Scan-EDF

    A variation of the Scan disk aceess
   algorithm for use in a real-time environment where, in
   general, requests are served according to Earliest Deadline
   First.  If two requests share the same deadline, they may be
   reorganised according to Scan.

   A typical example is a video server that retrieves video data
   from a hard disk.  The playback of a video impose tight
   real-time constraints but if the server retrieves data once
   every second for each video channel, Scan-EDF can be applied,
   reducing the seek overhead.

   (1995-11-15)