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Incidents/2021-10-29 graphite

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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)

Links to relevant documentation

Actionables

Scorecard

Incident Engagement™ ScoreCard Score
People Were the people responding to this incident sufficiently different than the previous N incidents? (0/5pt) 5
Were the people who responded prepared enough to respond effectively (0/5pt) 5
Did fewer than 5 people get paged (0/5pt)? 5
Were pages routed to the correct sub-team(s)? 1
Were pages routed to online (working hours) engineers (0/5pt)? (score 0 if people were paged after-hours) 5
Process Was the incident status section actively updated during the incident? (0/1pt) 0
If this was a major outage noticed by the community, was the public status page updated? If the issue was internal, was the rest of the organization updated with relevant incident statuses? (0/1pt) 1
Is there a phabricator task for the incident? (0/1pt) 1
Are the documented action items assigned? (0/1pt) 0.25
Is this a repeat of an earlier incident (-1 per prev occurrence) 0
Is there an open task that would prevent this incident / make mitigation easier if implemented? (0/-1p per task) -1
Tooling Did the people responding have trouble communicating effectively during the incident due to the existing or lack of tooling? (0/5pt) 5
Did existing monitoring notify the initial responders? (1pt) 0
Were all engineering tools required available and in service? (0/5pt) 2
Was there a runbook for all known issues present? (0/5pt) 5
Total Score 34.25