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From: Craig Ringer <craig@postnewspapers.com.au>
To: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Slow count(*) again...
Date: Wed, 13 Oct 2010 19:42:00 +0800
Message-ID: <4CB59B08.6030900@postnewspapers.com.au> (raw)
In-Reply-To: <4CB48EF4.5030502@krogh.cc>
References: <201010091626.18636.neil.whelchel@gmail.com>
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	<E2D86063-1FD0-48D0-BB0D-0E78F4A77FD2@richrelevance.com>
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	<4CB48EF4.5030502@krogh.cc>

On 13/10/2010 12:38 AM, Jesper Krogh wrote:

> If some clever postgres hacker could teach postgres to allocate blocks
> using posix_fallocate in quite large batches, say .. something like:
> fallocate(min(current_relation_size *0.1,1073741824))

There doesn't seem to be any use of posix_fallocate in the sources, at 
least according to git grep. The patch that introduced posix_fadvise use 
apparently had posix_fallocate in it, but that use appears to have been 
removed down the track.

It's worth noting that posix_fallocate sucks if your file system doesn't 
intelligent support for it. IIRC it's horrible on ext3, where it can 
take a while to return while it allocates (and IIRC zeroes!) all those 
blocks. This may be part of why it's not used. In past testing with 
posix_fallocate for other tools I've also found rather mixed performance 
results - it can slow things down rather than speed them up, depending 
on the file system in use and all sorts of other factors.

If Pg was to use posix_fallocate, it'd probably need control over it on 
a per-tablespace basis.

-- 
Craig Ringer

Tech-related writing at http://soapyfrogs.blogspot.com/



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