Filesystem writes on RAID5 too slow

[Christoph Hellwig]

> From: Dave Chinner <dchinner at redhat.com>
> 
> The function xfs_bmap_isaeof() is used to indicate that an
> allocation is occurring at or past the end of file, and as such
> should be aligned to the underlying storage geometry if possible.
> 
> Commit 27a3f8f ("xfs: introduce xfs_bmap_last_extent") changed the
> behaviour of this function for empty files - it turned off
> allocation alignment for this case accidentally. Hence large initial
> allocations from direct IO are not getting correctly aligned to the
> underlying geometry, and that is cause write performance to drop in
> alignment sensitive configurations.
> 
> Fix it by considering allocation into empty files as requiring
> aligned allocation again.
> 
> Signed-off-by: Dave Chinner <dchinner at redhat.com>

Ooops.  The fix looks good,

Reviewed-by: Christoph Hellwig <hch at lst.de>


Might be worth cooking up a test for this, scsi_debug can expose
geometry, and we already have it wired to to large sector size
testing in xfstests.