Postmortem: A Corrupted Loki 2.10 Log Store Caused 3 Days of Lost Debug Data
Check your Loki upgrade process for index_gateway misconfig and ensure backup jobs validate writes.
Patch the index_gateway flag in your Loki config, re‑enable s3:PutObject for backup service, add write‑success validation, and run a full backup validation test.
Summary
On 12 Oct 2024, a Loki 2.10 upgrade misconfigured the <code>index_gateway</code> flag, causing partial index writes under production‑scale load. The corrupted indexes triggered crash loops in ingester pods, propagating via Loki’s gossip protocol to all three regions and corrupting replicated copies. Meanwhile, a routine IAM role rotation on 5 Oct removed <code>s3:PutObject</code> from the backup service account, so incremental backups silently failed for seven days. The result was 4.2 TB of debug logs lost for 142 microservices, 217 failed deployments, and 892 high‑priority support tickets, dropping engineering velocity by 62%.
The remediation involved decommissioning the corrupted cluster, deploying a fresh Loki 2.10 instance with the corrected <code>index_gateway</code> setting, restoring IAM permissions, adding write‑success validation to backup jobs, and re‑configuring all services. Prevention measures now include production‑scale write‑load testing for upgrades, backup write‑validation, index checksum checks on startup, cross‑region backup replication, and a pre‑upgrade configuration check against release notes.
Key changes
- Loki 2.10 upgrade misconfigured <code>index_gateway</code> causing partial index writes
- IAM role rotation removed <code>s3:PutObject</code> leading to silent backup failures
- 4.2 TB of debug logs lost across 142 microservices
- Fresh Loki cluster deployed with corrected config and validated under load
- Backup jobs now validate write success and alert on failure
- Index file checksum validation added to ingester startup
- Cross‑region backup replication and monthly DR drills implemented
- Pre‑upgrade config check against release notes added