Re: New vacuum config to avoid anti wraparound vacuums

HI Mok

On Fri, Apr 24, 2026 at 2:16 PM Mok <gurmokh(at)protonmail(dot)com> wrote:

>
>
> On Thursday, April 23rd, 2026 at 3:10 PM, David Rowley <
> dgrowleyml(at)gmail(dot)com> wrote:
>
> > On Fri, 24 Apr 2026 at 01:04, Mok <gurmokh(at)protonmail(dot)com> wrote:
> > >
> > > On Thursday, April 23rd, 2026 at 4:44 AM, David Rowley <
> dgrowleyml(at)gmail(dot)com> wrote:
> > >
> > > > On Thu, 23 Apr 2026 at 08:19, Mok <gurmokh(at)protonmail(dot)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
> 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.
> I don't know where you got that idea from. For example have a table with 1
> billion records, autovacuum_vacuum_scale_factor = 0.01 ,
> 50+1000000000 *0.01 = 10000050 ,you can reduce
> autovacuum_vacuum_max_threshold substantially lower than 10000050 ，
> vacthresh = (float4) vac_base_thresh + vac_scale_factor * reltuples;
> if (vac_max_thresh >= 0 && vacthresh > (float4) vac_max_thresh)
> vacthresh = (float4) vac_max_thresh;
>
> There's no fundamental difference between this and your parameter
>