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XFS on 2.6.26: reading the first 4K of a large file takes ages

To: xfs@xxxxxxxxxxx
Subject: XFS on 2.6.26: reading the first 4K of a large file takes ages
From: Florian Weimer <fweimer@xxxxxx>
Date: Wed, 19 May 2010 11:33:27 +0000
We've got a couple of rather large files, and with a cold cache,
reading the first 4K bytes of the file (e.g., just running
"head --bytes 4096" on it) takes ages, up to several minutes,
sometimes triggering the hang check timer.

I wonder if XFS reads the whole extent information into RAM when the
file is opened.  Is this the case, at least on 2.6.26?  Has this
changed in later versions, perhaps?

The files in question are heavily fragmented (they have been created
with holes first, and the holes have been filled in subsequently).
I'll try to run xfs_fsr on those files, but it's going to be
tough. 8-/

-- 
Florian Weimer                <fweimer@xxxxxx>
BFK edv-consulting GmbH       http://www.bfk.de/
Kriegsstraße 100              tel: +49-721-96201-1
D-76133 Karlsruhe             fax: +49-721-96201-99

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