The elevator algorithm (also SCAN) is a disk-scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests. This algorithm is named after the behavior of a building elevator, where the elevator continues to travel in its current direction (up or down) until empty, stopping only to let individuals off or to pick up new individuals heading in the same direction. From an implementation perspective, the drive maintains a buffer of pending read/write requests, along with the associated cylinder number of the request, in which lower cylinder numbers generally indicate that the cylinder is closer to the spindle, and higher numbers indicate the cylinder is farther away. Description When a new request arrives while the drive is idle, the initial arm/head movement will be in the direction of the cylinder where the data is stored, either in or out. As additional requests arrive, requests are serviced only in the current direction of arm movement until the arm reaches the edge of the disk. When this happens, the direction of the arm reverses, and the requests that were remaining in the opposite direction are serviced, and so on. Variations One variation of this method ensures all requests are serviced in only one direction, that is, once the head has arrived at the outer edge of the disk, it returns to the beginning and services the new requests in this one direction only (or vice versa). This is known as the "Circular Elevator Algorithm" or C-SCAN. Although the time of the return seek is wasted, this results in more equal performance for all head positions, as the expected distance from the head is always half the maximum distance, unlike in the standard elevator algorithm where cylinders in the middle will be serviced as much as twice as often as the innermost or outermost cylinders. Other variations include: FSCAN LOOK (and C-LOOK) N-Step-SCAN 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