To: Bill Moran <wmoran@collaborativefusion.com>
Cc: Heikki Linnakangas <heikki@enterprisedb.com>,
	Jim Nasby <decibel@decibel.org>,
	Postgresql list Performance <pgsql-performance@postgresql.org>
Subject: Re: Filesystem fragmentation (Re: Fragmentation of WAL files)
References: <3BD44C5D-E402-43EB-8FFD-1A4BC97AFD0C@decibel.org>
	<20070426080050.522ef7b5.wmoran@collaborativefusion.com>
	<463096AD.4020707@enterprisedb.com>
	<20070426091432.274977fe.wmoran@collaborativefusion.com>
From: Florian Weimer <fweimer@bfk.de>
Date: Fri, 27 Apr 2007 09:39:28 +0200
In-Reply-To: <20070426091432.274977fe.wmoran@collaborativefusion.com> (Bill
	Moran's message of "Thu, 26 Apr 2007 09:14:32 -0400")
Message-ID: <82d51q9sgf.fsf@mid.bfk.de>
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* Bill Moran:

> To clarify my viewpoint:
> To my knowledge, there is no Unix filesystem that _suffers_ from
> fragmentation.  Specifically, all filessytems have some degree of
> fragmentation that occurs, but every Unix filesystem that I am aware of
> has built-in mechanisms to mitigate this and prevent it from becoming
> a performance issue.

One database engine tends to create a huge number of fragments because
the files are written with holes in them.  There is a significant
impact on sequential reads, but that doesn't matter much because the
engine doesn't implement fast, out-of-order B-tree scans anyway. 8-/

I still think that preallocating in reasonably sized chunks is
beneficial.

--=20
Florian Weimer                <fweimer@bfk.de>
BFK edv-consulting GmbH       http://www.bfk.de/
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