[Feature] Support Non-Evictable Databases (Pinned in Memory)

[zhangh43]

Description:
Add support for configuring certain databases (DBs) to be non-evictable, ensuring their data is never kicked out of memory by eviction, tiering, or memory pressure mechanisms.

This feature is useful for workloads that require predictable low-latency access to a subset of critical data, even when the system is under memory pressure.

Background / Motivation:

• Some DBs contain:

  • Hot metadata
  • Control-plane data
  • Latency-sensitive state

• Eviction of these DBs can cause:

  • Latency spikes due to storage reads
  • Unstable tail latency

• EloqKV supports multiple logical DBs; allowing per-DB memory policy improves flexibility

Proposed Behavior:

• Introduce a DB-level configuration flag:

  db.evictable = true | false
  

• When db.evictable = false:

  • Keys belonging to the DB:

    • Are never evicted from memory
    • Are excluded from eviction candidates
    • Are not tiered out to SSD due to memory pressure

• Eviction and memory reclamation apply only to evictable DBs

Scope / Requirements:

• Per-DB eviction policy

• Works with existing memory management and tiered storage logic

• Clear behavior under global memory pressure:

  • Evict evictable DBs first
  • If all DBs are non-evictable, return OOM or backpressure

• Redis protocol compatibility (logical DB semantics preserved)

Non-Goals:

• Per-key pinning (DB-level only)
• Automatic promotion/demotion between evictable and non-evictable

Configuration (Example):

db.0.evictable = false
db.1.evictable = true

Acceptance Criteria:

• Data in non-evictable DBs remains resident in memory
• No eviction events occur for those DBs
• Evictable DBs continue to function normally
• Memory pressure behavior is predictable and documented