender = queue.acquireAppender();
 
@@ -171,7 +172,7 @@ public void testDisplayRecord()
 
             //Read those written records
             List<String> actualRecords = new ArrayList<>();
-            AuditLogViewer.dump(ImmutableList.of(path.toString()), RollCycles.TEST_SECONDLY.toString(), false, false, actualRecords::add);
+            AuditLogViewer.dump(ImmutableList.of(path.toString()), TestRollCycles.TEST_SECONDLY.toString(), false, false, actualRecords::add);
 
             assertRecordsMatch(records, actualRecords);
         }
@@ -180,12 +181,12 @@ public void testDisplayRecord()
     @Test (expected = IORuntimeException.class)
     public void testRejectFutureVersionRecord()
     {
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptAppender appender = queue.acquireAppender();
             appender.writeDocument(createFutureRecord());
 
-            AuditLogViewer.dump(ImmutableList.of(path.toString()), RollCycles.TEST_SECONDLY.toString(), false, false, dummy -> {});
+            AuditLogViewer.dump(ImmutableList.of(path.toString()), TestRollCycles.TEST_SECONDLY.toString(), false, false, dummy -> {});
         }
         catch (Exception e)
         {
@@ -201,7 +202,7 @@ public void testIgnoreFutureVersionRecord()
         records.add("Test foo bar 1");
         records.add("Test foo bar 2");
 
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptAppender appender = queue.acquireAppender();
 
@@ -213,7 +214,7 @@ public void testIgnoreFutureVersionRecord()
 
             //Read those written records
             List<String> actualRecords = new ArrayList<>();
-            AuditLogViewer.dump(ImmutableList.of(path.toString()), RollCycles.TEST_SECONDLY.toString(), false, true, actualRecords::add);
+            AuditLogViewer.dump(ImmutableList.of(path.toString()), TestRollCycles.TEST_SECONDLY.toString(), false, true, actualRecords::add);
 
             // Assert all current records are present
             assertRecordsMatch(records, actualRecords);
@@ -223,12 +224,12 @@ public void testIgnoreFutureVersionRecord()
     @Test (expected = IORuntimeException.class)
     public void testRejectUnknownTypeRecord()
     {
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptAppender appender = queue.acquireAppender();
             appender.writeDocument(createUnknownTypeRecord());
 
-            AuditLogViewer.dump(ImmutableList.of(path.toString()), RollCycles.TEST_SECONDLY.toString(), false, false, dummy -> {});
+            AuditLogViewer.dump(ImmutableList.of(path.toString()), TestRollCycles.TEST_SECONDLY.toString(), false, false, dummy -> {});
         }
         catch (Exception e)
         {
@@ -244,7 +245,7 @@ public void testIgnoreUnknownTypeRecord()
         records.add("Test foo bar 1");
         records.add("Test foo bar 2");
 
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptAppender appender = queue.acquireAppender();
 
@@ -256,7 +257,7 @@ public void testIgnoreUnknownTypeRecord()
 
             //Read those written records
             List<String> actualRecords = new ArrayList<>();
-            AuditLogViewer.dump(ImmutableList.of(path.toString()), RollCycles.TEST_SECONDLY.toString(), false, true, actualRecords::add);
+            AuditLogViewer.dump(ImmutableList.of(path.toString()), TestRollCycles.TEST_SECONDLY.toString(), false, true, actualRecords::add);
 
             // Assert all supported records are present
             assertRecordsMatch(records, actualRecords);
diff --git a/test/unit/org/apache/cassandra/tools/BulkLoaderTest.java b/test/unit/org/apache/cassandra/tools/BulkLoaderTest.java
index 84a4b8a0975f..16edd9e8047f 100644
--- a/test/unit/org/apache/cassandra/tools/BulkLoaderTest.java
+++ b/test/unit/org/apache/cassandra/tools/BulkLoaderTest.java
@@ -18,6 +18,10 @@
 
 package org.apache.cassandra.tools;
 
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+
 import com.datastax.driver.core.exceptions.NoHostAvailableException;
 
 import org.hamcrest.CoreMatchers;
@@ -31,17 +35,31 @@
 
 public class BulkLoaderTest extends OfflineToolUtils
 {
+    private static final String[] ALLOWED_BULK_LOADER_THREADS = new String[]{
+            "ObjectCleanerThread",
+            "globalEventExecutor-[1-9]-[1-9]",
+            "globalEventExecutor-[1-9]-[1-9]",
+            "Shutdown-checker",
+            "cluster[0-9]-connection-reaper-[0-9]",
+            "Attach Listener",
+            "process reaper",
+            "JNA Cleaner",
+    };
+
+    // the driver isn't expected to terminate threads on close synchronously (CASSANDRA-19000)
+    private static final String SYNC_DRIVER_THREAD = "cluster[0-9]-nio-worker-[0-9]";
+
     @Test
     public void testBulkLoader_NoArgs()
     {
         ToolResult tool = ToolRunner.invokeClass(BulkLoader.class);
         assertEquals(1, tool.getExitCode());
         assertThat(tool.getCleanedStderr(), CoreMatchers.containsString("Missing sstable directory argument"));
-        
-        assertNoUnexpectedThreadsStarted(new String[] { "ObjectCleanerThread",
-                                                        "Shutdown-checker",
-                                                        "cluster[0-9]-connection-reaper-[0-9]" },
-                                         false);
+
+        assertNoUnexpectedThreadsStarted(false, "ObjectCleanerThread",
+                "Shutdown-checker",
+                "cluster[0-9]-connection-reaper-[0-9]");
+
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -63,15 +81,8 @@ public void testBulkLoader_WithArgs() throws Exception
         if (!(tool.getException().getCause().getCause().getCause() instanceof NoHostAvailableException))
             throw tool.getException();
 
-        assertNoUnexpectedThreadsStarted(new String[] { "ObjectCleanerThread",
-                                                        "globalEventExecutor-[1-9]-[1-9]",
-                                                        "globalEventExecutor-[1-9]-[1-9]",
-                                                        "Shutdown-checker",
-                                                        "cluster[0-9]-connection-reaper-[0-9]",
-                                                        "Attach Listener",
-                                                        "process reaper",
-                                                        "JNA Cleaner"},
-                                         false);
+
+        assertNoUnexpectedThreadsStarted(false, ALLOWED_BULK_LOADER_THREADS);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -95,21 +106,11 @@ public void testBulkLoader_WithArgs1() throws Exception
         if (!(tool.getException().getCause().getCause().getCause() instanceof NoHostAvailableException))
             throw tool.getException();
 
-        assertNoUnexpectedThreadsStarted(new String[] { "ObjectCleanerThread",
-                                                        "globalEventExecutor-[1-9]-[1-9]",
-                                                        "globalEventExecutor-[1-9]-[1-9]",
-                                                        "Shutdown-checker",
-                                                        "cluster[0-9]-connection-reaper-[0-9]",
-                                                        "Attach Listener",
-                                                        "process reaper",
-                                                        "JNA Cleaner",
-                                                        // the driver isn't expected to terminate threads on close synchronously (CASSANDRA-19000)
-                                                        "cluster[0-9]-nio-worker-[0-9]" },
-                                         false);
-    assertSchemaNotLoaded();
-    assertCLSMNotLoaded();
-    assertSystemKSNotLoaded();
-    assertKeyspaceNotLoaded();
+        assertNoUnexpectedThreadsStarted(false, appendToAllowedThreads(SYNC_DRIVER_THREAD));
+        assertSchemaNotLoaded();
+        assertCLSMNotLoaded();
+        assertSystemKSNotLoaded();
+        assertKeyspaceNotLoaded();
         assertServerNotLoaded();
     }
 
@@ -129,17 +130,7 @@ public void testBulkLoader_WithArgs2() throws Exception
         if (!(tool.getException().getCause().getCause().getCause() instanceof NoHostAvailableException))
             throw tool.getException();
 
