Postmortem: A Corrupted Loki 2.10 Log Store Caused 3 Days of Lost Debug Data

Overview

On October 12, 2024, at 09:15 UTC, our observability team detected anomalies in the Loki 2.10 log store cluster serving production debug data. By the time mitigation steps were fully effective, 3 full days of debug log data (October 12–14) had been corrupted and was unrecoverable, impacting incident response and root cause analysis for 14 concurrent production incidents.

Impact

The corrupted log store affected all production services that write debug-level logs to the Loki 2.10 cluster, including:

• 142 microservices across 3 cloud regions
• CI/CD pipeline debug logs for 217 failed deployments
• Customer support ticket debug traces for 892 high-priority tickets

Total unrecoverable data loss: ~4.2 TB of debug logs. No customer-facing services were disrupted, as debug logs are non-critical for production runtime, but engineering velocity dropped by 62% due to lack of debug context for incident resolution.

Timeline (All Times UTC)

• 2024-10-12 09:15: First alert fired for Loki ingester pod crash loops in us-east-1 region.
• 2024-10-12 09:30: On-call SRE identified 12 failed ingester pods, attempted to restart stateful sets with no success.
• 2024-10-12 10:45: Team discovered corrupted index files in the Loki object store (S3), traced to a failed rolling update of Loki 2.9 to 2.10 24 hours prior.
• 2024-10-12 12:20: Attempted to restore from the latest hourly backup, only to find backups had been failing silently for 7 days due to IAM permission rotation.
• 2024-10-12 14:00: Decision made to rebuild the Loki cluster from scratch, as index corruption had propagated to all 3 regions via replication.
• 2024-10-13 08:30: New Loki 2.10 cluster deployed, ingesting new logs successfully. Verification confirmed October 12–14 logs were unrecoverable.
• 2024-10-14 17:00: All 142 microservices reconfigured to point to the new cluster, full observability restored.

Root Cause

The incident was caused by a cascade of two unrelated failures:

1. Failed Loki 2.10 Upgrade: The rolling update from Loki 2.9 to 2.10 on 2024-10-11 included a misconfigured index_gateway flag that caused partial index writes to S3. The upgrade completed without errors in the CI environment, as the bug only manifested under production-scale write loads (120k logs/sec).
2. Silent Backup Failures: A routine IAM role rotation for the Loki backup service account on 2024-10-05 removed s3:PutObject permissions. Backup jobs ran without errors (the Loki backup tool does not validate write success for incremental backups), resulting in 7 days of invalid backups before the incident.

When ingester pods restarted on 2024-10-12, they attempted to read the corrupted index files, triggering crash loops that propagated to all nodes via the Loki gossip protocol, corrupting replicated index copies in eu-west-1 and ap-southeast-1.

Remediation

• Decommissioned the corrupted Loki 2.10 cluster and all associated S3 index buckets.
• Deployed a fresh Loki 2.10 cluster with the corrected index_gateway configuration, validated under production load in a staging environment first.
• Restored IAM permissions for the backup service account, added write success validation to the backup tool, and verified 24 hours of successful backups.
• Reconfigured all microservices to point to the new Loki endpoint, validated log ingestion for all 142 services.

Lessons Learned

What Went Well

• Alerting fired immediately when ingester pods crashed, reducing time to detection to 0 minutes (issue occurred during business hours).
• Cross-region replication of the Loki cluster prevented total data loss for pre-October 12 logs, as corruption only affected index files, not raw log chunks stored separately.
• Clear communication to engineering teams about the data loss minimized duplicate incident response efforts.

What Went Wrong

• Loki 2.10 upgrade was not tested under production-scale write loads in staging, allowing the index_gateway bug to go undetected.
• Backup job success criteria did not include validation of written backup files, leading to 7 days of silent failures.
• Index corruption propagated across regions via Loki's gossip protocol, as there were no safeguards to quarantine corrupted index files.

Where We Got Lucky

• Debug logs are non-critical for production runtime, so no customer impact occurred despite 3 days of data loss.
• Raw log chunks stored separately from indexes were intact, but reindexing 4.2 TB of data would take 14 days, so we accepted the loss to restore observability faster.

Prevention Measures

• All Loki upgrades must now be tested under production-scale write loads (minimum 100k logs/sec) in staging for 24 hours before production rollout.
• Backup jobs now validate write success by checking S3 object metadata after each backup, with alerts triggered on failure.
• Added index file checksum validation to Loki ingester startup, so corrupted indexes are quarantined instead of causing crash loops.
• Implemented cross-region backup replication, with monthly disaster recovery drills to validate backup integrity.
• Added a pre-upgrade check for Loki configuration flags against the official 2.10 release notes to catch misconfigurations.

All prevention measures were implemented by October 21, 2024, and validated during a subsequent Loki patch update on October 28, 2024.