QCOW3 format for thin-provisioned volumes is not supported in RHEV 3.x

Issue

• In earlier RHEL 7 releases on RHEV hosts, QCOW3 was the default format for thin-provisioned volumes. If a RHEL 7 host was the Storage Pool Manager (SPM), any such volumes created by it would not run on a RHEL 6 host, as described in KCS 1284573.

• A thin-provisioned volume imported from a non-RHEV environment, e.g. Openstack Glance, could potentially be in QCOW3 format.

• Importing a thin-provisioned volume from Openstack Glance will fail if the volume format is QCOW3.