-        assertNoUnexpectedThreadsStarted(new String[] { "ObjectCleanerThread",
-                                                        "globalEventExecutor-[1-9]-[1-9]",
-                                                        "globalEventExecutor-[1-9]-[1-9]",
-                                                        "Shutdown-checker",
-                                                        "cluster[0-9]-connection-reaper-[0-9]",
-                                                        "Attach Listener",
-                                                        "process reaper",
-                                                        "JNA Cleaner",
-                                                        // the driver isn't expected to terminate threads on close synchronously (CASSANDRA-19000)
-                                                        "cluster[0-9]-nio-worker-[0-9]" },
-                                         false);
+        assertNoUnexpectedThreadsStarted(false, appendToAllowedThreads(SYNC_DRIVER_THREAD));
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -216,4 +207,11 @@ public void testBulkLoader_WithArgs6() throws Throwable
         assertEquals(6, DatabaseDescriptor.getEntireSSTableInterDCStreamThroughputOutboundMebibytesPerSec(), 0.0);
         throw tool.getException().getCause().getCause().getCause();
     }
+
+    private String[] appendToAllowedThreads(String... optionalThreads)
+    {
+        var result = new ArrayList<>(Arrays.asList(ALLOWED_BULK_LOADER_THREADS));
+        Collections.addAll(result, optionalThreads);
+        return result.toArray(String[]::new);
+    }
 }
diff --git a/test/unit/org/apache/cassandra/tools/GetVersionTest.java b/test/unit/org/apache/cassandra/tools/GetVersionTest.java
index b3ddb58328a2..bb24dc747d74 100644
--- a/test/unit/org/apache/cassandra/tools/GetVersionTest.java
+++ b/test/unit/org/apache/cassandra/tools/GetVersionTest.java
@@ -29,7 +29,7 @@ public void testGetVersion()
     {
         ToolResult tool = ToolRunner.invokeClass(GetVersion.class);
         tool.assertOnCleanExit();
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/OfflineToolUtils.java b/test/unit/org/apache/cassandra/tools/OfflineToolUtils.java
index a8244f050c93..9c950fda802c 100644
--- a/test/unit/org/apache/cassandra/tools/OfflineToolUtils.java
+++ b/test/unit/org/apache/cassandra/tools/OfflineToolUtils.java
@@ -27,15 +27,13 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
-import java.util.Set;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
-import java.util.stream.StreamSupport;
-
-import com.google.common.collect.Iterables;
 
 import org.apache.commons.io.FileUtils;
+import org.junit.Assert;
 import org.junit.BeforeClass;
+import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import org.apache.cassandra.config.CassandraRelevantProperties;
@@ -44,7 +42,6 @@
 
 import static org.apache.cassandra.utils.FBUtilities.preventIllegalAccessWarnings;
 import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
 /**
@@ -52,6 +49,8 @@
  */
 public abstract class OfflineToolUtils
 {
+    private static final Logger logger = LoggerFactory.getLogger(OfflineToolUtils.class);
+
     static
     {
         preventIllegalAccessWarnings();
@@ -60,68 +59,77 @@ public abstract class OfflineToolUtils
     private static List<ThreadInfo> initialThreads;
 
     static final String[] OPTIONAL_THREADS_WITH_SCHEMA = {
-    "ScheduledTasks:[1-9]",
-    "ScheduledFastTasks:[1-9]",
-    "OptionalTasks:[1-9]",
-    "Reference-Reaper",
-    "LocalPool-Cleaner(-networking|-chunk-cache)",
-    "CacheCleanupExecutor:[1-9]",
-    "CompactionExecutor:[1-9]",
-    "ValidationExecutor:[1-9]",
-    "NonPeriodicTasks:[1-9]",
-    "Sampler:[1-9]",
-    "SecondaryIndexManagement:[1-9]",
-    "Strong-Reference-Leak-Detector:[1-9]",
-    "Background_Reporter:[1-9]",
-    "EXPIRING-MAP-REAPER:[1-9]",
-    "ObjectCleanerThread",
-    "process reaper",  // spawned by the jvm when executing external processes
-                       // and may still be active when we check
-    "Attach Listener", // spawned in intellij IDEA
-    "JNA Cleaner",     // spawned by JNA
-    "ThreadLocalMetrics-Cleaner", // spawned by org.apache.cassandra.metrics.ThreadLocalMetrics
-    "Native reference cleanup thread",
-    "^ForkJoinPool\\.commonPool-worker-\\d+$"
+        "ScheduledTasks:[1-9]",
+        "ScheduledFastTasks:[1-9]",
+        "OptionalTasks:[1-9]",
+        "Reference-Reaper",
+        "LocalPool-Cleaner(-networking|-chunk-cache)",
+        "CacheCleanupExecutor:[1-9]",
+        "CompactionExecutor:[1-9]",
+        "ValidationExecutor:[1-9]",
+        "NonPeriodicTasks:[1-9]",
+        "Sampler:[1-9]",
+        "SecondaryIndexManagement:[1-9]",
+        "Strong-Reference-Leak-Detector:[1-9]",
+        "Background_Reporter:[1-9]",
+        "EXPIRING-MAP-REAPER:[1-9]",
+        "ObjectCleanerThread",
+        "process reaper",               // spawned by the jvm when executing external processes and may still be active when we check
+                                        // and may still be active when we check
+        "Attach Listener",              // spawned in intellij IDEA
+        "JNA Cleaner",                  // spawned by JNA
+        "ThreadLocalMetrics-Cleaner",   // spawned by org.apache.cassandra.metrics.ThreadLocalMetrics
+        "Native reference cleanup thread",
+        "^ForkJoinPool\\.commonPool-worker-\\d+$"
     };
 
     static final String[] NON_DEFAULT_MEMTABLE_THREADS =
     {
-    "((Native|Slab|Heap)Pool|Logged)Cleaner"
+        "((Native|Slab|Heap)Pool|Logged)Cleaner"
     };
 
-    public void assertNoUnexpectedThreadsStarted(String[] optionalThreadNames, boolean allowNonDefaultMemtableThreads)
-    {
-        ThreadMXBean threads = ManagementFactory.getThreadMXBean();
+    /** We have some threads that show up in the presence of other JDK's; account for those here */
+    private static final String[] EXTRA_JDK_THREADS = new String[]{
+            "Native reference cleanup thread",      // Corretto janitor thread
+    };
 
-        Set<String> initial = initialThreads
-                              .stream()
-                              .map(ThreadInfo::getThreadName)
-                              .collect(Collectors.toSet());
-
-        Set<String> current = Arrays.stream(threads.getThreadInfo(threads.getAllThreadIds()))
-                                    .filter(Objects::nonNull)
-                                    .map(ThreadInfo::getThreadName)
-                                    .collect(Collectors.toSet());
-        Iterable<String> optionalNames = optionalThreadNames != null
-                                         ? Arrays.asList(optionalThreadNames)
-                                         : Collections.emptyList();
-        if (allowNonDefaultMemtableThreads && DatabaseDescriptor.getMemtableConfigurations().containsKey("default"))
-            optionalNames = Iterables.concat(optionalNames, Arrays.asList(NON_DEFAULT_MEMTABLE_THREADS));
+    /**
+     * When unexpected threads or core singleton objects are inadvertantly initialized during tool runs it can lead to
+     * the tool hanging on shutdown. We have a whitelisted number of threads above along with whatever initial threads
+     * were there on first tool creation we can diff against; if we find _anything_ new, it's an error condition - this
+     * needs to either be added to the list above or debugged and fixed.
+     */
+    public void assertNoUnexpectedThreadsStarted(boolean allowNonDefaultMemtableThreads, String... optionalThreadNames)
+    {
+        var allowedThreadNames = initialThreads.stream()
+                                               .map(ThreadInfo::getThreadName)
+                                               .collect(Collectors.toSet());
 
-        List<Pattern> optional = StreamSupport.stream(optionalNames.spliterator(), false)
-                                              .map(Pattern::compile)
-                                              .collect(Collectors.toList());
+        Collections.addAll(allowedThreadNames, EXTRA_JDK_THREADS);
+        Collections.addAll(allowedThreadNames, optionalThreadNames);
 
-        current.removeAll(initial);
+        if (allowNonDefaultMemtableThreads && DatabaseDescriptor.getMemtableConfigurations().containsKey("default"))
+            Collections.addAll(allowedThreadNames, NON_DEFAULT_MEMTABLE_THREADS);
 
