An embodiment of the present invention is a storage system including a plurality of non-volatile storage devices for storing user data, and a controller for controlling data transfer between the plurality of non-volatile storage devices and a host. The controller includes a processor core circuit, a processor cache, and a primary storage device including a cache area for temporarily storing user data. The processor core circuit ascertains contents of a command received from the host. The processor core circuit ascertains a retention storage device of data to be transferred in the storage system in operations responsive to the command. The processor core circuit determines whether to transfer the data via the processor cache in the storage system, based on a type of the command and the ascertained retention storage device.