Storage device and method for controlling storage device packet size

The present invention variably controls the packet size that is used within a storage device in accordance with the communication environment outside the storage device. The storage device comprises a CHA that controls data transfers with a host and a DKA that controls data transfers with a disk drive. The upper level I/F judgment unit of the CHA judges the type and communication band of the communication protocol used by the host and the packet size setting unit sets the optimum packet size on the basis of the type of communication protocol thus identified, and so forth. Likewise, the DKA sets the optimum packet size on the basis of the type and data processing speed of the disk drive that is used. As a result, data can be shifted rapidly to a packet in exchanges between the host and cache memory and between the disk drive and cache memory, whereby the data transfer efficiency can be improved.