Re: Introduce XID age and inactive timeout based replication slot invalidation

Hi,

On Tue, Mar 12, 2024 at 05:51:43PM +0530, Amit Kapila wrote:
> On Tue, Mar 12, 2024 at 1:24 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > On Fri, Mar 08, 2024 at 10:42:20PM +0530, Bharath Rupireddy wrote:
> > > On Wed, Mar 6, 2024 at 4:49 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > > You might want to consider its interaction with sync slots on standby.
> > > > Say, there is no activity on slots in terms of processing the changes
> > > > for slots. Now, we won't perform sync of such slots on standby showing
> > > > them inactive as per your new criteria where as same slots could still
> > > > be valid on primary as the walsender is still active. This may be more
> > > > of a theoretical point as in running system there will probably be
> > > > some activity but I think this needs some thougths.
> > >
> > > I believe the xmin and catalog_xmin of the sync slots on the standby
> > > keep advancing depending on the slots on the primary, no? If yes, the
> > > XID age based invalidation shouldn't be a problem.
> > >
> > > I believe there are no walsenders started for the sync slots on the
> > > standbys, right? If yes, the inactive timeout based invalidation also
> > > shouldn't be a problem. Because, the inactive timeouts for a slot are
> > > tracked only for walsenders because they are the ones that typically
> > > hold replication slots for longer durations and for real replication
> > > use. We did a similar thing in a recent commit [1].
> > >
> > > Is my understanding right? Do you still see any problems with it?
> >
> > Would that make sense to "simply" discard/prevent those kind of invalidations
> > for "synced" slot on standby? I mean, do they make sense given the fact that
> > those slots are not usable until the standby is promoted?
> >
>
> AFAIR, we don't prevent similar invalidations due to
> 'max_slot_wal_keep_size' for sync slots,

Right, we'd invalidate them on the standby should the standby sync slot restart_lsn
exceeds the limit.

> so why to prevent it for
> these new parameters? This will unnecessarily create inconsistency in
> the invalidation behavior.

Yeah, but I think wal removal has a direct impact on the slot usuability which
is probably not the case with the new XID and Timeout ones. That's why I thought
about handling them differently (but I'm also fine if that's not the case).

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com