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

On Thu, May 7, 2015 at 2:00 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
> Robert Haas wrote:
>> It seems to me that the most obvious places where
>> DetermineSafeOldestOffset() should be called are (1) at startup or
>> after recovery, to initialize the value; and (2) each time we truncate
>> the SLRU, to update the value. Other than that, this doesn't change.
>> The startup calls are there, in apparently reasonable places, but it's
>> not obvious to me how this gets called in the TruncateMultiXact path.
>> Instead it seems to get set via the SetMultiXactIdLimit path. Maybe
>> that's OK, but it would seem to imply that we're OK with overwriting
>> old members information if that information was slated for truncation
>> at the next checkpoint anyway, which seems scary.
>
> I considered this question in the previous commit: is it okay to
> overwrite a file that is no longer used (per the limits set by vacuum)
> but not yet removed (by checkpoint)? It seems to me that there is no
> data-loss issue with doing that -- which is why the advance-oldest code
> is called during vacuum and not during checkpoint.

I think the main question is whether truncation will be smart enough
to zap the non-overwritten part of the old stuff but not the part that
did get overwritten.

--
Robert Haas
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