In a storage subsystem, performance tuning is performed with respect to all of the logical devices including external storage subsystems that are not directly connected to host computers. The physical storage units presented by the external storage subsystems are defined as logical devices of the storage subsystem, and I/O processing requests from the host computers are relayed to the logical devices. At the time of relaying, I/O processing conditions are monitored. When the load of an external storage subsystem is high, then operating conditions of ports and processors are examined. When the load can be reduced by changing the configuration of those ports and processors, the configuration is changed to reduce the load. When 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.