-        Set<String> remain = current.stream()
-                                    .filter(threadName -> optional.stream().noneMatch(pattern -> pattern.matcher(threadName).matches()))
-                                    .collect(Collectors.toSet());
+        var allowedRegexes = allowedThreadNames.stream()
+                                               .map(Pattern::compile)
+                                               .collect(Collectors.toList());
 
-        if (!remain.isEmpty())
-            System.err.println("Unexpected thread names: " + remain);
+        var threads = ManagementFactory.getThreadMXBean();
+        var badThreads = Arrays.stream(threads.getThreadInfo(threads.getAllThreadIds()))
+                               .filter(Objects::nonNull)
+                               .filter(threadInfo -> allowedRegexes.stream().noneMatch(pattern -> pattern.matcher(threadInfo.getThreadName()).matches()))
+                               .collect(Collectors.toSet());
 
-        assertTrue("Wrong thread status, active threads unaccounted for: " + remain, remain.isEmpty());
+        if (!badThreads.isEmpty())
+        {
+            logger.error("Found unexpected disallowed threads during check. Printing thread details.");
+            badThreads.forEach(info -> logger.error(info.toString()));
+            var errorMessage = new StringBuilder("Bad threads found:");
+            badThreads.forEach(ti -> errorMessage.append("\n   ").append(ti.getThreadName()));
+            errorMessage.append("\n-Tools should not start threads that initialize singletons or other system resources");
+            Assert.fail(errorMessage.toString());
+        }
     }
 
     public void assertSchemaNotLoaded()
@@ -239,7 +247,7 @@ public static File copySSTables() throws IOException
     
     protected void assertCorrectEnvPostTest()
     {
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, true);
+        assertNoUnexpectedThreadsStarted(true, OPTIONAL_THREADS_WITH_SCHEMA);
         assertSchemaLoaded();
         assertServerNotLoaded();
     }
diff --git a/test/unit/org/apache/cassandra/tools/SSTableExpiredBlockersTest.java b/test/unit/org/apache/cassandra/tools/SSTableExpiredBlockersTest.java
index 95c76224ead3..ce529b01eea8 100644
--- a/test/unit/org/apache/cassandra/tools/SSTableExpiredBlockersTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTableExpiredBlockersTest.java
@@ -37,7 +37,7 @@ public void testNoArgsPrintsHelp()
         Assertions.assertThat(tool.getCleanedStderr()).isEmpty();
         assertEquals(1, tool.getExitCode());
 
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/SSTableExportSchemaLoadingTest.java b/test/unit/org/apache/cassandra/tools/SSTableExportSchemaLoadingTest.java
index 456756338ad2..48223914849c 100644
--- a/test/unit/org/apache/cassandra/tools/SSTableExportSchemaLoadingTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTableExportSchemaLoadingTest.java
@@ -184,7 +184,7 @@ public void testJSONLineArg() throws IOException
      */
     private void assertPostTestEnv()
     {
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, true);
+        assertNoUnexpectedThreadsStarted(true, OPTIONAL_THREADS_WITH_SCHEMA);
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
         assertKeyspaceNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/SSTableExportTest.java b/test/unit/org/apache/cassandra/tools/SSTableExportTest.java
index 0ec13abb654d..ee8e17981a62 100644
--- a/test/unit/org/apache/cassandra/tools/SSTableExportTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTableExportTest.java
@@ -108,7 +108,7 @@ public void testExcludePKArgOutOfOrder()
      */
     private void assertPostTestEnv()
     {
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, false);
+        assertNoUnexpectedThreadsStarted(false, OPTIONAL_THREADS_WITH_SCHEMA);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/SSTableLevelResetterTest.java b/test/unit/org/apache/cassandra/tools/SSTableLevelResetterTest.java
index cddfd6b7e102..c919cba5035b 100644
--- a/test/unit/org/apache/cassandra/tools/SSTableLevelResetterTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTableLevelResetterTest.java
@@ -36,7 +36,7 @@ public void testNoArgsPrintsHelp()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         Assertions.assertThat(tool.getCleanedStderr()).isEmpty();
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/SSTableMetadataViewerTest.java b/test/unit/org/apache/cassandra/tools/SSTableMetadataViewerTest.java
index ba53c0953bdb..ef359dedc5fa 100644
--- a/test/unit/org/apache/cassandra/tools/SSTableMetadataViewerTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTableMetadataViewerTest.java
@@ -54,7 +54,7 @@ public void testNoArgsPrintsHelp()
             assertThat(tool.getCleanedStderr(), CoreMatchers.containsStringIgnoringCase("Options:"));
             assertEquals(1, tool.getExitCode());
         }
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -218,7 +218,7 @@ public void testScanArg()
 
     private void assertGoodEnvPostTest()
     {
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, false);
+        assertNoUnexpectedThreadsStarted(false, OPTIONAL_THREADS_WITH_SCHEMA);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/SSTableOfflineRelevelTest.java b/test/unit/org/apache/cassandra/tools/SSTableOfflineRelevelTest.java
index dbe4d8a467d4..3e2ea7f61df2 100644
--- a/test/unit/org/apache/cassandra/tools/SSTableOfflineRelevelTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTableOfflineRelevelTest.java
@@ -36,7 +36,7 @@ public void testNoArgsPrintsHelp()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         Assertions.assertThat(tool.getCleanedStderr()).isEmpty();
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/SSTablePartitionsTest.java b/test/unit/org/apache/cassandra/tools/SSTablePartitionsTest.java
index fa1fda373c54..e039e880771d 100644
--- a/test/unit/org/apache/cassandra/tools/SSTablePartitionsTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTablePartitionsTest.java
@@ -101,7 +101,7 @@ private static void createBackupsAndSnapshots(String sstable)
     @After
     public void assertPostTestEnv()
     {
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, false);
+        assertNoUnexpectedThreadsStarted(false, OPTIONAL_THREADS_WITH_SCHEMA);
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
         assertKeyspaceNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/SSTableRepairedAtSetterTest.java b/test/unit/org/apache/cassandra/tools/SSTableRepairedAtSetterTest.java
index f895983c8ff6..1bd9dd01b889 100644
--- a/test/unit/org/apache/cassandra/tools/SSTableRepairedAtSetterTest.java
+++ b/test/unit/org/apache/cassandra/tools/SSTableRepairedAtSetterTest.java
@@ -41,7 +41,7 @@ public void testNoArgsPrintsHelp()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         Assertions.assertThat(tool.getCleanedStderr()).isEmpty();
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -79,7 +79,7 @@ public void testIsrepairedArg() throws Exception
                                                        "--is-repaired",
                                                        findOneSSTable("legacy_sstables", "legacy_ma_simple"));
         tool.assertOnCleanExit();
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, false);
+        assertNoUnexpectedThreadsStarted(false, OPTIONAL_THREADS_WITH_SCHEMA);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -95,7 +95,7 @@ public void testIsunrepairedArg() throws Exception
                                                  "--is-unrepaired",
                                                  findOneSSTable("legacy_sstables", "legacy_ma_simple"));
         tool.assertOnCleanExit();
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, false);
+        assertNoUnexpectedThreadsStarted(false, OPTIONAL_THREADS_WITH_SCHEMA);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -113,7 +113,7 @@ public void testFilesArg() throws Exception
         String file = tmpFile.absolutePath();
         ToolResult tool = ToolRunner.invokeClass(SSTableRepairedAtSetter.class, "--really-set", "--is-repaired", "-f", file);
         tool.assertOnCleanExit();
-        assertNoUnexpectedThreadsStarted(OPTIONAL_THREADS_WITH_SCHEMA, false);
+        assertNoUnexpectedThreadsStarted(false, OPTIONAL_THREADS_WITH_SCHEMA);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/ToolRunner.java b/test/unit/org/apache/cassandra/tools/ToolRunner.java
index 0495a5afdcaf..1bdd7cffac00 100644
--- a/test/unit/org/apache/cassandra/tools/ToolRunner.java
+++ b/test/unit/org/apache/cassandra/tools/ToolRunner.java
@@ -66,6 +66,7 @@ public class ToolRunner
     protected static final Logger logger = LoggerFactory.getLogger(ToolRunner.class);
 
