Disk Scheduling
In the realm of operating systems, a process demands two types of time: CPU time and I/O time. When it comes to I/O operations, processes request the operating system to access the disk. Balancing the fair allocation of resources while maintaining the efficiency and speed of process execution is a challenge. This is where disk scheduling algorithms come into play.
Key Concepts of Disk Scheduling
- Seek Time: Seek time refers to the duration required for the disk arm to position itself at a specific track where the read/write request can be fulfilled.
- Rotational Latency: Rotational latency is the time taken by the desired disk sector to rotate into a position where it becomes accessible by the read/write heads.
- Transfer Time: Transfer time is the time needed to transmit data.
- Disk Access Time: Disk access time can be calculated using the following formula: Disk Access Time = Rotational Latency + Seek Time + Transfer Time
- Disk Response Time: Disk response time represents the average time each request spends waiting for the I/O operation to complete.
Goals of Disk Scheduling Algorithms
- Fairness: Ensuring equitable allocation of disk access among processes.
- High Throughput: Maximizing the number of I/O requests processed.
- Minimal Traveling Head Time: Reducing the time the disk head spends moving between tracks.
Types of Disk Scheduling Algorithms
FCFS Scheduling Algorithm
- First-Come-First-Serve scheduling serves requests in the order they arrive.
- Simple but can lead to inefficient service for requests.
SSTF (Shortest Seek Time First) Algorithm
- SSTF selects the request with the shortest seek time.
- Improves efficiency but can cause starvation for certain requests.
SCAN Scheduling
- SCAN moves the disk arm in one direction, servicing requests along the way, and then reverses direction when it reaches the end.
- Balances fairness and efficiency.
C-SCAN Scheduling
- C-SCAN is similar to SCAN but services requests only in one direction.
- Reduces variations in service times.
LOOK Scheduling
- LOOK scans for requests in both directions, but it reverses direction when there are no pending requests in one direction.
- Provides a compromise between SCAN and C-SCAN.
C-LOOK Scheduling
- C-LOOK, like LOOK, searches in both directions but only service requests in one direction.
- Enhances fairness and reduces seek time variations.
Conclusion
Disk scheduling algorithms are crucial in optimizing the use of disk resources in an operating system. Each algorithm has its advantages and disadvantages, and the choice of algorithm depends on the specific system requirements. The evolution of these algorithms continues to address the limitations of previous approaches, aiming to strike a balance between fairness, efficiency, and reduced seek times.