A storage subsystem that has detected the necessity of the data relocation
determines whether the data relocation is possible or not from one
storage unit to another within the particular storage subsystem. A
virtualization device is notified in the case where the data relocation
in the particular subsystem is impossible. The manager of the
virtualization device gives an inquiry to the storage subsystems as to
whether any one of them can become a relocation destination or not. A
storage subsystem determines, based on the related information, whether
the data relocation requested is possible or not within the particular
storage subsystem. In the case where such a relocation is possible, the
copying process is carried out for data relocation from one storage
subsystem to another in compliance with an instruction from the
virtualization device.