A system and methods for building an efficient incremental data backup system capable of managing high frequency backups sessions, and capable of efficiently expiring backup revisions and locating the useless data elements is disclosed. A reduced set of data elements that have a non-zero probability of becoming redundant when a backup revision expires is prepared while each backup revision is being processed by the backup system. The backup system also maintains data structures, which reduce the number of searches that should be performed for each such data element before it can be realized that the data element is exclusively needed to support the expired backup revision, and therefore could be removed from the second tier storage.