What causes "Compact COLD log failed! :(" in microsoft/FASTER?

Retry the compaction/checkpoint — transient reader contention is the most common cause.

Compact COLD log failed! :(

Error message

Compact COLD log failed!  :(

What it means

The cold-log compaction step inside Checkpoint (CompactLog on the COLD store) returned false. The cold log was not compacted; like the hot variant this is logged and the checkpoint flow continues, but cold-log space was not reclaimed.

Solutions

Retry the compaction/checkpoint — transient reader contention is the most common cause.
Raise cold_log_compaction_config.num_threads.
Ensure until_address is a valid, flushed address within the cold log.
Verify no sessions are pinning old cold-log regions; stop idle sessions before compacting.

Example fix

// before
kv.CompactLog(until_address, 1);
// after
bool ok = kv.CompactLog(until_address, cold_cfg.num_threads);
if (!ok) { /* schedule retry */ }

Defensive patterns

Strategy: retry

Validate before calling

// before cold-log compaction
if (!cold_store.HasActiveSession()) cold_store.StartSession();
uint64_t until = cold_store.hlog.flushed_until_address;
if (until == 0) return;

Prevention

Stop idle sessions that pin old cold-log regions.
Use enough cold_log_compaction_config.num_threads.
Retry failed compaction cycles instead of aborting the checkpoint.
Monitor cold-log size trend to catch persistent compaction failure.

When it happens

Trigger: ShouldCompactHlog triggers on the cold store (cold_log_compaction_config thresholds exceeded) and CompactLog(cold_store, StoreType::COLD, ...) fails — invalid until_address, insufficient compaction threads, or an internal fold failure over cold-log segments.

Common situations: Auto-compaction of the cold log under sustained ingest; cold_log_compaction_config.num_threads too low; compacting a cold log with many immutable segments still referenced by active readers.

Understand the failure class

Background: Database query failed: Internal Server Error 500s wrapping SQL, Prisma, and connection failures — what to check first — this error's family across 16 libraries.

Related errors

AI-assisted analysis of microsoft/FASTER@321d872eab (2026-09-15). Data as JSON: /api/errors/83169bb3420ebe0b. Report an issue: GitHub.

Appendix: source

Thrown at cc/src/core/f2.h:945

    // Hot-Cold compaction (optional cold-log checkpoint)
    if (ShouldCompactHlog(hot_store, StoreType::HOT, hot_log_compaction_config, until_address)) {
      hot_store.StartSession();
      cold_store.StartSession();
      if (!CompactLog(hot_store, StoreType::HOT, until_address, true,
                      hot_log_compaction_config.num_threads, should_checkpoint_cold_store())) {
        log_error("Compact HOT log failed! :(");
      }
      cold_store.StopSession();
      hot_store.StopSession();
    }

    // Cold-Cold compaction (optional cold-log checkpoint)
    if (ShouldCompactHlog(cold_store, StoreType::COLD, cold_log_compaction_config, until_address)) {
      cold_store.StartSession();
      if (!CompactLog(cold_store, StoreType::COLD, until_address, true,
                      cold_log_compaction_config.num_threads, should_checkpoint_cold_store())) {
        log_error("Compact COLD log failed!  :(");
      }
      cold_store.StopSession();
    }

    wait = (checkpoint_.hot_store_status != StoreCheckpointStatus::REQUESTED &&
            !ShouldCompactHlog(hot_store, StoreType::HOT, hot_log_compaction_config, until_address) &&
            !ShouldCompactHlog(cold_store, StoreType::COLD, cold_log_compaction_config, until_address));
    if (wait) {
      compaction_scheduled_.store(false);
      std::this_thread::sleep_for(check_interval);
    }

  } while(background_worker_active_.load());

  compaction_scheduled_.store(false);
}

template<class K, class V, class D, class HHI, class CHI>

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