Support dual boot on same pool.

[HankB]

Provide capability to dual boot Ubuntu and Debian on the same root pool. See Richard Laager's email Apr 18, 2019, 4:29 PM "Grub error "compression algorithm inherit not supported" on reboot" (Sorry, I can't figure out how to find a link to that.)

On 4/18/19 3:46 PM, Hank Barta wrote:

I've given this some thought as a way to dual boot multiple Linux
variants from a single zpool. I have more questions than answers. Here
are some of mine.

• For all of the partitions that are automounted, is there a way to
  control which ones are mounted for a given distro? Or is it
  necessary to set them all to "legacy" and mount via fstab? Or a
  root=... in the linux grub command line?

If they aren't shared (e.g. /home is probably shared, though you noted
issues), then you need one of these two approaches:

A) Put them under the rootfs dataset. I believe the initramfs will mount
them, but you'll have to test. For example, if you had /opt
rpool/ROOT/ubuntu/opt
rpoot/ROOT/debian/opt

B) Make them all legacy and use different /etc/fstab per distro.

• Will grub (e.g. update-grub recognize more than the presently
  loaded distro? In this case, I think that EFI boot could come to the
  rescue. I have Win10, Ubuntu 19.04 (on EXT4) and Debian Buster (root
  on ZFS) on a laptop. Since the Ubuntu install (which followed
  Debian) I boot Debian via the BIOS boot selection menu. I could fix
  this by reinstalling grub when running Debian but it's so little
  trouble I haven't bothered. (I have become a huge fan of ZFS and
  secondarily, a fan of EFI.)

I'm not sure how well this will all work. You'll have to try. If it
doesn't work well now, hopefully this will be improved in the
not-too-distant future. People are looking into it.

• Not strictly related to ZFS but perhaps involved... Filesystems
  shared between distros (e.g. /home/$USER.) As an unrepentant distro
  hopper (or just distro upgrader) I have had a separate /home
  filesystem but found that some configuration files conflicted with
  the different versions of S/W installed by various distros. I
  resolved that by leaving $HOME in the root filesystem, mounting the
  old home filesystem at /mnt/home and linking directories I wanted to
  share (~/Documents, ~/Pictures etc.) to my home directory. It works
  well with a few quirks. (I need to follow directories with a
  trailing slash to get the result I want, such as “find ~/Documents/
  -name foo.bar” or “ls -l ~/Pictures/”.) This could be an issue if
  something in a newer version of Ubuntu puts things in the
  configuration files that would cause an older version to crash.
  (DAMHIK) This practice I describe is using EXT4 filesystems and is
  how my present desktop is organized. I wonder if there is a way to
  leverage ZFS filesystems to accomplish the same goal (or just
  migrate this practice to ZFS filesystems.) This should really be a
  subject for a different thread but could impact you if you get all
  of the other stuff to work.

You could do several things for /home/$USER, and then with any of them
have /home/$USER/Pictures as a ZFS dataset that is mounted normally or
is in all the /etc/fstabs:

A) /home/$USER exists in rpool/ROOT/{ubuntu,debian}
B) /home/$USER is rpool/ROOT/{ubuntu,debian}/home/$USER
C) /home/$USER is rpool/{ubuntu,debian}/home/$USER, mountpoint=legacy,
and in the separate /etc/fstabs