falloc vs reflink revisited

[Darrick J. Wong]

On Wed, Mar 02, 2016 at 07:50:07AM -0800, Christoph Hellwig wrote:
> Hi Darrick,
> 
> I know that I suggested unsharing blocks on fallocate, but it turns out
> this is causing problems.  Applications expect falloc to be a fast
> metadata operation, and copying a potentially large number of blocks
> is against that expextation.  This is especially bad for the NFS
> server, which should not be blocked for a long time in a synchronous
> operation.
> 
> I think we'll have to remove the unshare and just fail the fallocate
> for a reflinked region for now.  I still think it makes sense to expose
> an unshare operation, and we probably should make that anyother
> fallocate mode.
> 
> Opininions?

Back in the day I had a new FALLOC_FL_FUNSHARE_RANGE flag to force-cow
a bunch of file blocks.  I don't mind reintroducing it.

Just think of all the fun we can share! :P

--D