Horton

Re: New strategies for freezing, advancing relfrozenxid early

Peter Geoghegan <pg@bowt.ie>

View thread View on postgr.es
From: Peter Geoghegan <pg@bowt.ie>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>, John Naylor <john.naylor@enterprisedb.com>, Andres Freund <andres@anarazel.de>, Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-01-03T20:30:02Z
Lists: pgsql-hackers

Attachments

On Mon, Jan 2, 2023 at 6:26 PM Jeff Davis <pgsql@j-davis.com> wrote:
> On Mon, 2023-01-02 at 11:45 -0800, Peter Geoghegan wrote:
> > What do you think of the wording adjustments in the attached patch?
> > It's based on your suggested wording.
>
> Great, thank you.

Pushed that today.

Attached is v14.

v14 simplifies the handling of setting the visibility map at the end
of the blkno-wise loop in lazy_scan_heap(). And,
visibilitymap_snap_next() doesn't tell its caller (lazy_scan_heap)
anything about the visibility status of each returned block -- we no
longer need a all_visible_according_to_vm local variable to help with
setting the visibility map.

This new approach to setting the VM is related to hardening that I
plan on adding, which makes the visibility map robust against certain
race conditions that can lead to setting a page all-frozen but not
all-visible. I go into that here:

https://postgr.es/m/CAH2-WznuNGSzF8v6OsgjaC5aYsb3cZ6HW6MLm30X0d65cmSH6A@mail.gmail.com

(It's the second patch -- the first patch already became yesterday's
commit 6daeeb1f.)

In general I don't think that we should be using
all_visible_according_to_vm for anything, especially not anything
critical -- it is just information about how the page used to be in
the past, after all. This will be more of a problem with visibility
map snapshots, since all_visible_according_to_vm could be information
that is hours old by the time it's actually used by lazy_scan_heap().
But it is an existing issue.

BTW, it would be helpful if I could get a +1 to the visibility map
patch posted on that other thread. It's practically a bug fix -- the
VM shouldn't be able to show contradictory information about any given
heap page (i.e. "page is all-frozen but not all-visible"), no matter
what. Just on general principle.

--
Peter Geoghegan

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