Open Stack

Sometime soon I'm going to need to migrate a few hundred VMs from one 
(nova-network-using) cloud to a newer (neutron-having) cloud. Google 
searches for suggestions about inter-cloud migration get me a whole lot 
of pages that suggest should I do this by taking snapshots in the old 
cloud and importing those snapshots into Glance on the new cloud.

Doesn't that migration path result in a massive increase in storage 
needs in the new cloud?  Right now I have a small number of shared base 
images and a large number of copy-on-write instances.  If I create 
snapshots for each VM, won't my new Glance server need to have storage 
capacity equal to 100% of the expanded size of every one of my instances?

Or, alternatively, maybe people using this method are purging the 
imported Glance images after the migration -- if that's so, then they 
can't be using copy-on-write, so that would drastically increase my 
storage needs on the compute side instead.

Am I missing something?  Does glance support e.g. 
images-backed-by-other-images so that I can have nested copy-on-write 
images?

Right now I'm leaning towards doing my migrations the hard way instead, 
by simply copying the raw instance files across and then mucking around 
in the nova database.  I'd love it if someone could steer me to a proper 
API-based approach though!

Thanks!

-Andrew