cheapo3:/ # time rbd cp machine5 machine6
Image copy: 100% complete...done.

real    3m28.774s
user    0m14.581s
sys     0m42.147s
cheapo3:/ # rbd create --size 4200 machine7
cheapo3:/ # rbd showmapped
id pool image      snap device
1  rbd  sles_image -    /dev/rbd1
cheapo3:/ # rbd map machine7
cheapo3:/ # time dd if=/dev/rbd1 of=/dev/rbd2 bs=4M
1050+0 records in
1050+0 records out
4404019200 bytes (4.4 GB) copied, 102.456 s, 43.0 MB/s

real    1m42.481s
user    0m0.012s
sys     0m10.945s