Comment 10 for bug 1429794

code to check volume space is @ https://github.com/openstack/cinder/blob/stable/juno/cinder/volume/drivers/rbd.py#L350

this runs get_cluster_status from the rados client

>>> client = rados.Rados(rados_id='admin', conffile='')
>>> client.get_cluster_stats()
{'kb': 207172784L, 'num_objects': 59L, 'kb_avail': 198455288L, 'kb_used': 8717496L}

and would return 'unknown' for sizes if the command failed