Date: Tue, 22 Jul 2014 09:09:31 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@2ndquadrant.com>,
 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: <20140722130931.GA1563@momjian.us>
References: <20140720223010.GF5974@alap3.anarazel.de>
 <17935.1405895946@sss.pgh.pa.us>
 <20140720225653.GG5974@alap3.anarazel.de>
 <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>
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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?

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

  + Everyone has their own god. +


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