Quoting Daniel Carosone <dan@geek.com.au>:

> Even without addressing the filesystem issues at the layer above,
> I'd be interested in such a device.  Is this something you'll be
> able to release when you're done?

Yes.  The driver is available as an LKM for 1.6_STABLE.  I have a pkgsrc 
package which builds the LKM and includes some rc.d files to load it early 
enough, before fsck wants to run:
ftp://lola.pin.lu/pub/NetBSD/packages/device-mapper/pkgsrc-device-mapper-
0.03.tar.gz
There's only a very rough setup tool included, you almost definitely want to 
get some volume manager software[1].

> At the upper layer, it's probably something that will need direct
> support from the filesystems. In the softdep case, something that
> can be inserted in the dependency chain as a barrier - and when it
> finally hits the lower layer device in order, it triggers the actual
> snapshot to happen. That should avoid the need for locking, which
> you need to avoid so your snapshots remain transparent.

is there really such a single ordered dependency chain (in the softdep case) 
and does it include non-metadata accesses?

[1] ftp://lola.pin.lu/pub/NetBSD/packages/lvm2/pkgsrc-lvm2-1.95.15nb1.tar.gz
-- 
Christian Limpach <chris@pin.lu>