RBD Image Real Size

To get the real size used by a rbd image :

rbd diff $POOL/$IMAGE | awk '{ SUM += $2 } END { print SUM/1024/1024 " MB" }'

For exemple :

$rbd info myrbd
rbd image 'myrbd':
    size 2048 MB in 512 objects
    order 22 (4096 KB objects)
    block_name_prefix: rb.0.2c6a.238e1f29
    format: 1

$ rbd diff myrbd | awk '{ SUM += $2 } END { print SUM/1024/1024 " MB" }'
14.2812 MB

From Josh in Ceph User mailing list : http://permalink.gmane.org/gmane.comp.file-systems.ceph.user/3684