Horton

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

Alex Friedman <alexf01@gmail.com>

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From: Alex Friedman <alexf01@gmail.com>
To: John Naylor <johncnaylorls@gmail.com>
Cc: Sami Imseih <samimseih@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-02-26T16:50:40Z
Lists: pgsql-hackers

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Hi John,

Thanks for reviewing.

> It seems at a minimum this one-line patch is sufficient for the correction:
> 
> -     storage occupied by multixacts members exceeds 2GB, aggressive vacuum
> +     storage occupied by multixacts members exceeds about 10GB,
> aggressive vacuum
> 
> Commit c552e171d16e removed the percentage as part of a judgment call
> on clarity, and I'm not sure that was wrong.
> 
> We could add the proposed language on "can grow up to about 20GB" at
> the end of this paragraph, which seems more natural -- first mention
> the amount that triggers aggressive vacuum, then the maximum size.

Yes, I believe this can work.


> I'm on the fence about putting a hint in the C file, but the
> computation has changed in the past, see commit b4d4ce1d50bbdf , so
> it's a reasonable idea.

That's a good find about the change. Taken together with Bertrand's comments, I've added two reminders to multixact.c to update the docs, one for the threshold and another for the multixact storage scheme. Please see if it makes sense.

v4 patch attached.


Best regards,

Alex Friedman

Commits

  1. Doc: correct aggressive vacuum threshold for multixact members storage

  2. docs: clarify new aggressive vacuum mode for multi-xacts

  3. Increase threshold for multixact member emergency autovac to 50%.