Enable Gitaly Transactions on 2 regular Gitaly nodes

Production Change

Change Summary

Enable Gitaly's transaction support on 2 normal (non archive) Gitaly nodes.

This has been running on cny since 2025-04-14 and Gitaly has remained above the appdex.

Gitaly has recently gained support for ACID transactions. In summary:

• Each RPC runs in a transaction.
• Transactions are snapshot isolated from each other.
• Optimistic concurrency control is used with conflict detection on transaction commit.
• All writes go are write-ahead logged before applying them.

The goal of transactions is to improve Gitaly's reliability. Writes are atomic and durable. Gitaly can recover interrupted writes from the write-ahead log. Concurrency related issues are largely eliminated as transactions are snapshot isolated. Transactions can execute as if they were the only one executing.

As a storage level change, this affects every read and write operation in Gitaly. Unexpected issues can lead to errors, data corruption/loss, or performance problems.

The main uncertainty with the change is related to performance.

Change Details

1. Services Impacted - ServiceGitaly
2. Change Technician - @ahmadsherif
3. Change Reviewer -
4. Time tracking - 10 minutes
5. Downtime Component - None

Detailed steps for the change

Change Steps - steps to take to execute the change

Estimated Time to Complete (mins) - 10 minutes

• Set label changein-progress /label ~change::in-progress
• Merge https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/5944
• Merge https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/10935
• Run sudo google_metadata_script_runner startup on the targeted nodes
• 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) - 10 minutes

• Revert https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/10935
• Run sudo google_metadata_script_runner startup on the targeted nodes
• Set label changeaborted /label ~change::aborted

Monitoring

Key metrics to observe

The main metric to monitor is Gitaly's Apdex. Transcations affect all storage access in Gitaly, and this metric best covers the overall health of Gitaly.

• Metric: gitaly Service Apdex
  • Location: https://dashboards.gitlab.net/d/gitaly-main/gitaly3a-overview?orgId=1
  • What changes to this metric should prompt a rollback: If the Apdex decreases and shows an increase in errors.

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 change window has been agreed with Release Managers in advance of the change. If the change is planned for APAC hours, this issue has an agreed pre-change approval.
  • 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 prior to any C1, C2, or blocks deployments 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.