In a storage subsystem, performance tuning is performed with respect to whole logical devices including external storage subsystems that are not directly connected to host computers.Physical storage units presented by the external storage subsystems are defined as logical devices of the storage subsystem itself, and I/O processing requests from the host computers are relayed to those logical devices. At the time of relaying, I/O processing conditions are monitored. When there exists an external storage subsystem whose load is high, then, operating conditions of ports and processors are examined. In the case where the load can be reduced by changing the configuration of those ports and processors, the configuration is changed to reduce the load. In the case where the load can not be reduced, data is migrated from a logical device having a high load to a logical device having a sufficient performance.