From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Melanie Plageman <melanieplageman(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Andres Freund <andres(at)anarazel(dot)de> |
Subject: | Re: Emit fewer vacuum records by reaping removable tuples during pruning |
Date: | 2024-01-17 21:25:02 |
Message-ID: | CAH2-Wzkz=Wg+efGfUwPVoypUONkGKLi-tBmtoBwxFa3SjXpuZg@mail.gmail.com |
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Lists: | pgsql-hackers |
On Wed, Jan 17, 2024 at 3:58 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > Ah, I realize I was not clear. I am now talking about inconsistencies
> > in vacuuming the FSM itself. FreeSpaceMapVacuumRange(). Not updating
> > the freespace map during the course of vacuuming the heap relation.
>
> Fair enough, but I'm still not quite sure exactly what the question
> is. It looks to me like the current code, when there are indexes,
> vacuums the FSM after each round of index vacuuming. When there are no
> indexes, doing it after each round of index vacuuming would mean never
> doing it, so instead we vacuum the FSM every ~8GB. I assume what
> happened here is that somebody decided doing it after each round of
> index vacuuming was the "right thing," and then realized that was not
> going to work if no index vacuuming was happening, and so inserted the
> 8GB threshold to cover that case.
Note that VACUUM_FSM_EVERY_PAGES is applied against the number of
rel_pages "processed" so far -- *including* any pages that were
skipped using the visibility map. It would make a bit more sense if it
was applied against scanned_pages instead (just like
FAILSAFE_EVERY_PAGES has been since commit 07eef53955). In other
words, VACUUM_FSM_EVERY_PAGES is applied against a thing that has only
a very loose relationship with physical work performed/time elapsed.
I tend to suspect that VACUUM_FSM_EVERY_PAGES is fundamentally the
wrong idea. If it's such a good idea then why not apply it all the
time? That is, why not apply it independently of whether nindexes==0
in the current VACUUM operation? (You know, just like with
FAILSAFE_EVERY_PAGES.)
--
Peter Geoghegan
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