Logical volume management
Some volume managers also implement snapshotting by applying copy-on-write (COW) to each PE.  In this scheme, the volume manager will copy a PE to a COW table just before it is written to.  This preserves an old version of the LE—the snapshot—which can later be reconstructed by overlaying the copy-on-write table on the current LE.  Snapshots which are read-write are branching snapshots because they implicitly allow diverging versions of an LE.