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Incidents/2021-10-29 graphite

From Wikitech

document status: in-review

Summary and Metadata

The metadata is aimed at helping provide a quick snapshot of context around what happened during the incident.

Incident ID 2021-10-29 graphite UTC Start Timestamp: 2021-10-21 10:24:00
Incident Task T294355 UTC End Timestamp (no end- data lost forever. Mitigated on: 2021-12-13, 09:12:00)
People Paged 0 Responder Count 3: Lucas_Werkmeister_WMDE, fgiunchedi, Jcrespo
Coordinator(s) fgiunchedi Relevant Metrics / SLO(s) affected Data loss - No SLO defined
Summary: 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

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)

Actionables

Scorecard

Question Score Notes
People Were the people responding to this incident sufficiently different than the previous five incidents? (score 1 for yes, 0 for no) 1
Were the people who responded prepared enough to respond effectively (score 1 for yes, 0 for no) 1
Were more than 5 people paged? (score 0 for yes, 1 for no) 1
Were pages routed to the correct sub-team(s)? (score 1 for yes, 0 for no) 1
Were pages routed to online (business hours) engineers? (score 1 for yes,  0 if people were paged after business hours) 1
Process Was the incident status section actively updated during the incident? (score 1 for yes, 0 for no) 0
Was the public status page updated? (score 1 for yes, 0 for no) 1
Is there a phabricator task for the incident? (score 1 for yes, 0 for no) 1
Are the documented action items assigned?  (score 1 for yes, 0 for no) 0
Is this a repeat of an earlier incident (score 0 for yes, 1 for no) 0
Tooling Was there, before the incident occurred, open tasks that would prevent this incident / make mitigation easier if implemented? (score 0 for yes, 1 for no) 0
Were the people responding able to communicate effectively during the incident with the existing tooling? (score 1 for yes, 0 or no) 1
Did existing monitoring notify the initial responders? (score 1 for yes, 0 for no) 0
Were all engineering tools required available and in service? (score 1 for yes, 0 for no) 0
Was there a runbook for all known issues present? (score 1 for yes, 0 for no) 1
Total score 9
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