Hi,

I'm using ceph 0.56.6 and kernel 3.9.7 and it looks like modifying RBD image 
also changes it's snapshot. I can reproduce this as follows:

# create and map an image
rbd create --size 128 test-1
rbd map test-1

# write some data to the image
dd if=/dev/zero of=/dev/rbd/rbd/test-1 bs=1M count=128

# create and map a snapshot
rbd snap create test-1@snap-1
rbd map test-1@snap-1

# verify that image and snapshot are identical
md5sum /dev/rbd/rbd/test-1 /dev/rbd/rbd/test-1@snap-1 
fde9e0818281836e4fc0edfede2b8762  /dev/rbd/rbd/test-1
fde9e0818281836e4fc0edfede2b8762  /dev/rbd/rbd/test-1@snap-1

# modify the image
dd if=/dev/urandom of=/dev/rbd/rbd/test-1 bs=512 count=1

# compare checksums again
md5sum /dev/rbd/rbd/test-1 /dev/rbd/rbd/test-1@snap-1 
1d942c8a5bc7480cecb945ea0d020eed  /dev/rbd/rbd/test-1
1d942c8a5bc7480cecb945ea0d020eed  /dev/rbd/rbd/test-1@snap-1

Checksums are identical although the snapshot isn't supposed to be modified.

-- 
Karol Jurak
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