Re: persist logical slots to disk during shutdown checkpoint

On Tue, Aug 22, 2023 at 9:48 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > Another idea is to record the confirm_flush_lsn at the time of
> > persisting the slot. We can use it in two different ways 1. to mark a
> > slot dirty and persist if the last confirm_flush_lsn when slot was
> > persisted was too far from the current confirm_flush_lsn of the slot.
> > 2. at shutdown checkpoint, persist all the slots which have these two
> > confirm_flush_lsns different.
> >
>
> I think using it in the second (2) way sounds advantageous as compared
> to storing another dirty flag because this requires us to update
> last_persisted_confirm_flush_lsn only while writing the slot info.
> OTOH, having a flag dirty_for_shutdown_checkpoint will require us to
> update it each time we update confirm_flush_lsn under spinlock at
> multiple places. But, I don't see the need of doing what you proposed
> in (1) as the use case for it is very minor, basically this may
> sometimes help us to avoid decoding after crash recovery.

Once we have last_persisted_confirm_flush_lsn, (1) is just an
optimization on top of that. With that we take the opportunity to
persist confirmed_flush_lsn which is much farther than the current
persisted value and thus improving chances of updating restart_lsn and
catalog_xmin faster after a WAL sender restart. We need to keep that
in mind when implementing (2). The problem is if we don't implement
(1) right now, we might just forget to do that small incremental
change in future. My preference is 1. Do both (1) and (2) together 2.
Do (2) first and then (1) as a separate commit. 3. Just implement (2)
if we don't have time at all for first two options.

--
Best Wishes,
Ashutosh Bapat