Optimal performance tuning is enabled by avoiding the deterioration in the performance of a storage system caused by an erroneous setting in a tuning parameter. The storage system has a primary volume provided as an operational volume to a host computer, and a secondary volume capable of forming a copy-pair with the primary volume. When the load demanded in executing I/O processing to the secondary volume set with a second tuning parameter is lighter than the load demanded in executing I/O processing to the primary volume set with a first tuning parameter, the storage controller switches the primary/secondary relationship between the primary volume and secondary volume and provides the secondary volume as the operational volume to the host computer.