Ceph » rbd

From the design doc:

In addition to knowing which parent a given image has, we want to be able to tell if a protected snapshot still has children. This is accomplished with a new per-pool object, rbd_children, which maps (parent pool id, parent image id, parent snapshot id) to a list of child image ids. This is stored in the same pool as the child image because the client creating a clone already has read/write access to everything in this pool, but may not have write access to the parent’s pool. This lets a client with read-only access to one pool clone a snapshot from that pool into a pool they have full access to. It increases the cost of unprotecting an image, since this needs to check for children in every pool, but this is a rare operation. It would likely only be done before removing old images, which is already much more expensive because it involves deleting every data object in the image.