Re: 24.1.5.1. Multixacts And Wraparound

On Mon, 2021-06-21 at 22:50 -0400, Bruce Momjian wrote:
> On Fri, Jun 18, 2021 at 08:51:00AM +0200, Laurenz Albe wrote:
> > On Thu, 2021-06-17 at 18:31 +0000, PG Doc comments form wrote:
> > > In section "24.1.5.1. Multixacts And Wraparound" of
> > > https://www.postgresql.org/docs/current/routine-vacuuming.html we find the
> > > following sentence towards the end of the section:
> > >
> > > "...If the amount of used member storage space exceeds the amount 50% of the
> > > addressable storage space."
> > >
> > > I am having trouble parsing "...exceeds the amount 50% of the
> > > addressable..." part.
> >
> > I think that the second "the amount" in the sentence should be removed.
>
> I think it is worse than that. Here is the full paragraph:
>
> As a safety device, an aggressive vacuum scan will occur for any table
> whose multixact-age is greater than
> <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>. Aggressive
> vacuum scans will also occur progressively for all tables, starting with
> those that have the oldest multixact-age, if the amount of used member
> storage space exceeds the amount 50% of the addressable storage space.
> Both of these kinds of aggressive scans will occur even if autovacuum is
> nominally disabled.
>
> What does "the amount of used member storage space exceeds 50% of the
> addressable storage space" mean?

You are right. See MultiXactMemberFreezeThreshold for the whole story.

What about:

As a safety device, an aggressive vacuum scan will occur for any table
whose multixact-age (see <xref linkend="vacuum-for-multixact-wraparound"/>)
is greater than <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>.
Also, if the storage occupied by multixacts exceeds 2GB, aggressive vacuum
scans will occur more often for all tables, starting with those that have
the oldest multixact-age.
Both of these kinds ...

I hope I read the source right concerning the 2GB.

Yours,
Laurenz Albe