FSCTL_DUPLICATE_EXTENTS_TO_FILE is a new SMB3 ioctl introduced with
Windows Server 2016 (currently in technical preview), which allows for a client to
remotely request that the server perform a COW clone of a specified file:
https://msdn.microsoft.com/en-us/library/mt182620.aspx

The cifs.ko Linux kernel SMB client includes support for
FSCTL_DUPLICATE_EXTENTS_TO_FILE, which is invoked for cp --reflink
initiated COW clone operations. 

Samba has existing support for COW clones atop Btrfs via
FSCTL_SRV_COPYCHUNK:
https://wiki.samba.org/index.php/Server-Side_Copy

This functionality should be extended to support the new
FSCTL_DUPLICATE_EXTENTS_TO_FILE ioctl, to allow for remote COW clone
requests from new (Windows10+) clients, and the cifs.ko kernel client.

Time Plan:

• Install Windows Server 2016 Technical Preview
• Test cifs.ko fsctl support
• Implement Samba client/smbtorture functionality
• Implement Samba server support
• Plumb functionality into Btrfs backend, for remote COW clones
• Submit upstream