We are upgrading the operating system on our Postgres database clusters

This post details the upgrade of the operating system on GitLab.com's Postgres database clusters from Ubuntu 16.04 to Ubuntu 20.04. The upgrade required a maintenance window due to glibc changes affecting Postgres indexes. The approach involved setting up new Ubuntu 20.04 clusters with streaming replication, promoting them during downtime, and reindexing potentially corrupt indexes. Performance improvements were made to the `amcheck` script for faster index corruption detection and to the reindexing process itself. Specific strategies were outlined for reindexing B-Tree, GiST, Hash, and GIN indexes.