Patent ID: 7877359

Claim:
A computer system comprising: a storage system coupled to a network, wherein the storage system having one or more data storage devices containing a plurality of data storage areas, and a controller for controlling the data storage devices, wherein the plurality of data storage areas including a first storage area and a second storage area for storing a copy of data that is stored in the first storage area at one point in time, and wherein the storage system is configured to delete, only when the differential amount is smaller than the predetermined threshold, the data stored in the second storage area and newly store a copy of the data stored in the first storage area in the second; and a backup judging module, wherein the backup judging module is configured to judge whether or not a differential amount indicating a difference between data stored in the first storage area and data stored in the second storage area is smaller than a predetermined threshold, and wherein the backup judging module is further configured to: count keywords contained in at least one of data stored in the first storage area and data stored in the second storage area to calculate, for each of the keywords, a difference between an appearance count of the keyword in the data stored in the first storage area and an appearance count of the keyword in the data stored in the second storage area; add up one of an absolute value and square of the difference calculated for each of the keywords; and use the added up value as a differential amount indicating a difference between the data stored in the first storage area and the data stored in the second storage area.