     public static final ImmutableList<String> DEFAULT_CLEANERS = ImmutableList.of("(?im)^picked up.*\\R",
+                                                                                  "(?im)^.*package jdk.internal.util.jar not in java.base*\\R",
                                                                                   "(?im)^.*Not generating a deterministic id for table.*\\R",
                                                                                   "(?im)^.*`USE <keyspace>` with prepared statements is.*\\R");
 
diff --git a/test/unit/org/apache/cassandra/tools/ToolsSchemaLoadingTest.java b/test/unit/org/apache/cassandra/tools/ToolsSchemaLoadingTest.java
index 103fa508f77a..1ddc8eb0ed95 100644
--- a/test/unit/org/apache/cassandra/tools/ToolsSchemaLoadingTest.java
+++ b/test/unit/org/apache/cassandra/tools/ToolsSchemaLoadingTest.java
@@ -33,7 +33,7 @@ public void testNoArgsPrintsHelpStandaloneVerifier()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         assertThat(tool.getCleanedStderr(), CoreMatchers.containsStringIgnoringCase("Missing arguments"));
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -48,7 +48,7 @@ public void testNoArgsPrintsHelpStandaloneScrubber()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         assertThat(tool.getCleanedStderr(), CoreMatchers.containsStringIgnoringCase("Missing arguments"));
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -63,7 +63,7 @@ public void testNoArgsPrintsHelpStandaloneSplitter()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         assertThat(tool.getCleanedStderr(), CoreMatchers.containsStringIgnoringCase("No sstables to split"));
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -78,7 +78,7 @@ public void testNoArgsPrintsHelpStandaloneSSTableUtil()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         assertThat(tool.getCleanedStderr(), CoreMatchers.containsStringIgnoringCase("Missing arguments"));
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
@@ -93,7 +93,7 @@ public void testNoArgsPrintsHelpStandaloneUpgrader()
         assertThat(tool.getStdout(), CoreMatchers.containsStringIgnoringCase("usage:"));
         assertThat(tool.getCleanedStderr(), CoreMatchers.containsStringIgnoringCase("Missing arguments"));
         assertEquals(1, tool.getExitCode());
-        assertNoUnexpectedThreadsStarted(null, false);
+        assertNoUnexpectedThreadsStarted(false);
         assertSchemaNotLoaded();
         assertCLSMNotLoaded();
         assertSystemKSNotLoaded();
diff --git a/test/unit/org/apache/cassandra/tools/nodetool/GetAuditLogTest.java b/test/unit/org/apache/cassandra/tools/nodetool/GetAuditLogTest.java
index 816c7031b4e3..3ff266059dc6 100644
--- a/test/unit/org/apache/cassandra/tools/nodetool/GetAuditLogTest.java
+++ b/test/unit/org/apache/cassandra/tools/nodetool/GetAuditLogTest.java
@@ -112,7 +112,7 @@ private void testChangedOutputSimple(final String getAuditLogOutput)
         final String output = getAuditLogOutput.replaceAll("( )+", " ").trim();
         assertThat(output).startsWith("enabled true");
         assertThat(output).contains("logger BinAuditLogger");
-        assertThat(output).contains("roll_cycle HOURLY");
+        assertThat(output).contains("roll_cycle FAST_HOURLY");
         assertThat(output).contains("block true");
         assertThat(output).contains("max_log_size 17179869184");
         assertThat(output).contains("max_queue_weight 268435456");
@@ -131,7 +131,7 @@ private void testChangedOutputComplex(final String getAuditLogOutput)
         final String output = getAuditLogOutput.replaceAll("( )+", " ").trim();
         assertThat(output).startsWith("enabled true");
         assertThat(output).contains("logger BinAuditLogger");
-        assertThat(output).contains("roll_cycle HOURLY");
+        assertThat(output).contains("roll_cycle FAST_HOURLY");
         assertThat(output).contains("block true");
         assertThat(output).contains("max_log_size 17179869184");
         assertThat(output).contains("max_queue_weight 268435456");
@@ -150,7 +150,7 @@ private void testDefaultOutput(final String getAuditLogOutput)
         final String output = getAuditLogOutput.replaceAll("( )+", " ").trim();
         assertThat(output).startsWith("enabled false");
         assertThat(output).contains("logger BinAuditLogger");
-        assertThat(output).contains("roll_cycle HOURLY");
+        assertThat(output).contains("roll_cycle FAST_HOURLY");
         assertThat(output).contains("block true");
         assertThat(output).contains("max_log_size 17179869184");
         assertThat(output).contains("max_queue_weight 268435456");
diff --git a/test/unit/org/apache/cassandra/transport/MessagePayloadTest.java b/test/unit/org/apache/cassandra/transport/MessagePayloadTest.java
index 395b5b85ff6a..e1dee0e6c971 100644
--- a/test/unit/org/apache/cassandra/transport/MessagePayloadTest.java
+++ b/test/unit/org/apache/cassandra/transport/MessagePayloadTest.java
@@ -17,14 +17,11 @@
  */
 package org.apache.cassandra.transport;
 
-import java.lang.reflect.Field;
-import java.lang.reflect.Modifier;
 import java.nio.ByteBuffer;
 import java.util.Collections;
 import java.util.Map;
 
 import org.junit.After;
-import org.junit.AfterClass;
 import org.junit.Assert;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -48,8 +45,8 @@
 import org.apache.cassandra.transport.messages.ResultMessage;
 import org.apache.cassandra.utils.ByteArrayUtil;
 import org.apache.cassandra.utils.MD5Digest;
-import org.apache.cassandra.utils.ReflectionUtils;
 
+import static org.apache.cassandra.config.CassandraRelevantProperties.CUSTOM_QUERY_HANDLER_CLASS;
 import static org.apache.cassandra.utils.ByteBufferUtil.bytes;
 
 public class MessagePayloadTest extends CQLTester
@@ -57,47 +54,11 @@ public class MessagePayloadTest extends CQLTester
     public static Map<String, ByteBuffer> requestPayload;
     public static Map<String, ByteBuffer> responsePayload;
 
-    private static Field cqlQueryHandlerField;
-    private static boolean modifiersAccessible;
-
     @BeforeClass
-    public static void makeCqlQueryHandlerAccessible()
+    public static void setUpClass()
     {
-        try
-        {
-            cqlQueryHandlerField = ClientState.class.getDeclaredField("cqlQueryHandler");
-            cqlQueryHandlerField.setAccessible(true);
-
-            Field modifiersField = ReflectionUtils.getModifiersField();
-            modifiersAccessible = modifiersField.isAccessible();
-            modifiersField.setAccessible(true);
-            modifiersField.setInt(cqlQueryHandlerField, cqlQueryHandlerField.getModifiers() & ~Modifier.FINAL);
-        }
-        catch (IllegalAccessException | NoSuchFieldException e)
-        {
-            throw new RuntimeException(e);
-        }
-    }
-
-    @AfterClass
-    public static void resetCqlQueryHandlerField()
-    {
-        if (cqlQueryHandlerField == null)
-            return;
-        try
-        {
-            Field modifiersField = ReflectionUtils.getModifiersField();
-            modifiersField.setAccessible(true);
-            modifiersField.setInt(cqlQueryHandlerField, cqlQueryHandlerField.getModifiers() | Modifier.FINAL);
-
-            cqlQueryHandlerField.setAccessible(false);
-
-            modifiersField.setAccessible(modifiersAccessible);
-        }
-        catch (IllegalAccessException | NoSuchFieldException e)
-        {
-            throw new RuntimeException(e);
-        }
+        CUSTOM_QUERY_HANDLER_CLASS.setString("org.apache.cassandra.transport.MessagePayloadTest$TestQueryHandler");
+        CQLTester.setUpClass();
     }
 
