1. A storage system, comprising: a plurality of storage devices which are based upon a plurality of logical volumes; and a controller which receives from an external device an I/O (Input/Output) request designating any one among said plurality of logical volumes; wherein: said controller comprises an external interface for receiving I/O requests from said external device, and a plurality of microprocessors; said plurality of microprocessors comprises a synchronous processor and an asynchronous processor; said synchronous processor is a microprocessor whose duty is to perform synchronous processing and not to perform asynchronous processing; said asynchronous processor is a microprocessor whose duty is to perform asynchronous processing and not to perform synchronous processing; said synchronous processing is processing which must be performed during the time interval between the receipt of an I/O request from said external device and the returning of a response to said I/O request to said external device; and said asynchronous processing is specific processing, among processing other than said synchronous processing.