Re: 24.1.5.1. Multixacts And Wraparound

On Tue, Jun 22, 2021 at 08:32:18AM +0200, Laurenz Albe wrote:
> > 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 ...

Yes, very good. There were three problems with the original paragraph:

* Had duplicate words
* Had awkward phrasing
* Used undefined terms

Your version fixes all three of those. Patch attached.

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

I can confirm the 2GB. do_vacuum() calls
MultiXactMemberFreezeThreshold() to set its
effective_multixact_freeze_max_age. MultiXactMemberFreezeThreshold()
compares the number of members to 2^32 (0xFFFFFFFF) / 2 or 2GB:

#define MaxMultiXactOffset ((MultiXactOffset) 0xFFFFFFFF)
#define MULTIXACT_MEMBER_SAFE_THRESHOLD (MaxMultiXactOffset / 2)

/* If member space utilization is low, no special action is required. */
if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
return autovacuum_multixact_freeze_max_age;

If that test fails, autovacuum_multixact_freeze_max_age is set much more
aggressively.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

If only the physical world exists, free will is an illusion.