help me understand ceph snapshot (also for gfarnum@redhat.com)

good morning, i am trying to understand how ceph snapshot works. i have read that snap are cow, which means if i am correct that if a new write update an exising block on a volume, the "old" block is copied to snap before overwrite it on original volume, am i right? so, i creted a volume say 10 GB in size, empty, then created a snap. so, coming to my doubt, why the snap is 10GB in size? it should be 0, because no new write update were done, am i right? thank you VERY much for your time.