What is disk scheduling?

Overview

This process needs two types of time required to process a request:

• CPUCentral Processing Unit time

• I/Oinput/output time

At run-time I/O requests for disk tracks to come from the process.

It is the responsibility of OSOperating System to choose which request is served first.

Disk scheduling is important, without it a lot of time could be wasted reading a large amounts of data.

Disk architecture

• The disk consists of PlatterThe circular disk on which magnetic data is stored in a hard disk drive. which consists of tracksA track is a physical division of data in a disk drive..
• Each track consists of sectorsa sector is a subdivision of a track on a magnetic disk or optical disc. in which data is present.
• There is a Read-write pointer that keeps on changing the position of the head as the disk keeps rotating.

Important terminology

• Seek Time: The time taken to reach a specified track where read/write request will be satisfied.
• Rotational delay: The time required to rotate the disk to a required sector. The main goal of the disk scheduling algorithm is to reduce the seek time.
• Data transfer time: The time required to read/write data from the disk.

• Total access time $=$ Seek time $+$ Rotational delay $+$ Data transfer time
• Data Size $=$ Platter $\times$ Surface $\times$ Track $\times$ Sectors $\times$ Data

Algorithms

The following are the disk scheduling algorithms that can be used:

1. FCFS
2. SSTF
3. Elevator algorithm (SCAN)
4. C-SCAN
5. LOOK
6. C-LOOK