Re: Re: BUG #12990: Missing pg_multixact/members files (appears to have wrapped, then truncated)

On Fri, May 8, 2015 at 9:55 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
> Thomas Munro wrote:
>> I think I see why, and I think it's a bug: if you vacuum freeze all
>> your databases, MultiXactState->oldestMultiXactId finishes up equal to
>> MultiXactState->nextMXact.
>
> Uhm, that's rather silly.
>
>> But that's not actually a multixact that exists yet, so when when
>> DetermineSafeOldestOffset calls find_multixact_start, it reads a
>> garbage offset (all zeros in practice since pages start out zeroed)
>> and produces a garbage value for offsetStopLimit which might
>> incorrectly stop you from creating any more multixacts even though
>> member space is entirely empty (but it depends on where your
>> nextOffset happens to be at the time).
>
> Right.
>
>> I think the fix is something like "if nextMXact == oldestMultiXactId,
>> then there are no active multixacts, so the offsetStopLimit should be
>> set to nextOffset - (a segment's worth)".
>
> Makes sense.

Here's a patch that attempts to implement this.

--
Robert Haas
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