sudo rbd diff rbd/<poolname> --from-snap snap1 --debug-rbd 20 --debug-ms 1 >& log.txt



some part of log
2015-02-13 16:56:54.916704 87b7b000 20 librbd: diff_iterate object rb.0.10ce.238e1f29.00000001e780
2015-02-13 16:56:54.916761 87b7b000  1 -- 192.168.200.196:0/1024018 --> 192.168.200.209:6818/10685 -- osd_op(client.4566.0:124806 rb.0.10ce.238e1f29.00000001e780@snapdir [list-snaps] 2.d6c5009d ack+read e158) v4 -- ?+0 0x20ad6548 con 0x20a385e0
2015-02-13 16:56:54.918597 86b76420  1 -- 192.168.200.196:0/1024018 <== osd.22 192.168.200.209:6818/10685 6548 ==== osd_op_reply(124806 rb.0.10ce.238e1f29.00000001e780 [list-snaps] v0'0 uv0 ack = -2 ((2) No such file or directory)) v6 ==== 198+0+0 (1472486878 0 0) 0x82000468 con 0x20a385e0
2015-02-13 16:56:54.918756 87b7b000 20 librbd: diff_iterate object rb.0.10ce.238e1f29.00000001e781