Comment 8 for bug 1374256

If you’re using glance and cinder with rbd and show_direct_url=False, cinder has to download the image (you can see it at [1]). That isn't efficient.
Using show_direct_url=true and rbd_flatten_volume_from_image=True, cinder can directly fetch the image from rbd using the url.
[1] http://paste.openstack.org/show/492690/ - image_utils.TemporaryImages.fetch