Ceph has an ability to create CoW clones from RBD snapshots. This mechanism is known as snapshot layering. The layering feature allows Ceph to create multiple clones of an RBD image. This feature is extremely useful to private cloud and virtualization platforms such as OpenStack, CloudStack, and Qemu/KVM. These platforms allow the use of snapshots to protect RBD images that contain OS/VM images. The OS images can then be cloned to spin up new VM instances. Ceph snapshots are read-only but image clones are writeable.
Every cloned image stores a reference to its parent snapshot. It is thus important that the parent snapshot be protected before it can be used for cloning. The protection is necessary to prevent deletion ...