     @After
@@ -116,13 +77,9 @@ public void dropCreatedTable()
     @Test
     public void testMessagePayloadBeta() throws Throwable
     {
-        QueryHandler queryHandler = (QueryHandler) cqlQueryHandlerField.get(null);
-        cqlQueryHandlerField.set(null, new TestQueryHandler());
-        try
-        {
-            requireNetwork();
+        requireNetwork();
 
-            Assert.assertSame(TestQueryHandler.class, ClientState.getCQLQueryHandler().getClass());
+        Assert.assertSame(TestQueryHandler.class, ClientState.getCQLQueryHandler().getClass());
 
             SimpleClient client = new SimpleClient(nativeAddr.getHostAddress(),
                                                    nativePort,
@@ -133,239 +90,216 @@ public void testMessagePayloadBeta() throws Throwable
             {
                 client.connect(false);
 
-                Map<String, ByteBuffer> reqMap;
-                Map<String, ByteBuffer> respMap;
-
-                QueryOptions queryOptions = QueryOptions.create(
-                  QueryOptions.DEFAULT.getConsistency(),
-                  QueryOptions.DEFAULT.getValues(),
-                  QueryOptions.DEFAULT.skipMetadata(),
-                  QueryOptions.DEFAULT.getPageSize(),
-                  QueryOptions.DEFAULT.getPagingState(),
-                  QueryOptions.DEFAULT.getSerialConsistency(),
-                  ProtocolVersion.V5,
-                  KEYSPACE);
-                QueryMessage queryMessage = new QueryMessage("CREATE TABLE atable (pk int PRIMARY KEY, v text)",
-                                                             queryOptions);
-                PrepareMessage prepareMessage = new PrepareMessage("SELECT * FROM atable", KEYSPACE);
-
-                reqMap = Collections.singletonMap("foo", bytes(42));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(42));
-                queryMessage.setCustomPayload(reqMap);
-                Message.Response queryResponse = client.execute(queryMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, queryResponse.getCustomPayload());
-
-                reqMap = Collections.singletonMap("foo", bytes(43));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(43));
-                prepareMessage.setCustomPayload(reqMap);
-                ResultMessage.Prepared prepareResponse = (ResultMessage.Prepared) client.execute(prepareMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, prepareResponse.getCustomPayload());
-
-                ExecuteMessage executeMessage = new ExecuteMessage(prepareResponse.statementId, prepareResponse.resultMetadataId, QueryOptions.DEFAULT);
-                reqMap = Collections.singletonMap("foo", bytes(44));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(44));
-                executeMessage.setCustomPayload(reqMap);
-                Message.Response executeResponse = client.execute(executeMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, executeResponse.getCustomPayload());
-
-                BatchMessage batchMessage = new BatchMessage(BatchStatement.Type.UNLOGGED,
-                                                             Collections.<Object>singletonList("INSERT INTO " + KEYSPACE + ".atable (pk,v) VALUES (1, 'foo')"),
-                                                             Collections.singletonList(ByteArrayUtil.EMPTY_ARRAY_OF_BYTE_ARRAYS),
-                                                             queryOptions);
-                reqMap = Collections.singletonMap("foo", bytes(45));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(45));
-                batchMessage.setCustomPayload(reqMap);
-                Message.Response batchResponse = client.execute(batchMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, batchResponse.getCustomPayload());
-            }
-            finally
-            {
-                client.close();
-            }
+            Map<String, ByteBuffer> reqMap;
+            Map<String, ByteBuffer> respMap;
+
+            QueryOptions queryOptions = QueryOptions.create(
+              QueryOptions.DEFAULT.getConsistency(),
+              QueryOptions.DEFAULT.getValues(),
+              QueryOptions.DEFAULT.skipMetadata(),
+              QueryOptions.DEFAULT.getPageSize(),
+              QueryOptions.DEFAULT.getPagingState(),
+              QueryOptions.DEFAULT.getSerialConsistency(),
+              ProtocolVersion.V5,
+              KEYSPACE);
+            QueryMessage queryMessage = new QueryMessage("CREATE TABLE atable (pk int PRIMARY KEY, v text)",
+                                                         queryOptions);
+            PrepareMessage prepareMessage = new PrepareMessage("SELECT * FROM atable", KEYSPACE);
+
+            reqMap = Collections.singletonMap("foo", bytes(42));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(42));
+            queryMessage.setCustomPayload(reqMap);
+            Message.Response queryResponse = client.execute(queryMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, queryResponse.getCustomPayload());
+
+            reqMap = Collections.singletonMap("foo", bytes(43));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(43));
+            prepareMessage.setCustomPayload(reqMap);
+            ResultMessage.Prepared prepareResponse = (ResultMessage.Prepared) client.execute(prepareMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, prepareResponse.getCustomPayload());
+
+            ExecuteMessage executeMessage = new ExecuteMessage(prepareResponse.statementId, prepareResponse.resultMetadataId, QueryOptions.DEFAULT);
+            reqMap = Collections.singletonMap("foo", bytes(44));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(44));
+            executeMessage.setCustomPayload(reqMap);
+            Message.Response executeResponse = client.execute(executeMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, executeResponse.getCustomPayload());
+
+            BatchMessage batchMessage = new BatchMessage(BatchStatement.Type.UNLOGGED,
+                                                         Collections.<Object>singletonList("INSERT INTO " + KEYSPACE + ".atable (pk,v) VALUES (1, 'foo')"),
+                                                         Collections.singletonList(ByteArrayUtil.EMPTY_ARRAY_OF_BYTE_ARRAYS),
+                                                         queryOptions);
+            reqMap = Collections.singletonMap("foo", bytes(45));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(45));
+            batchMessage.setCustomPayload(reqMap);
+            Message.Response batchResponse = client.execute(batchMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, batchResponse.getCustomPayload());
         }
         finally
         {
-            cqlQueryHandlerField.set(null, queryHandler);
+            client.close();
         }
     }
 
     @Test
     public void testMessagePayload() throws Throwable
     {
-        QueryHandler queryHandler = (QueryHandler) cqlQueryHandlerField.get(null);
-        cqlQueryHandlerField.set(null, new TestQueryHandler());
-        try
-        {
-            requireNetwork();
+        requireNetwork();
 
-            Assert.assertSame(TestQueryHandler.class, ClientState.getCQLQueryHandler().getClass());
+        Assert.assertSame(TestQueryHandler.class, ClientState.getCQLQueryHandler().getClass());
 
