copy chunk preliminary results

[Christoph Hellwig]

On Thu, Nov 21, 2013 at 11:45:41AM +0100, David Disseldorp wrote:
> It depends on how the SMB server interprets the copy-chunk wire request.
> On Btrfs, Samba can translate the request into a BTRFS_IOC_CLONE_RANGE
> ioctl, in which case the same CoW semantics are observed[1]. See:
> 
> https://wiki.samba.org/index.php/Server-Side_Copy#Btrfs_Enhanced_Server-Side_Copy_Offload
> 
> By default however, Samba (and Windows) will perform the copy on the
> server-side using regular reads/writes. A generic cp --offload or
> similar would probably make more sense on the client side.

I don't think it really matters what the optimal case is, it matters
what the worst case is.  Think about it - a reflink really just is
a smart shortcut for copy + dedup, which a filesystem on the server
could do anyway.

On the other hand a user of cp --reflink expects it to be a quick
operation.

So it's time folks finally get the damn copyfile system call in, use
that for CIFS, NFS and co, as well as letting btrfs optimize it.