[PATCH v2] xfs: increase prealloc size to double that of the previous extent

[Ben Myers]

On Tue, Feb 19, 2013 at 10:24:41AM -0500, Brian Foster wrote:
> The updated speculative preallocation algorithm for handling sparse
> files can becomes less effective in situations with a high number of
> concurrent, sequential writers. The number of writers and amount of
> available RAM affect the writeback bandwidth slicing algorithm,
> which in turn affects the block allocation pattern of XFS. For
> example, running 32 sequential writers on a system with 32GB RAM,
> preallocs become fixed at a value of around 128MB (instead of
> steadily increasing to the 8GB maximum as sequential writes
> proceed).
> 
> Update the speculative prealloc heuristic to base the size of the
> next prealloc on double the size of the preceding extent. This
> preserves the original aggressive speculative preallocation
> behavior and continues to accomodate sparse files at a slight cost
> of increasing the size of preallocated data regions following holes
> of sparse files.
> 
> Signed-off-by: Brian Foster <bfoster at redhat.com>
> Reviewed-by: Dave Chinner <dchinner at redhat.com>

Excuse me.  This is the version I applied.  Not v1.

-Ben