Incident documentation/2021-10-25 s3 db recentchanges replica

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document status: in-review

Summary

The s3 replica (db1112.eqiad.wmnet) that handles recentchanges/watchlist/contributions queries went down, triggering an icinga alert for the host being down, and a few minutes later an alert for increased appserver latency on GET requests. Confusion over the role of db1112, as it's also the s3 sanitarium master, didn't appropriately recognize the severity. Only while investigating the latency alerts was it realized that the database server was down, leading it to be depooled and restarted via mgmt. Once the host came back, a page was sent out. The incident was resolved by pooling a different s3 replica in its place.

s3 replication to WMCS wikireplicas was broken until it was restarted at 2021-10-26 09:15. s3 is the default database section for smaller wikis, which currently accounts for 92% of wikis (905/981 wikis).

Impact:

  • For ~30min (from 18:25 until 19:06) average HTTP GET latency for mediawiki backends was higher than usual.
  • For ~12 hours, database replicas of many wikis were stale for Wikimedia Cloud Services such as Toolforge.

Actionables

  • T294490: db1112 being down did not trigger any alert that paged until the host was brought back up (we get paged for replication lag but not for host down, Marostegui said for DB hosts we should start paging on HOST down which we normally don't do. This would require a puppet change.)