denning developed the scan disk scheduling strategy to reduce the unfairness
and variance of response times exhibited by sstf.  scan chooses the request that
requires the shortes seek distance in a preferred direction.  thus, if the
preferred direction is currently outward, the scan strategy chooses the shortest
seek distance in the outward direction.  scan does not change its preferred
direction until it reaches the outermost cylinder or the innermost cylinder.
in this sense, it is called the elevator algorithm, because an elevator 
continues in one direction servicing requests before reversing direction.
scan behaves much like sstf in terms of high trhoughput and good mean
response times.  however, because scan ensures that all requests in a given direction
will be serviced before the requests in the opposite direction, it offers a lower variance
of response times than sstf.  scan, like sstf, is a cylinder oriented strategy.
because the read write heads oscillate between opposite ends of each platter in
scan, the outer tracks are visited less often than the midrange tracks, but generally
moe often than they would be using sstf.  because arriving requests can be serviced
before waiting requests, both sstf and scan can suffer indefinite postponement.
the look variation of the scan strategy looks ahead to the end of the current
sweep to determine the next request to service.  if there are no more requests in the
current direction, look changes the preferred direction and begins the next
sweep.  in this sense, it is appropriate to call this the elevator algorithm,
because an elevator continues in one direction until it reaches the last request in
that direction, then reverses direction'  this strategy eliminates unnecessary seek
operations exhibited by other variations of the scan strategy compare the left
side of to the corresponding location in.
circular look variation of the look strategy uses the same technique as cscan to
reduce the bias against requests located at the extreme ends of the platters.
book quote