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

To: Florian Weimer <fweimer@xxxxxx>
Subject: Re: XFS on 2.6.26: reading the first 4K of a large file takes ages
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Wed, 19 May 2010 07:48:26 -0400
Cc: xfs@xxxxxxxxxxx
In-reply-to: <8239xojfco.fsf@xxxxxxxxxx>
References: <8239xojfco.fsf@xxxxxxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
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.

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