US Patent 7831782 Roll-back log to provide data consistency

A system or method for reestablishing data consistency between data volume P and its real time or near real time copy volume S. When volumes P and S enter a state of data inconsistency, data consistency can be restored through use of roll-back and/or roll-forward logs. The roll-forward log stores new data to be written to volume P. The roll-back log stores existing data of volume S before the existing data of volume S is overwritten with new data. As will be more fully described below, the roll-back log can be used to incrementally restore volume S to a prior data state in reverse chronological order, while a roll-forward log can be used to incrementally restore volume S in forward chronological to equal the data state of volume P.