perf(storage): volatile-ttl evicts the exact nearest-expiry victim via the expiry index (#551)

[TinDang97]

Summary

volatile-ttl eviction picked its victim by sampling maxmemory-samples random volatile keys and evicting the sample's minimum expiry — an approximation that can miss the globally soonest key entirely. With 5 samples over 5,001 volatile keys the true nearest-deadline key is chosen ~0.1% of the time: the policy evicts a key hours from expiring while one expiring in seconds survives, expires naturally moments later, and the eviction is wasted.

The #541 deadline-ordered expiry index already holds every hot volatile key sorted by (expires_at_ms, key), so the exact answer is the index head:

• Database::peek_nearest_expiry() — first index pair regardless of due-ness, O(log n); None when no hot key is volatile. Cold-spilled keys are not indexed, matching the old sampler which also only saw hot entries.
• find_victim_volatile_ttl(db) drops the samples parameter and returns the index head's key. maxmemory-samples still governs LRU/LFU/random, which remain sampling-based.

Red/green

New test volatile_ttl_evicts_globally_nearest_expiry buries one now+60s key among 5,000 keys expiring in an hour — red under the sampling picker (probabilistically guaranteed miss), green on the index head.

Validation

• Eviction module 42/42; full host lib suite 4688/4688
• clippy -D warnings clean, fmt clean, host audits green
• Rebased onto main post-fix(storage): #541 review follow-ups — GETEX overflow guard, stale-pair discipline, latch-at-promotion #550 with an identical tree hash (content-preserving)
• ci-local VM legs + full dispatch matrix to run before merge (first ci-local attempt died to a VM outage mid-compile, not a test failure; re-run pending)

Closes #551

Summary by CodeRabbit

• Bug Fixes

  • Improved volatile-ttl eviction to consistently remove the volatile key with the nearest expiration time.
  • Prevented distant-expiration keys from being selected due to random sampling.
  • Preserved maxmemory-samples behavior for LRU, LFU, and random eviction policies.
  • Added coverage confirming accurate selection across large sets of volatile keys.

• Documentation

  • Updated the changelog to describe the improved eviction behavior.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7b9940a and b45c86f.

📒 Files selected for processing (3)

• CHANGELOG.md
• src/storage/db/mod.rs
• src/storage/eviction.rs

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📝 Walkthrough

Walkthrough

volatile-ttl eviction now reads the expiry-index head to select the globally nearest-expiring volatile key. Database exposes this lookup, sampling is removed from this policy, and a regression test covers 5,000 far-future keys.

Changes

Volatile-TTL eviction

Layer / File(s)	Summary
Nearest expiry lookup src/storage/db/mod.rs	Database::peek_nearest_expiry returns the earliest expiry-index entry, including deadlines that have not elapsed.
Exact eviction integration and validation src/storage/eviction.rs, CHANGELOG.md	volatile-ttl uses the exact expiry lookup instead of sampling. The regression test verifies selection among 5,000 far-future keys. The changelog records the new behavior and sampling rules for other policies.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to b45c8

This PR changes volatile-TTL eviction to select the nearest-expiry key directly and reports passing targeted and full-suite validation. No actionable merge-blocking risk remains beyond normal checks and review.

Possibly related PRs

• pilotspace/moon#549: Introduced the expiry index used by exact volatile-ttl victim selection.

Suggested reviewers: pilotspacex-byte

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the exact nearest-expiry selection change for volatile-ttl eviction.
Description check	✅ Passed	The description clearly explains the change, design, testing, validation, and policy scope, although it does not mirror every template heading.
Linked Issues check	✅ Passed	The changes satisfy issue #551 by using the expiry index for volatile-ttl while preserving sampling for volatile-random.
Out of Scope Changes check	✅ Passed	The changelog, database accessor, eviction update, and regression test directly support the linked issue objectives.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/551-volatile-ttl-exact

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