Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts

On 2014-07-22 09:09:31 -0400, Bruce Momjian wrote:
> On Sun, Jul 20, 2014 at 11:19:30PM -0400, Tom Lane wrote:
> > I wrote:
> > > Here's a draft patch for this.  I think this will fix all cases where
> > > the "1" minmxid inserted by previous pg_upgrade versions is actually
> > > in the future at the time we run VACUUM.  We would still be at risk if
> > > it had been in the future when pg_upgrade ran but no longer is now,
> > > since that would mean there could be non-lock-only mxids on disk that
> > > are older than "1".  However, for the reasons discussed upthread, it
> > > seems fairly unlikely to me that people would actually get burnt in
> > > practice, so I'm satisfied with doing this much and no more.
> >
> > Ah, belay that: as coded, that would allow truncation of clog/multixact
> > as soon as any one relation in any one database had sane
> > frozenxid/minmxid.  If we want to have any pretense of being safe, we have
> > to postpone truncation until *all* relations have been vacuumed.  So more
> > like the attached, instead.
>
> Should we conclude that the multi-xact code is hopelessly complex and
> needs a redesign?

That might be true, but I don't see the above as evidence of it. It's a
nontrivial workaround for a bug; it's not surprising that it needs a
couple iterations to make sense. Without the pg_upgrade bug there'd be
no need to make those adjustments.

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services