On Tue, 2023-04-18 at 10:20 +0200, Robert Sjöblom wrote: > At the moment where we would switch to pg15 being the primary/lead we > will stop writes to pg10; at that point we will validate that we are > fully in sync, tear down pg10 and send writes to pg15. Our question is > how we can validate our sync status. Given that there have been no DDL > changes on pg10 (new tables, for instance), would the following check be > sufficient? > > Compare byte diff between WAL LSNs: > SELECT abs(pg_wal_lsn_diff(write_lsn, flush_lsn)) AS lag FROM > pg_stat_replication; > > If the byte diff is 0, I would assume that we're fully in sync. Is this > understanding correct? I would compare the last replayed log sequence number with the current position on the primary: SELECT abs(pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn)) AS lag FROM pg_stat_replication; If that is 0, you are good. Yours, Laurenz Albe
