Horton

Re: New strategies for freezing, advancing relfrozenxid early

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Peter Geoghegan <pg@bowt.ie>, Jeff Davis <pgsql@j-davis.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, John Naylor <john.naylor@enterprisedb.com>, Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-01-26T14:20:57Z
Lists: pgsql-hackers 
On Wed, Jan 25, 2023 at 10:56 PM Andres Freund <andres@anarazel.de> wrote:
> but that's only true because page level freezing neutered
> vacuum_freeze_min_age. Compared to <16, it's a *huge* change.

Do you think that page-level freezing
(1de58df4fec7325d91f5a8345757314be7ac05da) was improvidently
committed?

I have always been a bit skeptical of vacuum_freeze_min_age as a
mechanism. It's certainly true that it is a waste of energy to freeze
tuples that will soon be removed anyway, but on the other hand,
repeatedly dirtying the same page for various different freezing and
visibility related reasons *really sucks*, and even repeatedly reading
the page because we kept deciding not to do anything yet isn't great.
It seems possible that the page-level freezing mechanism could help
with that quite a bit, and I think that the heuristic that patch
proposes is basically reasonable: if there's at least one tuple on the
page that is old enough to justify freezing, it doesn't seem like a
bad bet to freeze all the others that can be frozen at the same time,
at least if it means that we can mark the page all-visible or
all-frozen. If it doesn't, then I'm not so sure; maybe we're best off
deferring as much work as possible to a time when we *can* mark the
page all-visible or all-frozen.

In short, I think that neutering vacuum_freeze_min_age at least to
some degree might be a good thing, but that's not to say that I'm
altogether confident in that patch, either.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

  1. Revert "Add eager and lazy freezing strategies to VACUUM."

  2. Add eager and lazy freezing strategies to VACUUM.

  3. Refine the definition of page-level freezing.

  4. Avoid special XID snapshotConflictHorizon values.

  5. Add page-level freezing to VACUUM.

  6. Remove overzealous MultiXact freeze assertion.

  7. Refactor how VACUUM passes around its XID cutoffs.

  8. Deduplicate freeze plans in freeze WAL records.

  9. Set relfrozenxid to oldest extant XID seen by VACUUM.

  10. Only skip pages marked as clean in the visibility map, if the last 32

  11. Add vacuum_freeze_table_age GUC option, to control when VACUUM should