[update] Since Ceph octopus (v15.2)
Use command "rbd du"
$ rbd du rbd/myrbd
NAME PROVISIONED USED
myrbd 4 TiB 1023 GiB
For previous version
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 …