I'm not familiar with Ubuntu Cloud, so take this with a huge grain of salt. GlusterFS can be configured to provide the storage like you want. There may be performance implicates for doing this as well.
If you're trying to use up every last little gigabyte you've got left, it's probably time for a new HD (or three). If you're just looking for a clustered filesystem so VMs can be bounced around to different hosts easily, Gluster is probably what you're looking for.
No you can't. This may be possible with very specific datastore technology but I'm not even sure.
I'm not familiar with Ubuntu Cloud, so take this with a huge grain of salt. GlusterFS can be configured to provide the storage like you want. There may be performance implicates for doing this as well.
If you're trying to use up every last little gigabyte you've got left, it's probably time for a new HD (or three). If you're just looking for a clustered filesystem so VMs can be bounced around to different hosts easily, Gluster is probably what you're looking for.
Glusterfs or ceph may be able to do that but indeed it's not possible out of the box, and would require intense hacking to get such a thing going.