-            SimpleClient client = new SimpleClient(nativeAddr.getHostAddress(), nativePort);
-            try
-            {
-                client.connect(false);
-
-                Map<String, ByteBuffer> reqMap;
-                Map<String, ByteBuffer> respMap;
-
-                QueryMessage queryMessage = new QueryMessage(
-                                                            "CREATE TABLE " + KEYSPACE + ".atable (pk int PRIMARY KEY, v text)",
-                                                            QueryOptions.DEFAULT
-                );
-                PrepareMessage prepareMessage = new PrepareMessage("SELECT * FROM " + KEYSPACE + ".atable", null);
-
-                reqMap = Collections.singletonMap("foo", bytes(42));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(42));
-                queryMessage.setCustomPayload(reqMap);
-                Message.Response queryResponse = client.execute(queryMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, queryResponse.getCustomPayload());
-
-                reqMap = Collections.singletonMap("foo", bytes(43));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(43));
-                prepareMessage.setCustomPayload(reqMap);
-                ResultMessage.Prepared prepareResponse = (ResultMessage.Prepared) client.execute(prepareMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, prepareResponse.getCustomPayload());
-
-                ExecuteMessage executeMessage = new ExecuteMessage(prepareResponse.statementId, prepareResponse.resultMetadataId, QueryOptions.DEFAULT);
-                reqMap = Collections.singletonMap("foo", bytes(44));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(44));
-                executeMessage.setCustomPayload(reqMap);
-                Message.Response executeResponse = client.execute(executeMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, executeResponse.getCustomPayload());
-
-                BatchMessage batchMessage = new BatchMessage(BatchStatement.Type.UNLOGGED,
-                                                             Collections.<Object>singletonList("INSERT INTO " + KEYSPACE + ".atable (pk,v) VALUES (1, 'foo')"),
-                                                             Collections.singletonList(ByteArrayUtil.EMPTY_ARRAY_OF_BYTE_ARRAYS),
-                                                             QueryOptions.DEFAULT);
-                reqMap = Collections.singletonMap("foo", bytes(45));
-                responsePayload = respMap = Collections.singletonMap("bar", bytes(45));
-                batchMessage.setCustomPayload(reqMap);
-                Message.Response batchResponse = client.execute(batchMessage);
-                payloadEquals(reqMap, requestPayload);
-                payloadEquals(respMap, batchResponse.getCustomPayload());
-            }
-            finally
-            {
-                client.close();
-            }
+        SimpleClient client = new SimpleClient(nativeAddr.getHostAddress(), nativePort);
+        try
+        {
+            client.connect(false);
+
+            Map<String, ByteBuffer> reqMap;
+            Map<String, ByteBuffer> respMap;
+
+            QueryMessage queryMessage = new QueryMessage(
+                                                        "CREATE TABLE " + KEYSPACE + ".atable (pk int PRIMARY KEY, v text)",
+                                                        QueryOptions.DEFAULT
+            );
+            PrepareMessage prepareMessage = new PrepareMessage("SELECT * FROM " + KEYSPACE + ".atable", null);
+
+            reqMap = Collections.singletonMap("foo", bytes(42));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(42));
+            queryMessage.setCustomPayload(reqMap);
+            Message.Response queryResponse = client.execute(queryMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, queryResponse.getCustomPayload());
+
+            reqMap = Collections.singletonMap("foo", bytes(43));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(43));
+            prepareMessage.setCustomPayload(reqMap);
+            ResultMessage.Prepared prepareResponse = (ResultMessage.Prepared) client.execute(prepareMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, prepareResponse.getCustomPayload());
+
+            ExecuteMessage executeMessage = new ExecuteMessage(prepareResponse.statementId, prepareResponse.resultMetadataId, QueryOptions.DEFAULT);
+            reqMap = Collections.singletonMap("foo", bytes(44));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(44));
+            executeMessage.setCustomPayload(reqMap);
+            Message.Response executeResponse = client.execute(executeMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, executeResponse.getCustomPayload());
+
+            BatchMessage batchMessage = new BatchMessage(BatchStatement.Type.UNLOGGED,
+                                                         Collections.<Object>singletonList("INSERT INTO " + KEYSPACE + ".atable (pk,v) VALUES (1, 'foo')"),
+                                                         Collections.singletonList(ByteArrayUtil.EMPTY_ARRAY_OF_BYTE_ARRAYS),
+                                                         QueryOptions.DEFAULT);
+            reqMap = Collections.singletonMap("foo", bytes(45));
+            responsePayload = respMap = Collections.singletonMap("bar", bytes(45));
+            batchMessage.setCustomPayload(reqMap);
+            Message.Response batchResponse = client.execute(batchMessage);
+            payloadEquals(reqMap, requestPayload);
+            payloadEquals(respMap, batchResponse.getCustomPayload());
         }
         finally
         {
-            cqlQueryHandlerField.set(null, queryHandler);
+            client.close();
         }
     }
 
     @Test
     public void testMessagePayloadVersion3() throws Throwable
     {
-        QueryHandler queryHandler = (QueryHandler) cqlQueryHandlerField.get(null);
-        cqlQueryHandlerField.set(null, new TestQueryHandler());
+        requireNetwork();
+
+        Assert.assertSame(TestQueryHandler.class, ClientState.getCQLQueryHandler().getClass());
+
+        SimpleClient client = new SimpleClient(nativeAddr.getHostAddress(), nativePort, ProtocolVersion.V3);
         try
         {
-            requireNetwork();
+            client.connect(false);
 
-            Assert.assertSame(TestQueryHandler.class, ClientState.getCQLQueryHandler().getClass());
+            Map<String, ByteBuffer> reqMap;
 
-            SimpleClient client = new SimpleClient(nativeAddr.getHostAddress(), nativePort, ProtocolVersion.V3);
+            QueryMessage queryMessage = new QueryMessage(
+                                                        "CREATE TABLE " + KEYSPACE + ".atable (pk int PRIMARY KEY, v text)",
+                                                        QueryOptions.DEFAULT
+            );
+            PrepareMessage prepareMessage = new PrepareMessage("SELECT * FROM " + KEYSPACE + ".atable", null);
+
+            reqMap = Collections.singletonMap("foo", bytes(42));
+            responsePayload = Collections.singletonMap("bar", bytes(42));
+            queryMessage.setCustomPayload(reqMap);
             try
             {
-                client.connect(false);
-
-                Map<String, ByteBuffer> reqMap;
-
-                QueryMessage queryMessage = new QueryMessage(
-                                                            "CREATE TABLE " + KEYSPACE + ".atable (pk int PRIMARY KEY, v text)",
-                                                            QueryOptions.DEFAULT
-                );
-                PrepareMessage prepareMessage = new PrepareMessage("SELECT * FROM " + KEYSPACE + ".atable", null);
-
-                reqMap = Collections.singletonMap("foo", bytes(42));
-                responsePayload = Collections.singletonMap("bar", bytes(42));
-                queryMessage.setCustomPayload(reqMap);
-                try
-                {
-                    client.execute(queryMessage);
-                    Assert.fail();
-                }
-                catch (RuntimeException e)
-                {
-                    Assert.assertTrue(e.getCause() instanceof ProtocolException);
-                }
-                // when a protocol exception is thrown by the server the connection is closed, so need to re-connect
-                client.close();
-                client.connect(false);
-
-                queryMessage.setCustomPayload(null);
                 client.execute(queryMessage);
+                Assert.fail();
+            }
+            catch (RuntimeException e)
+            {
+                Assert.assertTrue(e.getCause() instanceof ProtocolException);
+            }
+            // when a protocol exception is thrown by the server the connection is closed, so need to re-connect
+            client.close();
+            client.connect(false);
 
-                reqMap = Collections.singletonMap("foo", bytes(43));
-                responsePayload = Collections.singletonMap("bar", bytes(43));
-                prepareMessage.setCustomPayload(reqMap);
-                try
-                {
-                    client.execute(prepareMessage);
-                    Assert.fail();
-                }
-                catch (RuntimeException e)
-                {
-                    Assert.assertTrue(e.getCause() instanceof ProtocolException);
-                }
-                // when a protocol exception is thrown by the server the connection is closed, so need to re-connect
-                client.close();
-                client.connect(false);
+            queryMessage.setCustomPayload(null);
+            client.execute(queryMessage);
 
-                prepareMessage.setCustomPayload(null);
-                ResultMessage.Prepared prepareResponse = (ResultMessage.Prepared) client.execute(prepareMessage);
-
-                ExecuteMessage executeMessage = new ExecuteMessage(prepareResponse.statementId, prepareResponse.resultMetadataId, QueryOptions.DEFAULT);
-                reqMap = Collections.singletonMap("foo", bytes(44));
-                responsePayload = Collections.singletonMap("bar", bytes(44));
-                executeMessage.setCustomPayload(reqMap);
-                try
-                {
-                    client.execute(executeMessage);
-                    Assert.fail();
-                }
-                catch (RuntimeException e)
-                {
-                    Assert.assertTrue(e.getCause() instanceof ProtocolException);
-                }
-                // when a protocol exception is thrown by the server the connection is closed, so need to re-connect
-                client.close();
-                client.connect(false);
+            reqMap = Collections.singletonMap("foo", bytes(43));
+            responsePayload = Collections.singletonMap("bar", bytes(43));
+            prepareMessage.setCustomPayload(reqMap);
+            try
+            {
+                client.execute(prepareMessage);
+                Assert.fail();
+            }
+            catch (RuntimeException e)
+            {
+                Assert.assertTrue(e.getCause() instanceof ProtocolException);
+            }
+            // when a protocol exception is thrown by the server the connection is closed, so need to re-connect
+            client.close();
+            client.connect(false);
+
+            prepareMessage.setCustomPayload(null);
+            ResultMessage.Prepared prepareResponse = (ResultMessage.Prepared) client.execute(prepareMessage);
 
