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Date: Tue, 22 Oct 2024 12:43:34 +0300
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Subject: Re: POC: make mxidoff 64 bits
To: Maxim Orlov <orlovmg@gmail.com>, Alexander Korotkov <aekorotkov@gmail.com>
Cc: wenhui qiu <qiuwenhuifx@gmail.com>,
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From: Heikki Linnakangas <hlinnaka@iki.fi>
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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)