Disk scheduling in an operating system is the method used to determine the order in which disk I/O requests are serviced. Efficient disk scheduling is essential because disk I/O is relatively slow, and multiple requests may arrive simultaneously. The primary goal of disk scheduling is to reduce the total seek time, which is the time taken by the disk's read/write head to move to the correct track where the data resides.
Key Terms:
- Seek Time: Time required to move the disk arm to the desired track.
- Rotational Latency: Time for the disk to rotate the required sector under the read/write head.
- Transfer Time: Time taken to transfer data after the head is positioned.
Common Disk Scheduling Algorithms:
- First-Come, First-Served (FCFS):
- This algorithm services requests in the order they arrive.
- Pros: Simple and fair.
- Cons: Can lead to high seek times and inefficient disk utilization, as the head may have to move back and forth frequently (the “convoy effect”).
- Shortest Seek Time First (SSTF):
- SSTF selects the request closest to the current head position, minimizing seek time for each operation.
- Pros: Reduces average seek time compared to FCFS.
- Cons: Can cause starvation for requests far from the head, as closer requests are always prioritized.
- SCAN (Elevator Algorithm):
- The disk head moves in one direction (e.g., inward), servicing requests, and then reverses direction when it reaches the end.
- Pros: Reduces starvation and provides more efficient head movement than SSTF.
- Cons: Edge requests may wait longer since the head services requests in a sweeping motion.
- C-SCAN (Circular SCAN):
- The disk head moves in one direction to the end of the disk, then jumps back to the beginning without servicing requests on the way back.
- Pros: Provides more uniform wait times compared to SCAN.
- Cons: The jump-back can introduce a small delay for requests near the start of the disk.
- LOOK and C-LOOK:
- Variants of SCAN and C-SCAN where the disk head only goes as far as the furthest request, reducing unnecessary movement.
- Pros: More efficient in terms of minimizing head movement.
Efficient disk scheduling reduces seek times, improves system performance, and ensures fair access to disk resources for multiple processes.
