Disk Scheduling

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:

1. 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”).

1. 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.

1. 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.

1. 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.

1. 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.