Patent ID: 7984245

Claim:
A storage system, comprising: a first volume to be accessed by a host computer; a second volume mapped to the first volume; a cache memory for temporarily storing write data sent from the host computer to the first volume; a third volume to be used for saving write data of the cache memory; a controller for controlling the transfer of the write data; and a management memory storing a rule for deciding whether to transfer write data of the cache memory to the third volume; wherein the controller controls whether to transfer the write data, which was not transferred to the second volume and is remaining in the cache memory based on the rule stored in the management memory, to the third volume based on the transfer rule, wherein the controller transfers the write data from the cache memory to the third volume if the transfer of the write data from the cache memory to the second volume is a status that conforms to the rule, and transfers the write data from the cache memory to the second volume if the transfer is a status that does not conform to the rule, and wherein the controller determines that a status conforms to the rule when the capacity of the cache memory used by write data existing in the cache memory without being transferred to the second volume exceeds a threshold value, and thereby saves the write data in the third volume.