From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruce Momjian <bruce@momjian.us>
cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Andres Freund <andres@2ndquadrant.com>,
 PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
In-reply-to: <9879.1405877821@sss.pgh.pa.us>
References: <20140619020633.GA20469@momjian.us>
 <20140619015205.GD3806@momjian.us> <20140619014034.GC3806@momjian.us>
 <20140619220425.GQ18688@eldon.alvh.no-ip.org>
 <20140620010653.GS18688@eldon.alvh.no-ip.org>
 <20140620180725.GD29143@momjian.us> <20140624201346.GB17110@momjian.us>
 <20140627210520.GP7340@eldon.alvh.no-ip.org>
 <20140701161419.GA16033@momjian.us>
 <20140701190105.GA7340@eldon.alvh.no-ip.org>
 <20140701193047.GB16033@momjian.us> <9879.1405877821@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
 message dated "Sun, 20 Jul 2014 13:37:01 -0400"
Date: Sun, 20 Jul 2014 15:55:25 -0400
Message-ID: <13210.1405886125@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-bugs-owner@postgresql.org

I wrote:
> I started transcribing Bruce's proposed fix procedure at
> https://wiki.postgresql.org/wiki/20140702pg_upgrade_fix
> into the release notes, but I'm afraid it's all wet.

> He's suggesting copying the last checkpoint's NextMultiXactId into
> datminmxid/relminmxid, which is surely the wrong thing: that's likely to
> be newer than all mxids in the tables, not older than them.  I thought at
> first that this was a simple thinko and he meant to write oldestMultiXid,
> but here's the thing: if we're in the situation where we've got
> wraparound, isn't oldestMultiXid going to be 1?  The value recorded in the
> checkpoint isn't magic, it's just going to be extracted from whatever's in
> pg_database; and the whole problem here is that we can't trust that data.
> Where can we get a useful lower bound from?

Ugh: it's worse than that.  pg_upgrade itself is using this utterly
nonsensical logic to set datminmxid/relminmxid.  This is a stop-ship
issue for 9.3.5.

After some reflection it seems to me that we could estimate oldestmxid for
a pre-9.3 source cluster as the NextMultiXactId from its pg_control less
2000000000 or so.  This will nearly always be much older than the actual
oldest mxid, but that's okay --- the next vacuuming cycle will advance the
datminmxid/relminmxid values to match reality, so long as they aren't
wrapped around already.

Note that there's already an assumption baked into pg_upgrade that 2E9
xids or mxids back is safely past the oldest actual data; see where it
sets autovacuum_freeze_max_age and autovacuum_multixact_freeze_max_age
while starting the new cluster.

(Hm ... I guess "2000000000 or so" actually needs to be a bit less than
that, otherwise autovacuum might kick off while we're munging the new
cluster.)

We could recommend the same estimate in the instructions about cleaning
up a previous pg_upgrade by hand.

			regards, tom lane


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