Description of problem:

When performing a snapshot a local copy is made. In the case of RBD, it reads what libvirt thinks is a raw block device and then converts that to a local raw file. The file is then uploaded to glance, which reads the whole raw file and stores it in the backend, if the backend is Ceph this is completely unnecessary and defeats the whole point of having a Ceph cluster. The fix should go something like this:

1. Tell Ceph to make a snapshot of the RBD
2. Get Ceph metadata from backend, send that to Glance
3. Glance gets metadata, if it has Ceph backend no download is necessary
4. If it doesn't, download image from Ceph location, store in backend

Version-Release number of selected component (if applicable):
Affects Icehouse, Juno, and Havana

How reproducible:
Always