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Nice work!

Some comments:

I think the ext4 object backend should be renamed to 'Linux', as it just
requires basic Posix filesystems APIs + Linux xattrs.  In fact if it
used and abstraction for the attributes it could easily support all
modern Unix systems.

Also md5 seems a bit dangerous for an identity hash, I'd suggest to
use sha1 at least.

One thing I am a bit concerned about is the direct manipulation of the
qcow2 on disk format directly from cinder.  How does this cope with
updates to the format, e.g. from the qcow3 updates?  Also how does it
synchronize with concurrent I/O from qemu?

I'd love to see this functionality added directly to qemu an exposed
through the monitor.