The present invention provides a method for providing snapshots of data at different times and for performing rollbacks to a selected snapshot that does not result in any subsequent snapshots being rendered unusable. The method includes a mapping mechanism wherein when a map entry for a block being written to for the first time during a session exists, then another map entry is created and all later sessions are applied to it. When a map entry for the block does not exist, a new map entry is provided and all prior sessions are applied to it.