Re: POC: make mxidoff 64 bits

On 07/09/2024 07:36, Maxim Orlov wrote:
> Here is v3.

MultiXactMemberFreezeThreshold looks quite bogus now. Now that
MaxMultiXactOffset==2^64-1, you cannot get anywhere near the
MULTIXACT_MEMBER_SAFE_THRESHOLD and MULTIXACT_MEMBER_DANGER_THRESHOLD
values anymore. Can we just get rid of MultiXactMemberFreezeThreshold? I
guess it would still be useful to trigger autovacuum if multixacts
members grows large though, to release the disk space, even if you can't
run out of members as such anymore. What should the logic for that look
like?

I'd love to see some tests for the pg_upgrade code. Something like a
little perl script to generate test clusters with different wraparound
scenarios etc. using the old version, and a TAP test to run pg_upgrade
on them and verify that queries on the upgraded cluster works correctly.
We don't have tests like that in the repository today, and I don't know
if we'd want to commit these permanently either, but it would be highly
useful now as a one-off thing, to show that the code works.

On upgrade, are there really no changes required to
pg_multixact/members? I imagined that the segment files would need to be
renamed around wraparound, so that if you previously had files like this:

pg_multixact/members/FFFE
pg_multixact/members/FFFF
pg_multixact/members/0000
pg_multixact/members/0001

after upgrade you would need to have:

pg_multixact/members/00000000FFFE
pg_multixact/members/00000000FFFF
pg_multixact/members/000000010000
pg_multixact/members/000000010001

Thanks for working on this!

--
Heikki Linnakangas
Neon (https://neon.tech)