Date: Tue, 22 Jul 2014 16:14:01 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: Tom Lane <tgl@sss.pgh.pa.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
Message-ID: <20140722141401.GC4190@alap3.anarazel.de>
References: <17935.1405895946@sss.pgh.pa.us>
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 <18673.1405897470@sss.pgh.pa.us> <18851.1405897900@sss.pgh.pa.us>
 <20140720233125.GI5974@alap3.anarazel.de>
 <19458.1405899435@sss.pgh.pa.us>
 <20140720234925.GJ5974@alap3.anarazel.de>
 <22178.1405903964@sss.pgh.pa.us> <26504.1405912770@sss.pgh.pa.us>
 <20140722130931.GA1563@momjian.us>
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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


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