Date: Fri, 24 Apr 2026 06:15:48 +0000
To: David Rowley <dgrowleyml@gmail.com>
From: Mok <gurmokh@protonmail.com>
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: New vacuum config to avoid anti wraparound vacuums
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On Thursday, April 23rd, 2026 at 3:10 PM, David Rowley <dgrowleyml@gmail.co=
m> wrote:

> On Fri, 24 Apr 2026 at 01:04, Mok <gurmokh@protonmail.com> wrote:
> >
> > On Thursday, April 23rd, 2026 at 4:44 AM, David Rowley <dgrowleyml@gmai=
l.com> wrote:
> >
> > > On Thu, 23 Apr 2026 at 08:19, Mok <gurmokh@protonmail.com> wrote:
> > > > For example, set to 0.8 a 'standard' vacuum would be triggered when=
 the table reached 160million with a default 200million setting.
> > >
> > > If that's what you want, why wouldn't you set the
> > > autovacuum_freeze_max_age to 160million?
> >
> > Because that would trigger a 'to-prevent-wraparound' vacuum, which is w=
hat this change is trying to avoid.
>=20
> Yes, it would. Why do you want to prevent them? I believe a few people
> have been alarmed in the past about the "to prevent wraparound" text
> in pg_stat_activity or when they saw those words in the logs. The
> default 200 million autovacuum_freeze_max_age setting triggers an
> autovacuum when it's less than 10% of the way into exhausting the
> transaction space for the table. What you're proposing with an
> autovacuum_age_scale_factor of 0.1 sounds like it would result in an
> auto-vacuum when only 1% of the transaction ID space is consumed! I
> think you're under the false impression that these anti-wraparound
> vacuums are bad. They're not.
>=20
> There's some documentation that might be worthwhile reading in [1].
>=20
> David
>=20
> [1] https://www.postgresql.org/docs/18/routine-vacuuming.html#VACUUM-FOR-=
WRAPAROUND
>=20


On large tables they can be quite inconvenient so avoiding them is preferab=
le. My example of 0.1 is to test the patch if you tried it. The range for t=
his setting is 0.1 -> 1 with the latter effectively rendering the setting m=
oot.

Gurmokh