A data-mirroring method in which, while an I/O process is processing a first
I/O
request for writing data to a first device, control is obtained from that I/O process
executing on a host computer. A second I/O request, this one for writing the data
to the second device, is then created. Then, control is returned to the I/O process
by causing the first I/O request to be made available to a first data storage system
managing the first device.