Github user nathanejohnson commented on the issue:
https://github.com/apache/cloudstack/pull/1645
On KVM where primary storage is ceph / rbd, and a snapshot is backed up to
secondary, the comments seem to indicate that in this scenario the image will
be converted to qcow2 to save space. In implementation, however, it actually
"converts" the source format (RAW in this case) into RAW, which is effectly a
straight copy. This means that for a 20GB volume that is mostly empty, it will
copy a full 20GB file onto secondary storage as a backup. I have changed this
to it always sets the source type to that of the disk (which should always be
RAW for RBD), and sets the dest disk type to QCOW2.
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