Re: pg_upgrade instructions involving "rsync --size-only" might lead to standby corruption?

On Thu, Jun 29, 2023 at 1:50 PM Nikolay Samokhvalov <nik@postgres.ai> wrote:
> Does this make sense or I'm missing something and the current docs describe a reliable process? (As I said, we have
deviatedfrom the process, to involve logical replication, so I'm not 100% sure I'm right suspecting the original
procedurein having standby corruption risks.) 

I'm very suspicious about this section of the documentation. It
doesn't explain why --size-only is used or why --no-inc-recursive is
used.

> > 9. Prepare for standby server upgrades
> > If you are upgrading standby servers using methods outlined in section Step 11, verify that the old standby servers
arecaught up by running pg_controldata against the old primary and standby clusters. Verify that the “Latest checkpoint
location”values match in all clusters. (There will be a mismatch if old standby servers were shut down before the old
primaryor if the old standby servers are still running.) Also, make sure wal_level is not set to minimal in the
postgresql.conffile on the new primary cluster. 
>
> – admitting that there might be mismatch. But if there is mismatch, rsync --size-only is not going to help
synchronizeproperly, right? 

I think the idea is that you shouldn't use the procedure in this case.
But honestly I don't think it's probably a good idea to use this
procedure at all. It's not clear enough under what circumstances, if
any, it's safe to use, and there's not really any way to know if
you've done it correctly. You couldn't pay me enough to recommend this
procedure to anyone.

--
Robert Haas
EDB: http://www.enterprisedb.com