1:create an encrypted rbd volume A, which size is 2G.
2:create A's snapshot snap_A.
3:create new encrypted volume B form snap_A, the size of B is 2G too.

  we know that, currently create rbd encrypted volume, we use qume-img create to create an encrypted file, and then use rbd import into rbd cluster, and the encrypted head takes up the first 1 MB capacity, so in the rbd cluster, the real size of volume A is 2G + 1M.
  in rbd driver, at function def create_volume_from_snapshot(), after clo...

J4108-01, a 4U 108-Bay 12Gb/s SAS JBOD, ideal for high-availability storage and high performance app