Storage system and cache control method

According to the present invention, a plurality of switches are provided. A first cache unit in addition to a plurality of storage nodes is connected to each switch of the plurality of switches. Each of the plurality of storage nodes has a second cache unit. Each first cache unit and each second cache unit are mapped to a global cache recognized by the plurality of storage nodes. A local node (a storage node that has received an I/O request) writes N multiplexed I/O target data to corresponding N cache units (where N is an integer equal to or greater than 2). The N cache units include at least one second cache unit and at least one first cache unit. If data to be read is not present in the second cache unit of the local node, but is present in one of the first cache units, then the local node reads the data to be read from that first cache unit.