Re: Doc fix of aggressive vacuum threshold for multixact members storage

On Mon, Feb 3, 2025 at 11:29 AM Alex Friedman <alexf01(at)gmail(dot)com> wrote:
>
> Hi Sami,
>
> Thanks for the feedback.
>
> > 1/ Remove this as
> > "(50% of the maximum, which is about 20GB),"
> >
> > [1] tried to avoid explaining this level of detail, and I
> > agree with that.
>
> I feel it is critical for users to know what is the hard limit of
> multixact members. As PG doesn't (yet) expose how many multixact
> members are in use, the only way for users to know the distance to
> members wraparound is by monitoring the members directory space usage.
> So it seems to me that the 20 GB number is very important to have in
> the docs.

A few paragraphs up the docs, there is this mention:

". There is a separate storage area which holds the list of members
in each multixact, which also uses a 32-bit counter and which must
also be managed."

Maybe we can add more to this paragraph, such as:

"also be managed. This member can grow to 20GB"

And then in the proposed correction:

"
Also, if the storage occupied by multixacts members area exceeds 10GB
(50% of the maximum the members area can grow), aggressive vacuum
scans will occur more often for all tables
"

What do you think?

> > 2/ c/"about 10GB"/"10GB" the "about" does not seem necessary here.
>
> The threshold is actually ~10.015 GiB (due to the 12 bytes wasted per
> 8KB page), or ~10.75 GB, so to avoid confusion by users when
> aggressive autovacuum doesn't trigger exactly at 10GB, I believe we
> should either be exact, or say that we are not being exact. Being
> exact is difficult as it depends on the block size. And as I looked
> through the doc page in question, I noticed there are already several
> cases using the "about" wording, e.g. "about 50MB of pg_xact storage"
> and "about 2GB of pg_commit_ts storage", so here I went for
> consistency with the rest of the doc.

I really don't have a strong opinion about this, except it seemed unnecessary.
But if it helps clarify that it's not an exact 10GB, I am ok with keeping it.

Regards,

Sami