On Tue, 2 May 2023, 19:43 Julien Rouhaud, <rjuju...@gmail.com> wrote:
> 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). > if what you meant is that the slot may have to send a record generated by an older major version, then unless I'm missing something the same restriction could be added to such a feature as what's being discussed in this thread for the logical replication slots. so only a final shutdown checkpoint record would be present after the flushed WAL position. it may be possible to work around that, if there weren't all the other problems I mentioned. >
