From: Florian Weimer <fweimer@bfk.de>
To: Christoph Hellwig <hch@infradead.org>
Cc: xfs@oss.sgi.com
Subject: Re: XFS on 2.6.26: reading the first 4K of a large file takes ages
Date: Thu, 20 May 2010 12:11:00 +0000 [thread overview]
Message-ID: <82sk5m7oyz.fsf@mid.bfk.de> (raw)
In-Reply-To: <20100519114826.GA18224@infradead.org> (Christoph Hellwig's message of "Wed\, 19 May 2010 07\:48\:26 -0400")
* Christoph Hellwig:
> On Wed, May 19, 2010 at 11:33:27AM +0000, Florian Weimer wrote:
>> 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?
>
> Yes, XFS always reads in the extent map, and no this hasn't changed
> recently.
Okay, defragmenting seems to improve things considerably. But it's
going to take a while: "extents before:5309152 after:13" *sigh*
Thanks for confirming my hunch. I don't think it's worth fixing this
in XFS. The database should call posix_fallocate() before flushing
its internal cache to the file in essentially random order, but it's
difficult to get upstream to implement this (the source code is a bit
hard to follow, unfortunately).
--
Florian Weimer <fweimer@bfk.de>
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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next prev parent reply other threads:[~2010-05-20 12:08 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-19 11:33 XFS on 2.6.26: reading the first 4K of a large file takes ages Florian Weimer
2010-05-19 11:48 ` Christoph Hellwig
2010-05-19 23:27 ` Dave Chinner
2010-05-20 12:11 ` Florian Weimer [this message]
2010-05-21 6:20 ` Stewart Smith
2010-05-21 6:43 ` Florian Weimer
2010-05-21 8:26 ` Dave Chinner
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