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Re: 8.4 open item: copy performance regression?

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Smith <gsmith(at)gregsmith(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Alan Li <ali(at)truviso(dot)com>
Subject: Re: 8.4 open item: copy performance regression?
Date: 2009-06-21 08:52:42
Message-ID: 1245574362.31430.18.camel@ebony.2ndQuadrant (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Sun, 2009-06-21 at 10:28 +0200, Stefan Kaltenbrunner wrote:
> I did some limited testing on that but I was unable to measure any 
> significant effect - especially since the difference between
> wal-logged and not is rather small for a non-parallel COPY (ie in the
> above example you get around 6m20s runtime for wal-logged and ~5m40s
> in the other case).

This is a common confusion for small tests.

Non-WAL logged case causes all buffers to be written to disk at end of
COPY. This is roughly the same size as the volume of WAL written. In
logged case we do not write data blocks, they get written at next
checkpoint. So the reduction in I/O is not apparent, since during the
period of the test the I/O is about the same in both cases and less I/O
in the non-WAL logged case. On longer tests the difference shows more
clearly because the data blocks start to migrate out of shared buffers
while the COPY is still running, effecting the test results. 

 Simon Riggs 
 PostgreSQL Training, Services and Support

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