Abstract: Disk drives are mirrored through duplication controlled by disk controllers. Each disk controller controls writing to a set of disk drives. A disk write request to one disk controller causes that disk controller to write to one of its disks and to transmit the write request to another controller that in turn writes to its disk. The second controller then acknowledges the write to the first controller, which in turn acknowledges the write to the computer issuing the request. The first controller further logs the writes in a log file. This allows efficient resynchronization after mirroring is broken and reestablished, as well as removing cable length restrictions between controllers.