In the case of Nova instances, by default, a copy of the image will be made to the compute node where the instance runs. A qcow2 file is then created with the image copy as backing file; conceptually: qemu-img -f qcow2 -o backing_file=copy-of-image instancedisk.qcow2. The instance is the launched with instancedisk.qcow2 as root disk.
In the case of Nova instances, by default, a copy of the image will be made to the compute node where the instance runs. A qcow2 file is then created with the image copy as backing file; conceptually: qemu-img -f qcow2 -o backing_file=copy-of-image instancedisk.qcow2. The instance is the launched with instancedisk.qcow2 as root disk.