Date: Mon, 21 Jul 2014 18:12:22 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
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
Message-ID: <20140721161222.GA17049@alap3.anarazel.de>
References: <20140720223010.GF5974@alap3.anarazel.de>
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 <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 2014-07-20 23:19:30 -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.

Looks basically good to me. Two minor things:
1) How about adding a elog(WARNING, "relation %u has invalid freeze
limits", ...)  to vac_update_datfrozenxid()? Otherwise it's a bit hard
to understand what's going on. I wouldn't want to invest too much effort
into the precision of the message, but some log bleat seems justified.

2) From a pedantic POV lastSane* should be initialized to
ReadNewTransactionId()/ReadNextMultiXactId(). On a busy system we could
otherwise consider concurrently updated values as being too new if some
longrunning transaction finishes. Also GetOldestXmin() can sometimes go
backwards...

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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