2020-05-04: possible data loss via external diffs migration

Summary

More detail in #1851 (comment 335804164) - we seem to be migrating external MR diffs incorrectly in some cases. Of 11 million migrated diffs, half a million claim to be on disk, when they should be in object storage.

It's not necessarily the case that any data is lost - we need to find out. If it's actually on disk, it can be moved to object storage. If it's lost on disk, it can often be recovered from the repository.

I think we need to pause the migration while we work out what's happening and determine next steps.

More information will be added as we investigate the issue.

Timeline

All times UTC.

YYYY-MM-DD

• 00:00 - ...

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Incident Review

Summary

1. Service(s) affected :
2. Team attribution :
3. Minutes downtime or degradation :

Metrics

Customer Impact

1. Who was impacted by this incident? (i.e. external customers, internal customers)
2. What was the customer experience during the incident? (i.e. preventing them from doing X, incorrect display of Y, ...)
3. How many customers were affected?
4. If a precise customer impact number is unknown, what is the estimated potential impact?

Incident Response Analysis

1. How was the event detected?
2. How could detection time be improved?
3. How did we reach the point where we knew how to mitigate the impact?
4. How could time to mitigation be improved?

Post Incident Analysis

1. How was the root cause diagnosed?
2. How could time to diagnosis be improved?
3. Do we have an existing backlog item that would've prevented or greatly reduced the impact of this incident?
4. Was this incident triggered by a change (deployment of code or change to infrastructure. If yes, have you linked the issue which represents the change?)?

Timeline

• YYYY-MM-DD XX:YY UTC: action X taken
• YYYY-MM-DD XX:YY UTC: action Y taken

5 Whys

Lessons Learned

Corrective Actions

Guidelines

• Blameless RCA Guideline