1. A storage system comprising: a plurality of SCSI (Small Computer System Interface) disk devices storing data received from a host computer; a plurality of ATA (Advanced Technology Attachment) disk devices storing data received from the host computer; a plurality of non-volatile devices storing data received from the host computer; and a processing unit controlling read/write requests received from the host computer to a plurality of logical volumes configured by at least one to the plurality of SCSI disk devices, the plurality of ATA disk devices and the plurality of non-volatile devices, and managing relation between storage hierarchy devices including the plurality of SCSI disk devices, the plurality of ATA disk devices and the plurality of non-volatile devices, and attribution information of each of the plurality of SCSI disk devices, the plurality of ATA disk devices and the plurality of non-volatile devices, wherein the processing unit determines a first storage layer of the storage hierarchy devices as a source storage layer and a second storage layer of the storage hierarchy devices as a destination storage layer, based on a user policy of I/O performance and power consumption, and migrates data stored in at least one of the logical volumes of the first storage layer to at least one of the logical volumes of the second storage layer, based on the user policy.