Re: Problem with pg_upgrade?

Robert Haas wrote:
> > So, we set the cluster xid while we do this schema-only restore. ?I
> > belive it might be possible for autovacuum to run while the schema is
> > restored, see an empty table, and set the relfrozenxid to be the current
> > xid, when in fact we are about to put a heap file in place of the
> > current empty file. ?I thought the autovacuum_freeze_max_age=2000000000
> > would prevent this but now I am not sure. ?I assumed that since the gap
> > between the restored relfrozenxid and the current counter would
> > certainly be < 2000000000 that autovacuum would not touch it. ?It is
> > possible these users had drastically modified autovacuum_freeze_max_age
> > to cause 3-billion gaps --- again, I have no direct contact with the
> > reporters, but I figured being paranoid is a good thing where pg_upgrade
> > is involved.
> 
> It does seem possible that that could happen, but I'm not sure exactly
> what would be causing autovacuum to fire in the first place.  It
> wouldn't have to be triggered by the anti-wraparound machinery - if
> the table appeared to be in need of vacuuming, then we'd vacuum it,
> discover that is was empty, and update relfrozenxid.  Hmm... could it
> fire just because the table has no stats?  But if that were the case
> you'd think we'd be seeing this more often.

Well, autovacuum=off, so it should only run in freeze mode, and I can't
see how that could happen.  I am thinking I have to study autovacuum.c.

I wonder if datfrozenxid could be incremented because the database is
originally empty.  It would just need to scan pg_class, not actually
vacuum anything.  I wonder if we do that.  The bottom line is I am
hanging too much on autovacuum_freeze_max_age causing autovacuum to do
nothing.

--  Bruce Momjian          http://momjian.us EnterpriseDB                             http://enterprisedb.com
 + It's impossible for everything to be true. +