GitLab database decomposition: Final migration and results

What this post added

This post details the final migration process for decomposing the GitLab backend database, specifically focusing on the separation of CI-related data. It contrasts a planned zero-downtime approach with the chosen two-hour downtime strategy, highlighting the advantages of a simpler rollback mechanism and the necessity of extensive rehearsals. The post presents the positive results of the migration, including decreased primary database CPU utilization, significant storage savings, reduced dead tuples and vacuuming saturation, and a substantial reduction in average query duration for Sidekiq PGBouncer query pools.