[tommyzli]

How would that have worked with multiple replicas cascading from the new primary? Streaming replication doesn't work across versions, so would we have had to build out a tree of new instances, then pg_upgrade them all at the same time?

[craigkerstiens]

I believe it is possible to repoint the replicas with re-wind and then repoint to a new timeline. This is something we looked at at Heroku a long time back. It wasn't trivial to fix, but Heroku eventually improved some of this. This post drills into some of that - https://blog.keikooda.net/2017/10/18/battle-with-a-phantom-w...

[cconstantine]

I've done a few postgres upgrades, the first using pg_upgrade, and the last doing effectively what you did (it was even 9.x -> 12).

My experience was that it needs to rewrite all the data for some tables/indexes under some circumstances, and the db won't be available while that happens. So, unless your db can be down for the time it takes to rewrite all that data it isn't really an option. After having done the upgrade through streaming logical replication I'm not sure I would try pg_upgrade again.

I did the pg_upgrade style update a long time ago, so most details are fuzzy, but I remember setting up a string of replicas something like:

primary -> [read_replica, backups_replica]

read_replica -> [upgrade_replica]

upgrade_replica -> [read_replica_upgraded, backups_replica_upgraded]

This allowed us to do multiple practice rounds without putting any unnecessary load on the primary. I think we needed to re-initiate replication off 'upgrade_replica' after the upgrade, but we did the live update during low-load so the extra read load wasn't an issue.

[ants_a]

In pg_upgrade documentation there is documented a way to use rsync to quickly replicate the upgraded contents of the new primary to replicas. So you would first move to upgraded base VMs running the old version streaming from old primary, which can be done one host at a time if need be.

The new cluster can then be pg_upgraded and rsynced all at once.

[paulryanrogers]

Good questions. As logical replication matures it may someday be possible to replicate among versions.

[skunkworker]

Logical replication would be nice, but at the moment it's far too slow to replicate for heavy day-to-day use, other than doing a pg upgrade between primaries and replicas.

[sharadov]

I've replicated across different PG versions, there is an extension called mimeo, which is fantastic for logical replication https://pgxn.org/dist/mimeo/1.5.1/doc/howto_mimeo.html