Only cache git clones that occur more than once

Production Change

Change Summary

As part of scalability#2222 we will set GITALY_PACK_OBJECTS_CACHE_MIN_OCCURRENCES=1 on our Gitaly fleet. This environment variable was introduced in gitlab-org/gitaly!5501 (merged). The intended effect is that unique Git clones no longer get cached by the pack-objects cache, reducing IO and storage churn.

This is essentially a feature flag change, only it uses an environment variable because we have no suitable feature flags for this in Gitaly.

Change Details

1. Services Impacted - ServiceGit ServiceGitaly
2. Change Technician - @jacobvosmaer-gitlab
3. Change Reviewer - @igorwwwwwwwwwwwwwwwwwwww
4. Time tracking - 105m
5. Downtime Component - N/A

Detailed steps for the change

Change Steps - steps to take to execute the change

Estimated Time to Complete (mins) - 75m

• Set label changein-progress /label ~change::in-progress
• Merge https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/3087 for gstg
• Merge https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/3088 for gprd-cny
• Run sudo chef-client on file-cny-01
• Observe impact on file-cny-01
• Merge https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/3089 for gprd
• Set label changecomplete /label ~change::complete

Rollback

Rollback steps - steps to be taken in the event of a need to rollback this change

Estimated Time to Complete (mins) - 30m

• Revert https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/3089 (gprd)
• Revert https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/3088 (gprd-cny)
• Revert https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/3087 (gstg)
• Set label changeaborted /label ~change::aborted

Monitoring

Key metrics to observe

• Metric: Apdex/Errors/RPS
  • Location: Gitaly Host Detail
  • What changes to this metric should prompt a rollback: Should not change
• Metric: Pack-objects hit rate weighed by bytes
  • Location: thanos
  • What changes to this metric should prompt a rollback: Should not drop below 50%
• Metric: Pack-objects cache disk usage
  • Location: thanos
  • What changes to this metric should prompt a rollback: Should not increase

Change Reviewer checklist

C4 C3 C2 C1:

• Check if the following applies:
  • The scheduled day and time of execution of the change is appropriate.
  • The change plan is technically accurate.
  • The change plan includes estimated timing values based on previous testing.
  • The change plan includes a viable rollback plan.
  • The specified metrics/monitoring dashboards provide sufficient visibility for the change.

C2 C1:

• Check if the following applies:
  • The complexity of the plan is appropriate for the corresponding risk of the change. (i.e. the plan contains clear details).
  • The change plan includes success measures for all steps/milestones during the execution.
  • The change adequately minimizes risk within the environment/service.
  • The performance implications of executing the change are well-understood and documented.
  • The specified metrics/monitoring dashboards provide sufficient visibility for the change.
    • If not, is it possible (or necessary) to make changes to observability platforms for added visibility?
  • The change has a primary and secondary SRE with knowledge of the details available during the change window.
  • The labels blocks deployments and/or blocks feature-flags are applied as necessary

Change Technician checklist

• Check if all items below are complete:
  • The change plan is technically accurate.
  • This Change Issue is linked to the appropriate Issue and/or Epic
  • Change has been tested in staging and results noted in a comment on this issue.
  • A dry-run has been conducted and results noted in a comment on this issue.
  • The change execution window respects the Production Change Lock periods.
  • For C1 and C2 change issues, the change event is added to the GitLab Production calendar.
  • For C1 and C2 change issues, the SRE on-call has been informed prior to change being rolled out. (In #production channel, mention @sre-oncall and this issue and await their acknowledgement.)
  • For C1 and C2 change issues, the SRE on-call provided approval with the eoc_approved label on the issue.
  • For C1 and C2 change issues, the Infrastructure Manager provided approval with the manager_approved label on the issue.
  • Release managers have been informed (If needed! Cases include DB change) prior to change being rolled out. (In #production channel, mention @release-managers and this issue and await their acknowledgment.)
  • There are currently no active incidents that are severity1 or severity2
  • If the change involves doing maintenance on a database host, an appropriate silence targeting the host(s) should be added for the duration of the change.