1. A system, comprising: a plurality of computers coupled together, each computer comprising a local volatile memory; an array of redundant storage devices accessible to the plurality of computers; and at least one bitmap set that includes a master bitmap and at least one local bitmap; wherein the at least one local bitmap is stored in the local volatile memory of one of the plurality of computers to log modifications made to the array only by the particular computer of the plurality of computers where the local bitmap is stored; wherein the master bitmap logs modifications made by each of the plurality of computers to the array of redundant storage devices, said modifications logged in the master bitmap identifying regions of storage in the storage devices that have been modified; and wherein at least one of the computers stores a bitmap policy that specifies a plurality of bitmap sets and, for each bitmap set, identifies one or more computers that is to host a master bitmap.