I was asked off-list to explain this a bit better:

The file system suspension API is responsible for the preparation of snapshots.
It stops all new operations on a file system, waits for current operations to
complete and then syncs the (now silent) file system to disk.

This API is kernel-only.

The only user-visible change is the removal of snapshot support for
non-ffs file systems.

On Fri, Jan 19, 2007 at 04:56:07PM +0100, Juergen Hannken-Illjes wrote:
> Just committed the new file system suspension API.  It replaces the
> current vn_start_write/vn_finished_write API.  The suspension helpers
> are now put into file system specific operations.  Therefore snapshots
> of non-ffs file systems are currently unsupported.
> 
> Those using file system snapshots are encouraged to try this new API.
> Take your favorite -current kernel and build with `options NEWVNGATE'.
> 
> Plan is to make the new API default in mid february 2007.
> 
> If the machine deadlocks and you have DDB compiled in, please
> `call fstrans_dump' from DDB before reboot.
> 
> If you encounter problems, please send-pr with subject `fstrans ...'.
> 
> -- 
> Juergen Hannken-Illjes - hannken@eis.cs.tu-bs.de - TU Braunschweig (Germany)

-- 
Juergen Hannken-Illjes - hannken@eis.cs.tu-bs.de - TU Braunschweig (Germany)