A host computer and primary and secondary storage devices are connected as capable
of communicating with each other via a network, the primary and secondary storage
devices functioning as data storage means of the host computer. The host computer
transmits data to be written and a write request Wa to the primary storage device
in writing of data into the primary storage device and transmits the data to be
written and a write request Wb to the secondary storage device at any point in
time after the foregoing transmission. The primary storage device receives the
write request Wa and stores the data to be written. The secondary storage device
receives the write request Wb and stores the data to be written.