Storage system and back-up method for storage system

1. A storage system configured to be coupled to a plurality of computers, the system comprising:

a primary volume in a primary site, the primary volume being configured to store data received from at least one of the plurality of computers;

a first controller in the primary site, the first controller coupled to the primary volume and configured to store data in the primary volume, store storage contents of the primary volume at a prescribed point in time in the form of differential data, generate a plurality of generational management information each corresponding to the differential data of the prescribed point in time and manage the plurality of generational management information as a plurality of generations of the primary volume;

a secondary volume in a secondary site, the secondary volume being configured to store backup data of data stored in the primary volume; and

a second controller in a secondary site, the second controller coupled to the secondary volume and configured to receive data sent from the first controller based on an asynchronous remote copy procedure, store data to the secondary volume, store the plurality of generational management information received by the first controller, receive a request for a certain generation of the plurality of generations, and provide data of the primary volume corresponding to the requested generation based on the request by: 1) using data stored in the secondary volume; and 2) the generational management information corresponding to the requested generation,

wherein the first controller determines to send differential data and the generational management information to the second controller if an amount of differential data reaches a prescribed amount.

2. A storage system according to claim 1 , wherein said at least one of the plurality of computers sending the request is at least one host computer.

3. A storage system according to claim 1 , wherein the second controller receives the request from said at least one of the plurality of computers.

4. A storage system according to claim 1 , wherein the second controller creates a virtual volume for the requested generation based upon the retrieved generational management information.

5. A storage system according to claim 4 , wherein the second controller transits data of said virtual volume created for the requested generation to the primary site.

6. A storage system according to claim 5 , wherein the first controller in the primary site stores the data received from the second controller in the primary volume.

7. A storage system according to claim 1 , wherein each generational management information includes a differential bitmap information and a destination address information of corresponding differential data in the secondary volume.

8. A storage system located in a secondary site and configured to be coupled to a plurality of computers comprising a primary volume in a primary site, the primary volume being configured to store data received from at least one of the plurality of computers, a first controller in the primary site, the first controller coupled to the primary volume and configured to store data in the primary volume, store storage contents of the primary volume at a prescribed point in time in the form of differential data, generate a plurality of generational management information each corresponding to the differential data of the prescribed point in time and manage the plurality of generational management information as a plurality of generations of the primary volume, the storage system comprising:

a secondary volume in a secondary site, the secondary volume being configured to store backup data of data stored in the primary volume; and

a second controller in a secondary site, the second controller coupled to the secondary volume and configured to receive data sent from the first controller based on an asynchronous remote copy procedure, store data to the secondary volume, store the plurality of generational management information received by the first controller, receive a request for a certain generation of the plurality of generations, and provide data of the primary volume corresponding to the requested generation based on the request by: 1) using data stored in the secondary volume; and 2) the generational management information corresponding to the requested generation,

wherein the first controller determines to send differential data and the generational management information to the second controller if an amount of differential data reaches a prescribed amount.

9. A storage system according to claim 8 , wherein said at least one of the plurality of computers sending the request is at least one host computer.

10. A storage system according to claim 8 , wherein the second controller receives the request from said at least one of the plurality of computers.

11. A storage system according to claim 8 , wherein the second controller creates a virtual volume for the requested generation based upon the retrieved generational management information.

12. A storage system according to claim 11 , wherein the second controller transits data of said virtual volume created for the requested generation to the primary site.

13. A storage system according to claim 12 , wherein the first controller in the primary site stores the data received from the second controller in the primary volume.

14. A storage system according to claim 8 , wherein each generational management information includes a differential bitmap information and a destination address information of corresponding differential data in the secondary volume.