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From: Mok <[email protected]>
To: David Rowley <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: New vacuum config to avoid anti wraparound vacuums
Date: Fri, 24 Apr 2026 06:15:48 +0000
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On Thursday, April 23rd, 2026 at 3:10 PM, David Rowley <[email protected]> wrote:

> On Fri, 24 Apr 2026 at 01:04, Mok <[email protected]> wrote:
> >
> > On Thursday, April 23rd, 2026 at 4:44 AM, David Rowley <[email protected]> wrote:
> >
> > > On Thu, 23 Apr 2026 at 08:19, Mok <[email protected]> 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 what this change is trying to avoid.
> 
> 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.
> 
> There's some documentation that might be worthwhile reading in [1].
> 
> David
> 
> [1] https://www.postgresql.org/docs/18/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND
> 


On large tables they can be quite inconvenient so avoiding them is preferable. My example of 0.1 is to test the patch if you tried it. The range for this setting is 0.1 -> 1 with the latter effectively rendering the setting moot.

Gurmokh

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