-                BatchMessage batchMessage = new BatchMessage(BatchStatement.Type.UNLOGGED,
-                                                             Collections.<Object>singletonList("INSERT INTO " + KEYSPACE + ".atable (pk,v) VALUES (1, 'foo')"),
-                                                             Collections.singletonList(ByteArrayUtil.EMPTY_ARRAY_OF_BYTE_ARRAYS),
-                                                             QueryOptions.DEFAULT);
-                reqMap = Collections.singletonMap("foo", bytes(45));
-                responsePayload = Collections.singletonMap("bar", bytes(45));
-                batchMessage.setCustomPayload(reqMap);
-                try
-                {
-                    client.execute(batchMessage);
-                    Assert.fail();
-                }
-                catch (RuntimeException e)
-                {
-                    Assert.assertTrue(e.getCause() instanceof ProtocolException);
-                }
+            ExecuteMessage executeMessage = new ExecuteMessage(prepareResponse.statementId, prepareResponse.resultMetadataId, QueryOptions.DEFAULT);
+            reqMap = Collections.singletonMap("foo", bytes(44));
+            responsePayload = Collections.singletonMap("bar", bytes(44));
+            executeMessage.setCustomPayload(reqMap);
+            try
+            {
+                client.execute(executeMessage);
+                Assert.fail();
+            }
+            catch (RuntimeException e)
+            {
+                Assert.assertTrue(e.getCause() instanceof ProtocolException);
+            }
+            // when a protocol exception is thrown by the server the connection is closed, so need to re-connect
+            client.close();
+            client.connect(false);
+
+            BatchMessage batchMessage = new BatchMessage(BatchStatement.Type.UNLOGGED,
+                                                         Collections.<Object>singletonList("INSERT INTO " + KEYSPACE + ".atable (pk,v) VALUES (1, 'foo')"),
+                                                         Collections.singletonList(ByteArrayUtil.EMPTY_ARRAY_OF_BYTE_ARRAYS),
+                                                         QueryOptions.DEFAULT);
+            reqMap = Collections.singletonMap("foo", bytes(45));
+            responsePayload = Collections.singletonMap("bar", bytes(45));
+            batchMessage.setCustomPayload(reqMap);
+            try
+            {
+                client.execute(batchMessage);
+                Assert.fail();
             }
-            finally
+            catch (RuntimeException e)
             {
-                client.close();
+                Assert.assertTrue(e.getCause() instanceof ProtocolException);
             }
         }
         finally
         {
-            cqlQueryHandlerField.set(null, queryHandler);
+            client.close();
         }
     }
 
diff --git a/test/unit/org/apache/cassandra/utils/MerkleTreeTest.java b/test/unit/org/apache/cassandra/utils/MerkleTreeTest.java
index 658ca2d6b63b..d118e5cb21ae 100644
--- a/test/unit/org/apache/cassandra/utils/MerkleTreeTest.java
+++ b/test/unit/org/apache/cassandra/utils/MerkleTreeTest.java
@@ -50,6 +50,8 @@
 import org.apache.cassandra.utils.MerkleTree.TreeRangeIterator;
 
 import static com.google.common.collect.Lists.newArrayList;
+import static org.apache.cassandra.config.CassandraRelevantProperties.JAVA_VERSION;
+import static org.apache.cassandra.utils.JavaUtils.parseJavaVersion;
 import static org.apache.cassandra.utils.MerkleTree.RECOMMENDED_DEPTH;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
@@ -619,8 +621,12 @@ public void testEstimatedSizes()
         Assert.assertEquals(12, MerkleTree.estimatedMaxDepthForBytes(Murmur3Partitioner.instance,
                                                                      1048576, 32));
 
-        // With 100 mebibytes we should get a limit of 19
-        Assert.assertEquals(19, MerkleTree.estimatedMaxDepthForBytes(Murmur3Partitioner.instance,
+        // With 100 mebibytes we should get a limit of 19 on JDK's < 21, 18 on JDK21
+        // Object size impl details can be expected to change on different JDK's so this impl will be brittle
+        // in the face of future JDK changes.
+        int version = parseJavaVersion(JAVA_VERSION.getString());
+        int expectedLimit = version < 21 ? 19 : 18;
+        Assert.assertEquals(expectedLimit, MerkleTree.estimatedMaxDepthForBytes(Murmur3Partitioner.instance,
                                                                      100 * 1048576, 32));
 
         // With 300 mebibytes we should get the old limit of 20
diff --git a/test/unit/org/apache/cassandra/utils/binlog/BinLogTest.java b/test/unit/org/apache/cassandra/utils/binlog/BinLogTest.java
index 806ad38d6910..1247b660a5e2 100644
--- a/test/unit/org/apache/cassandra/utils/binlog/BinLogTest.java
+++ b/test/unit/org/apache/cassandra/utils/binlog/BinLogTest.java
@@ -30,8 +30,8 @@
 
 import net.openhft.chronicle.queue.ChronicleQueue;
 import net.openhft.chronicle.queue.ExcerptTailer;
-import net.openhft.chronicle.queue.RollCycles;
 import net.openhft.chronicle.queue.impl.single.SingleChronicleQueueBuilder;
+import net.openhft.chronicle.queue.rollcycles.TestRollCycles;
 import net.openhft.chronicle.wire.WireOut;
 
 import org.junit.After;
@@ -75,7 +75,7 @@ public void setUp() throws Exception
     {
         path = tempDir();
         binLog = new BinLog.Builder().path(path)
-                                     .rollCycle(RollCycles.TEST_SECONDLY.toString())
+                                     .rollCycle(TestRollCycles.TEST_SECONDLY.toString())
                                      .maxQueueWeight(10)
                                      .maxLogSize(1024 * 1024 * 128)
                                      .blocking(false)
@@ -110,13 +110,13 @@ public void testConstructorNullRollCycle() throws Exception
     @Test(expected = IllegalArgumentException.class)
     public void testConstructorZeroWeight() throws Exception
     {
-        new BinLog.Builder().path(tempDir()).rollCycle(RollCycles.TEST_SECONDLY.toString()).maxQueueWeight(0).build(false);
+        new BinLog.Builder().path(tempDir()).rollCycle(TestRollCycles.TEST_SECONDLY.toString()).maxQueueWeight(0).build(false);
     }
 
     @Test(expected = IllegalArgumentException.class)
     public void testConstructorLogSize() throws Exception
     {
-        new BinLog.Builder().path(tempDir()).rollCycle(RollCycles.TEST_SECONDLY.toString()).maxLogSize(0).build(false);
+        new BinLog.Builder().path(tempDir()).rollCycle(TestRollCycles.TEST_SECONDLY.toString()).maxLogSize(0).build(false);
     }
 
     /**
@@ -383,7 +383,7 @@ public void writeMarshallablePayload(WireOut wire)
     public void testCleanupOnOversize() throws Exception
     {
         tearDown();
-        binLog = new BinLog.Builder().path(path).rollCycle(RollCycles.TEST_SECONDLY.toString()).maxQueueWeight(1).maxLogSize(10000).blocking(false).build(false);
+        binLog = new BinLog.Builder().path(path).rollCycle(TestRollCycles.TEST_SECONDLY.toString()).maxQueueWeight(1).maxLogSize(10000).blocking(false).build(false);
         for (int ii = 0; ii < 5; ii++)
         {
             binLog.put(record(String.valueOf(ii)));
@@ -496,7 +496,7 @@ public void writeMarshallablePayload(WireOut wire)
     List<String> readBinLogRecords(Path path)
     {
         List<String> records = new ArrayList<String>();
-        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(RollCycles.TEST_SECONDLY).build())
+        try (ChronicleQueue queue = SingleChronicleQueueBuilder.single(path.toFile()).rollCycle(TestRollCycles.TEST_SECONDLY).build())
         {
             ExcerptTailer tailer = queue.createTailer();
             while (true)
diff --git a/test/unit/org/apache/cassandra/utils/concurrent/RefCountedTest.java b/test/unit/org/apache/cassandra/utils/concurrent/RefCountedTest.java
index b8224bc90003..a85655274329 100644
--- a/test/unit/org/apache/cassandra/utils/concurrent/RefCountedTest.java
+++ b/test/unit/org/apache/cassandra/utils/concurrent/RefCountedTest.java
@@ -52,6 +52,8 @@
 import org.apache.cassandra.utils.Pair;
 import org.apache.cassandra.utils.concurrent.Ref.Visitor;
 
