Circular Scan - TarisMajor/5143-OpSystems GitHub Wiki

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Circular SCAN (C-SCAN) is a disk scheduling algorithm that extends the principles of SCAN (Elevator) scheduling to provide a more uniform wait time for disk I/O requests. In C-SCAN, the disk arm moves in one direction, servicing requests, and upon reaching the end of the disk, it quickly returns to the beginning and resumes servicing in the same direction. This helps in maintaining a consistent performance across different areas of the disk.

Key Characteristics of C-SCAN Scheduling

  1. Unidirectional Service: The disk arm moves in a single direction while servicing requests. When it reaches the end of the disk, it returns to the start without servicing any requests on the return trip.
  2. Uniform Wait Times: By treating the disk as a circular buffer, C-SCAN ensures more consistent and uniform wait times for requests, regardless of their location on the disk.

Advantages of C-SCAN Scheduling

  1. Reduced Wait Times: By servicing requests in one direction only and returning quickly to the start, C-SCAN reduces the wait times for requests compared to SCAN, especially for those near the edges of the disk.
  2. Fairness: C-SCAN provides a fairer distribution of service times for requests spread across the disk, minimizing the risk of starvation.
  3. Improved Throughput: The algorithm enhances throughput by reducing the number of times the disk arm has to move back and forth, focusing instead on moving in a single direction efficiently.

Disadvantages of C-SCAN Scheduling

  1. Increased Seek Time for Return Trip: The quick return of the disk arm to the start without servicing requests can increase the overall seek time, as this return trip is not used for servicing any I/O operations.
  2. Complex Implementation: Implementing C-SCAN requires additional logic to handle the quick return of the disk arm and ensure the algorithm maintains its unidirectional service pattern.

Use Cases for C-SCAN Scheduling

  1. Multi-User Systems: Suitable for multi-user systems where fair and consistent service times are essential for user satisfaction.
  2. Database Systems: Effective in database systems that require predictable and uniform access times for data retrieval and updates.
  3. High-Performance Environments: Useful in high-performance computing environments where disk I/O efficiency and fairness are critical for overall system performance.

Sources for Further Reading