Re: [PoC] pg_upgrade: allow to upgrade publisher node

Hi,

On Tue, May 02, 2023 at 12:55:18PM +0200, Alvaro Herrera wrote:
> On 2023-Apr-07, Julien Rouhaud wrote:
>
> > That being said, I have a hard time believing that we could actually preserve
> > physical replication slots.  I don't think that pg_upgrade final state is fully
> > reproducible:  not all object oids are preserved, and the various pg_restore
> > are run in parallel so you're very likely to end up with small physical
> > differences that would be incompatible with physical replication.  Even if we
> > could make it totally reproducible, it would probably be at the cost of making
> > pg_upgrade orders of magnitude slower.  And since many people are already
> > complaining that it's too slow, that doesn't seem like something we would want.
>
> A point on preserving physical replication slots: because we change WAL
> format from one major version to the next (adding new messages or
> changing format for other messages), we can't currently rely on physical
> slots working across different major versions.

I don't think anyone suggested to do physical replication over different major
versions.  My understanding was that it would be used to pg_upgrade a
"physical cluster" (e.g. a primary and physical standby server) at the same
time, and then simply starting them up again would lead to a working physical
replication on the new version.

I guess one could try to keep using the slots for other needs (PITR backup with
pg_receivewal or something similar), and then you would indeed have to be aware
that you won't be able to do anything with the new WAL records until you do a
fresh base backup, but that's a problem that you can already face after a
normal pg_upgrade (although in most cases it's probably quite obvious for now
as the timeline isn't preserved).