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

On Wed, Feb 5, 2025 at 2:23 AM Sami Imseih <samimseih@gmail.com> wrote:
>
> I confirmed the 20GB value as is described here [1].
> 8k page can hold 409 member groups and each
> member group can hold 4 members, thus
> (2^32/(409 *4))*8192 = 20GB.
>
> I also fixed whitespace issues in v3.

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.

On Fri, Feb 21, 2025 at 9:14 PM Sami Imseih <samimseih@gmail.com> wrote:
>
> > Maybe we could also add a comment in multixact.c to update the doc accordingly if
> > the computation changes? (I think that will be easy to miss).
>
> Thanks for the comments!
>
> I rather we not touch the .c file for this update. It's unlikely the actual
> computation will change.

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.

-- 
John Naylor
Amazon Web Services