SCAN Elevator - TarisMajor/5143-OpSystems GitHub Wiki

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SCAN (Elevator) scheduling is a disk scheduling algorithm designed to handle disk I/O requests in an efficient manner. The algorithm gets its name because it works similarly to an elevator, moving in one direction until it reaches the end, then reversing direction. This method helps in reducing the average seek time compared to simpler algorithms like First-Come, First-Served (FCFS).

Key Characteristics of SCAN Scheduling

  1. Bi-Directional Movement: The disk arm moves in one direction, servicing all requests until it reaches the last request in that direction. It then reverses and services requests in the opposite direction.
  2. Orderly Processing: Requests are processed in an orderly manner based on their physical location on the disk, minimizing the overall seek time.

Advantages of SCAN Scheduling

  1. