Patent attributes
A problem of prior art is that normal backup data is lost as a result of taking a backup by overwriting the normal backup data with data that has been changed greatly from a virus infection or the like. This invention provides a computer system having a storage system. The storage system contains 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. Only when the differential amount indicating the difference between data stored in the first storage area and data stored in the second storage area is smaller than a predetermined threshold, the storage system deletes the data which is stored in the second storage area and newly stores a copy of the data stored in the first storage area in the second storage area.
