I have several streaming replication pairs running under postgres 10, and I'm looking at upgrading them to postgres 13. Doing that on the active side is pretty quick and easy: just run pg_upgrade once for each of versions 11, 12, and 13, then start things up again. That's tested (on non-replicated instances) and seems to be working great. The standby side looks a little bit more interesting, so I wanted some feedback about whether my approach looks safe. It looks like the most correct approach would be to discard the standby systems and just do a new pg_basebackup, but quite a few of the pairs have a standby on AWS and limited upstream bandwidth, so the basebackup can take several days. I'd like to avoid that if I can. So, my thought is to just upgrade the active side as normal, and once that's done and the master is back in use, stop the standby's postgres, call "pg_start_backup(...)" on the master, rsync the changed files from the running master's data dir to the standby's 10.x data directory, call "pg_stop_backup()", migrate the recovery.conf into a postgresql.auto.conf, touch the standby.signal file, and then start the new postgres 13 on the standby. That seems to work, but I want to be sure I'm not just having good luck due to the relatively low activity on my test systems. I use named replication slots with my streaming replication, so I think that the rsync'd pg_wal directory should have everything I need (or maybe this isn't guaranteed?), and the pg_start/stop_backup will ensure that all the damage done by rsync'ing an active database can be repaired from the pg_wal files that rsync fetches. Are those assumptions right, or am I missing anything else? Is this idea workable at all? Thanks!
