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Elevator algorithm
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. Lower cylinder numbers 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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