Ceph » rbd

As I see it, we have two options that we need to choose between.
1) We can add fencing to librbd and let anybody do it.
2) We can make fencing part of the rbd tool.

Either way, the implementation is going to be yucky. We need to (in probably not this order) break their locks, take our own, and blacklist the entity in the OSDMap. Then we need to do a null write to every OSD which hosts objects for that RBD image (in order to force a map update which includes the blacklist).
This obviously has the potential to break a great many things since blacklisting happens at the address level right now — a KVM host is probably going to have a lot of images mounted and this is a very expensive operation that only handles one of them. I think we want to rethink this feature — since it will need to be admin-coordinated anyway we're probably better off making it somebody else's responsibility to handle blacklisting before breaking the locks.