From: Andres Freund <andres@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
Date: Mon, 21 Jul 2014 18:34:09 +0200
Message-ID: <20140721163409.GB14989@alap3.anarazel.de> (raw)
In-Reply-To: <3655.1405959656@sss.pgh.pa.us>
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<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>
<20140721161222.GA17049@alap3.anarazel.de>
<3655.1405959656@sss.pgh.pa.us>
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On 2014-07-21 12:20:56 -0400, Tom Lane wrote:
> Andres Freund <andres@2ndquadrant.com> writes:
> > 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...
>
> If it's wrong then that's a pre-existing bug, but I don't think it's
> wrong.
I'm not wondering so much about vac_update_relstats(). There indeed the
calls aren't new and the worst that can happen is a slightly older
freeze limit. I'm more wondering about vac_update_datfrozenxid(). Afaics
we very well could hit
newFrozenXid = lastSaneFrozenXid = GetOldestXmin(NULL, true);
newMinMulti = lastSaneMinMulti = GetOldestMultiXactId();
...
/*
* If things are working properly, no relation should have a
* relfrozenxid or relminmxid that is "in the future". However, such
* cases have been known to arise due to bugs in pg_upgrade. If we
* see any entries that are "in the future", chicken out and don't do
* anything. This ensures we won't truncate clog before those
* relations have been scanned and cleaned up.
*/
if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid) ||
MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))
{
bogus = true;
break;
}
for a relation that has just been vacuumed by another backend. In a
database with a high number of pg_class entries/much bloat it's not that
unrealistic that relfrozenxid/relminmxid have been updated since the
GetOldest* calls above. Since vac_update_relstats() writes them using
heap_inplace_update() the window for that isn't particularly small. That
could potentially lead to never updating the database freeze limits,
right?
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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To: pgsql-bugs@postgresql.org
Cc: andres@2ndquadrant.com, tgl@sss.pgh.pa.us, bruce@momjian.us, alvherre@2ndquadrant.com
Subject: Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
In-Reply-To: <20140721163409.GB14989@alap3.anarazel.de>
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