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Re: Recalculating OldestXmin in a long-running vacuum

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, pgsql-patches(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Gregory Stark <gsstark(at)mit(dot)edu>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Subject: Re: Recalculating OldestXmin in a long-running vacuum
Date: 2007-03-27 16:57:54
Message-ID: 46094D12.6080909@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-patches
Gregory Stark wrote:
> "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> writes:
>> Yes, at least for now. I can't believe the patch actually hurts performance,
>> but I'm not going to spend time investigating it.
> 
> Isn't this exactly what you would expect? It will clean up more tuples so
> it'll dirty more pages. Especially given the pessimal way vacuum's dirty
> buffers are handled until Simon's patch to fix that goes in.

Hmm. Yeah, maybe it'll get better when we get that fixed..

> The benefit of the patch that we would expect to see is that you won't need to
> run VACUUM as often. In the long term we would expect the stock table to grow
> less too but I doubt these tests were long enough to demonstrate that effect.

The size did reach a steady state about half-way through the test, see 
the logs here:

patched
http://community.enterprisedb.com/oldestxmin/92/server/relsizes.log

unpatched
http://community.enterprisedb.com/oldestxmin/93/server/relsizes.log

The test was a success in that sense, the patch did reduce the steady 
state size of the stock table.

Maybe we would see a gain in transactions per minute or response times 
if we traded off the smaller table size to run vacuum slightly less 
frequently.. But as I said I don't want to spend time running more tests 
for what seems like a small benefit.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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