16 /* * Seek sort for disks. * * There are actually two queues, sorted in ascendening order. The first * queue holds those requests which are positioned after the current block; * the second holds requests which came in after their position was passed. * Thus we implement a one-way scan, retracting after reaching the end of * the drive to the first request on the second queue, at which time it * becomes the first queue. * * A one-way scan is natural because of the way UNIX read-ahead blocks are * allocated. */