a storage system includes a primary volume and a first controller that stores data in a primary volume and stores content at a prescribed point in time in the form of differential data . the first controller generates generational management information corresponding to the differential data of the prescribed point in time and manages the generational management information . the storage system also includes a secondary volume configured to store backup data and a second controller configured to receive data sent from the first controller , store data to the secondary volume , store the generational management information , receive a request for a certain generation of the generations and provide data of the primary volume corresponding to the requested generation based on the request . the first controller determines to send differential data and the generational management information to the second controller when an amount of differential data reaches a prescribed amount .