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

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

--
With Regards,
Amit Kapila.