Some volume managers also implement snapshots by applying copy-on-write to each LE. In this scheme, the volume manager will copy the LE to a copy-on-write table just before it is written to. This preserves an old version of the LV—the snapshot—which systems can later reconstruct by overlaying the copy-on-write table atop the current LV. Read-write snapshots are branching snapshots because they implicitly allow diverging versions of an LV.
Snapshots can be useful for backing up self-consistent versions of volatile data like table files from a busy database, or for rolling back large changes (such as an operating system upgrade) in a single operation. Some Linux-based Live CD systems also use snapshots to simulate read-write access on a read-only compact disc.
Read more about this topic: Logical Volume Management