Abstract: In a system for storing same data in first and second storages, a central controller for producing a duplex write control signal on a bus. An I/O processor processes the duplex write control signal received from the bus into first and second write control signals to deliver the first and the second write control signals to first and second DMA controllers connected to the first and the second storages. A request controller selects, as a first selected request, that one of first and second data transfer requests simultaneously produced by the first and the second DMA controllers in response to the first and the second write control signals. Supposing that the first selected request is the first data transfer request, a data buffer controller controls the first DMA controller in response to the first selected request to read the data from a main memory on the bus and to write the data in the first storage. Simultaneously, the data buffer controller makes a data buffer memory memorize the data.