Re: [Cluster-devel] [PATCH][try6] VFS: new want_holesize and got_holesize buffer_head flags for fiemap

[Christoph Hellwig <hch@infradead.org>]

On Wed, Oct 22, 2014 at 08:28:53AM -0400, Bob Peterson wrote:
> Yes, I thought about that.
> One of my early prototypes had a separate function used by fiemap.
> Function __generic_block_fiemap would call get_block() which
> returned an indication of a hole as it does today. When it saw
> the hole, fiemap called a new function get_hole_size() that was
> passed in like get_block. The problem is: it's grossly inefficient,
> since the new function get_hole_size() has to redo most of the work
> that get_block just did (at least in the case of GFS2). (Which in the
> case of a 1PB sparse file is non-trivial, since it involves several
> levels of metadata indirection). Combining it with get_block made it
> much more efficient.
> 
> Making a separate get_block_map_fiemap() function just seems like an
> exercise in redundancy.

I was thinking of replacing get_blocks entirely.  We're not actually
using a buffer_head in fiemap, so the interface seems somewhat awkward.
If it used something like the iomap interface proposed by Dave long
time ago we'd have a much saner interface that for example XFS could use
as well.