Message-ID: <ad54b2b4c7c84b35cfa20066f4c2c2f163edca2f.camel@cybertec.at>
Subject: Re: 24.1.5.1. Multixacts And Wraparound
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Bruce Momjian <bruce@momjian.us>
Cc: eric.mutta@gmail.com, pgsql-docs@lists.postgresql.org
Date: Tue, 22 Jun 2021 08:32:18 +0200
In-Reply-To: <20210622025023.GH22121@momjian.us>
References: <162395467510.686.11947486273299446208@wrigleys.postgresql.org>
	 <8b8fed1d71a692d9cd72116e42ddde776a12be7e.camel@cybertec.at>
	 <20210622025023.GH22121@momjian.us>
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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