Incident documentation/2021-10-29 graphite

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document status: in-review

Summary

Impact: The backfill process for Graphite metrics silently failed during the Bullseye migration. A subset of metrics experienced loss for data points before October 11th 2021

The process of reimaging a Graphite host is as follows:

  1. reimage host
  2. let metrics flow for a few days to validate the host is working
  3. backfill the rest of the data (online, no downtime) from the other Graphite host following https://wikitech.wikimedia.org/wiki/Graphite#Merge_and_sync_metrics

During the Bullseye migration the backfill process failed (undetected) for a subset of metrics, leading to metric data loss once the Bullseye migration was complete (i.e. graphite2003 first and then graphite1004 were reimaged and put back in service)

Timeline

All times in UTC.

Detection

Some Grafana dashboards backed by Graphite showed partial data (starting Oct 11 or Oct 21) for a subset of metrics, as reported by Lucas Werkmeister in https://phabricator.wikimedia.org/T294355

Conclusions

The whisper-sync backfill process is not as reliable as previously thought, no visible errors were logged and/or detected.

What went well?

  • Only a subset of metric files experienced data loss

What went poorly?

  • The data loss was not detected by automated means or during spot-check validation
  • The data loss was only detected after both hosts had been reimaged, at which point lost data could no longer be recovered

Where did we get lucky?

  • Only a subset of metric files experienced data loss

How many people were involved in the remediation?

  • 1 SRE (Filippo Giunchedi)

Links to relevant documentation

Actionables