Editing Elevator algorithm (section)

== Example ==
The following is an example of how to calculate average disk seek times for both the SCAN and C-SCAN algorithms.

* Example list of pending disk requests (listed by track number): 100, 50, 10, 20, 75.
* The starting track number for the examples will be 35.
* The list will need to be sorted in ascending order: 10, 20, 50, 75, 100.

Both SCAN and C-SCAN behave in the same manner until they reach the last track queued. For the sake of this example let us assume that the SCAN algorithm is currently going from a lower track number to a higher track number (like the C-SCAN is doing). For both methods, one takes the difference in magnitude (i.e. absolute value) between the next track request and the current track.

* '''Seek 1:''' 50 − 35 = 15
*  '''Seek 2:''' 75 − 50 = 25
*  '''Seek 3:''' 100 − 75 = 25

At this point both have reached the highest (end) track request. SCAN will just reverse direction and service the next closest disk request (in this example, 20) and C-SCAN will always go back to track 0 and start going to higher track requests.

*  '''Seek 4 (SCAN):''' 20 − 100 = 80
*  '''Seek 5 (SCAN):''' 10 − 20 = 10
*  '''Total (SCAN):''' 155
*  '''Average (SCAN):''' 155 ÷ 5 = 31
*  '''Seek 4 (C-SCAN):''' 0 − 100 = 0 head movement as cylinders are treated as a  circular list (C-SCAN always goes back to the first track)
*  '''Seek 5 (C-SCAN):''' 10 − 0 = 10
*  '''Seek 6 (C-SCAN):''' 20 − 10 = 10
*  '''Total (C-SCAN):''' 85
*  '''Average (C-SCAN):''' 85 ÷ 5 = 17

Even though six seeks were performed using the C-SCAN algorithm, only five I/Os were actually done.