From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@2ndquadrant.com>
cc: Bruce Momjian <bruce@momjian.us>,
 Alvaro Herrera <alvherre@2ndquadrant.com>,
 PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
In-reply-to: <20140720223010.GF5974@alap3.anarazel.de>
References: <20140701193047.GB16033@momjian.us>
 <9879.1405877821@sss.pgh.pa.us> <20140720200225.GA5974@alap3.anarazel.de>
 <13755.1405887378@sss.pgh.pa.us> <20140720204852.GB5974@alap3.anarazel.de>
 <15685.1405891368@sss.pgh.pa.us> <20140720213258.GD5974@alap3.anarazel.de>
 <16274.1405892584@sss.pgh.pa.us> <20140720220253.GE5974@alap3.anarazel.de>
 <17255.1405894611@sss.pgh.pa.us> <20140720223010.GF5974@alap3.anarazel.de>
Comments: In-reply-to Andres Freund <andres@2ndquadrant.com>
 message dated "Mon, 21 Jul 2014 00:30:10 +0200"
Date: Sun, 20 Jul 2014 18:39:06 -0400
Message-ID: <17935.1405895946@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-bugs-owner@postgresql.org

Andres Freund <andres@2ndquadrant.com> writes:
> On 2014-07-20 18:16:51 -0400, Tom Lane wrote:
>> My point is that the cutoff multi xid won't be new enough to remove
>> non-LOCKED_ONLY (ie, post-9.3) mxids.

> Why not? Afaics this will continue to happen until multixacts are
> wrapped around once? So the cutoff multi will be new enough for that at
> some point after the pg_upgrade?

Before that happens, nextMultiXid will catch up with the minmxid = 1
values, and they'll be in the past, and then we're at the same point
that we're at to begin with if we used 9.3.5 pg_upgrade.

> Luckily in most cases full table vacuums triggered due to normal xids
> will prevent bad problems though.

Yeah.  While it's not that comfortable to rely on that, we were reliant
on that effect in every pre-9.3 branch, so I'm not terribly upset about
it continuing to be the case in existing 9.3 installations.  As long as
they're not consuming mxids at a rate much greater than xids, they'll be
all right; and things will be unconditionally safe once the freezing
process has advanced far enough.

			regards, tom lane


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