The SCAN disk scheduling algorithm is also known as the:

Question

BlackTom AI · Accepted Answer

SCAN is called the Elevator algorithm because the disk arm moves back and forth across the disk, servicing requests in one direction until it reaches the end (or the last pending request), then reverses direction—just like an elevator that moves up serving floors, then moves down serving floors.

1. Relevant concepts:
- SCAN algorithm: disk head services requests in increasing (or decreasing) cylinder order while moving in one direction, then reverses direction and continues servicing requests.
- Elevator analogy: an elevator moves in one direction serving all requests (stops) on the way, then reverses; this mirrors SCAN’s operation.
- Other algorithms for contrast: C-SCAN (Circular SCAN) treats the disk as circular and jumps to the start without servicing on the return; FCFS serves requests in arrival order; SSTF serves the nearest pending request.

2. Step-by-step reasoning:
- Suppose the head is at cylinder X and moving outward. SCAN will service all requests with cylinder ≥ X in increasing order until the outermost pending request (or end) is reached.
- Then the head reverses direction and services remaining requests in decreasing order.
- This continuous back-and-forth motion is exactly like an elevator that goes up servicing floors, then goes down servicing floors.

3. Intermediate steps / illustration (conceptual):
- Head at 30, requests at 40, 70, 20, 10. Moving outward: service 40, 70. Reverse: service 20, 10. Order follows a single-direction sweep then reverse.

4. Why the correct answer is correct:
- The “Elevator algorithm” is the widely used nickname for SCAN because of identical servicing behavior (sweep and reverse). The SCAN name itself comes from this sweeping motion.

5. Why other options are incorrect:
- b. Circular SCAN algorithm describes C-SCAN, which does not reverse but jumps to the start—different behavior.
- c. FCFS serves strictly by arrival order, not by head direction or position.
- d. SSTF selects the nearest request each time, not a directional sweep.

Conclusion: The correct answer is a. Elevator algorithm, which matches the provided answer.

The SCAN disk scheduling algorithm is also known as the:单项选择题

类似问题

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Consider a disk with tracks numbered 0 to 199. The current head position is 50 (Head Movement is Right). The request queue is: 98, 183, 37, 122, 14, 124, 65, 67. How Many Head movements are there in servicing requests using FCFS?Tip: Enter numerical data ONLY. E.g: "5" and NOT "5 Head Movements".

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