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

On Wed, Jun 18, 2014 at 09:52:05PM -0400, Bruce Momjian wrote:
> On Wed, Jun 18, 2014 at 06:51:31PM -0400, Alvaro Herrera wrote:
> > > This is a bug in 9.3 pg_upgrade as well?  Why has no one reported it
> > > before?
> >
> > I think one reason is that not all upgrades see an issue here; for old
> > clusters that haven't gone beyond the 0000 offset file, there is no
> > problem.  For clusters that have gone beyond 0000 but not by much, the
> > file will be deleted during the first truncation.  It only becomes a
> > problem if the cluster is close enough to 2^31.  Another thing to keep
> > in consideration is that initdb initializes all databases' datminmxid to
> > 1.  If the old cluster was past the 2^31 point, it means the datminmxid
> > doesn't move from 1 until the actual wraparound.
>
> OK, so the xid has to be beyond 2^31 during pg_upgrade to trigger a
> problem?  That might explain the rare reporting of this bug.  What would
> the test query look like so we can tell people when to remove the '0000'
> files?  Would we need to see the existence of '0000' and high-numbered
> files?  How high?  What does a 2^31 file look like?

Also, what would a legitimate 0000 file at wrap-around time look like?
Would there have to be an 'ffff' or 'ffffff' file?

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +