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

[Dave Chinner]

On Wed, May 19, 2010 at 07:48:26AM -0400, Christoph Hellwig wrote:
> 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.

And demand paging the in-memory information is hard. It's on a to-do
list somewhere 'round here....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com