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

On May 7, 2015, at 6:21 PM, Thomas Munro <thomas.munro@enterprisedb.com> wro=
te:
> I may have the details wrong there, but in general I can't see how you
> can delete just the right segment files while the head and tail
> pointers of this circular buffer are both moving, unless you hold the
> lock to stop that while you make the decision and unlink the file
> (which I assume is out of the question), or say 'well if we keep the
> head and tail pointers at least 20 pages apart, that's unlikely to
> happen' (which I assume is also out of the question). =20

That is exactly the sort of thing I was worried about, but I couldn't put my=
 finger on it. I think your analysis is right.

> Now I
> understand the suggestion that the checkpoint code could be in charge
> of advancing the oldest multixact + offset.

Yeah. I think we need to pursue that angle unless somebody has a better idea=
.  It would also address another issue I am concerned about: the current sys=
tem seems to advance the oldest multixact information in shared memory diffe=
rently on the primary and the standby. I'm not sure the logic actually works=
 on the standby at all at this point, but even if it does, it seems unlikely=
 be right to rely on redo to do on the standby what is being done by a compl=
etely different, not-WAL-logged operation on the master.  Making the checkpo=
int do it in both cases would fix that.

> But if we did that, our new autovacuum code would get confused and
> keep trying to vacuum stuff that it's already dealt with, until the
> next checkpoint... so maybe we'd need to track both (1) the offset of
> the oldest multixact: the one that we use to trigger autovacuums, even
> though there might be even older segments still on disk, and (2)
> offset_of_start_of_oldest_segment_on_disk (suitably compressed): the
> one that we use to block wraparound in GetNewMultiXactId, so that we
> never write into a file that checkpoint might decide to delete.

That sounds plausible.


...Robert=