+import static org.apache.cassandra.config.CassandraRelevantProperties.JAVA_VERSION;
+import static org.apache.cassandra.utils.JavaUtils.parseJavaVersion;
 import static org.assertj.core.api.Assertions.assertThat;
 
 @SuppressWarnings({"unused", "unchecked", "rawtypes"})
@@ -468,10 +470,10 @@ Runnable getRunOnCloseLambdaWithThis()
     private Set<Ref.GlobalState> testCycles(Function<LambdaTestClass, Runnable> runOnCloseSupplier)
     {
         LambdaTestClass test = new LambdaTestClass();
-        Runnable weakRef = runOnCloseSupplier.apply(test);
+        Runnable supplierRef = runOnCloseSupplier.apply(test);
         RefCounted.Tidy tidier = new RefCounted.Tidy()
         {
-            Runnable ref = weakRef;
+            Runnable ref = supplierRef;
 
             public void tidy()
             {
@@ -505,7 +507,17 @@ public String name()
     public void testCycles()
     {
         assertThat(testCycles(LambdaTestClass::getRunOnCloseLambdaWithThis)).isNotEmpty(); // sanity test
-        assertThat(testCycles(LambdaTestClass::getRunOnCloseInner)).isNotEmpty(); // sanity test
+
+        // Behavior changed in JDK21; this was a sanity check to explore the way things worked but doesn't impact
+        // the correctness of the final actual check of the test.
+        int version = parseJavaVersion(JAVA_VERSION.getString());
+        boolean runOnCloseInnerState = version >= 21;
+
+        var result = testCycles(LambdaTestClass::getRunOnCloseInner);
+        if (runOnCloseInnerState)
+            assertThat(result).isEmpty();
+        else
+            assertThat(result).isNotEmpty();
 
         assertThat(testCycles(LambdaTestClass::getRunOnCloseLambda)).isEmpty();
     }
diff --git a/tools/bin/cassandra.in.sh b/tools/bin/cassandra.in.sh
index 79bbfcd34a64..083c324c200e 100644
--- a/tools/bin/cassandra.in.sh
+++ b/tools/bin/cassandra.in.sh
@@ -85,57 +85,37 @@ if [ -z $JAVA ] ; then
 fi
 
 # Matches variable 'java.supported' in build.xml
-java_versions_supported=11,17
+java_versions_supported="11 17 21"
+java_version_string=$(IFS=" "; echo "${java_versions_supported}")
 
 # Determine the sort of JVM we'll be running on.
-java_ver_output=`"${JAVA:-java}" -version 2>&1`
-jvmver=`echo "$java_ver_output" | grep '[openjdk|java] version' | awk -F'"' 'NR==1 {print $2}' | cut -d\- -f1`
-JVM_VERSION=${jvmver%_*}
-short=$(echo "${jvmver}" | cut -c1-2)
-
-# Unsupported JDKs below the upper supported version are not allowed
-if [ "$short" != "$(echo "$java_versions_supported" | cut -d, -f1)" ] && [ "$JVM_VERSION" \< "$(echo "$java_versions_supported" | cut -d, -f2)" ] ; then
-    echo "Unsupported Java $JVM_VERSION. Supported are $java_versions_supported"
-    exit 1;
-fi
+JAVA_VERSION=$(java -version 2>&1 | grep '[openjdk|java] version' | cut -d '"' -f2 | cut -d '.' -f1)
 
-# Allow execution of supported Java versions, and newer if CASSANDRA_JDK_UNSUPPORTED is set
-is_supported_version=$(echo "$java_versions_supported" | tr "," '\n' | grep -F -x "$short")
-if [ -z "$is_supported_version" ] ; then
-   if [ -z "$CASSANDRA_JDK_UNSUPPORTED" ] ; then
-        echo "Unsupported Java $JVM_VERSION. Supported are $java_versions_supported"
-        echo "If you would like to test with newer Java versions set CASSANDRA_JDK_UNSUPPORTED to any value (for example, CASSANDRA_JDK_UNSUPPORTED=true). Unset the parameter for default behavior"
-        exit 1;
-    else
-        echo "######################################################################"
-        echo "Warning! You are using JDK$short. This Cassandra version only supports $java_versions_supported."
-        echo "######################################################################"
-    fi
+supported=0
+for version in ${java_versions_supported}; do
+  if [ "$version" -eq "$JAVA_VERSION" ]; then
+      supported=1
+  fi
+done
+
+if [ "$supported" -eq 0 ]; then
+  if [ -z "$CASSANDRA_JDK_UNSUPPORTED" ]; then
+    echo "Unsupported Java $JAVA_VERSION. Supported are $java_version_string"
+    echo "If you would like to test with newer Java versions set CASSANDRA_JDK_UNSUPPORTED to any value (for example, CASSANDRA_JDK_UNSUPPORTED=true). Unset the parameter for default behavior"
+    exit 1
+  else
+    echo "######################################################################"
+    echo "Warning! You are using JDK$JAVA_VERSION. This Cassandra version only supports $java_version_string"
+    echo "######################################################################"
+  fi
 fi
-JAVA_VERSION=$short
-
-jvm=`echo "$java_ver_output" | grep -A 1 '[openjdk|java] version' | awk 'NR==2 {print $1}'`
-case "$jvm" in
-    OpenJDK)
-        JVM_VENDOR=OpenJDK
-        # this will be "64-Bit" or "32-Bit"
-        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $2}'`
-        ;;
-    "Java(TM)")
-        JVM_VENDOR=Oracle
-        # this will be "64-Bit" or "32-Bit"
-        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $3}'`
-        ;;
-    *)
-        # Help fill in other JVM values
-        JVM_VENDOR=other
-        JVM_ARCH=unknown
-        ;;
-esac
+
 
 # Read user-defined JVM options from jvm-server.options file
 JVM_OPTS_FILE=$CASSANDRA_CONF/jvm${jvmoptions_variant:--clients}.options
-if [ $JAVA_VERSION -ge 17 ] ; then
+if [ $JAVA_VERSION -ge 21 ] ; then
+    JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm21${jvmoptions_variant:--clients}.options
+elif [ $JAVA_VERSION -ge 17 ] ; then
     JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm17${jvmoptions_variant:--clients}.options
 elif [ $JAVA_VERSION -ge 11 ] ; then
     JVM_DEP_OPTS_FILE=$CASSANDRA_CONF/jvm11${jvmoptions_variant:--clients}.options
diff --git a/tools/fqltool/src/org/apache/cassandra/fqltool/commands/Dump.java b/tools/fqltool/src/org/apache/cassandra/fqltool/commands/Dump.java
index cae00062af03..7234fc3e2873 100644
--- a/tools/fqltool/src/org/apache/cassandra/fqltool/commands/Dump.java
+++ b/tools/fqltool/src/org/apache/cassandra/fqltool/commands/Dump.java
@@ -60,8 +60,8 @@ public class Dump implements Runnable
     @Parameters(paramLabel = "path", description = "Path containing the full query logs to dump.", arity = "1..*")
     private List<String> arguments = new ArrayList<>();
 
-    @Option(paramLabel = "roll_cycle", names = { "--roll-cycle" }, description = "How often to roll the log file was rolled. May be necessary for Chronicle to correctly parse file names. (MINUTELY, HOURLY, DAILY). Default HOURLY.")
-    private String rollCycle = "HOURLY";
+    @Option(paramLabel = "roll_cycle", names = { "--roll-cycle" }, description = "How often to roll the log file was rolled. May be necessary for Chronicle to correctly parse file names. (FAST_MINUTELY, FAST_HOURLY, FAST_DAILY). Default FAST_HOURLY.")
+    private String rollCycle = "FAST_HOURLY";
 
     @Option(paramLabel = "follow", names = { "--follow" }, description = "Upon reacahing the end of the log continue indefinitely waiting for more records")
     private boolean follow = false;