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

[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.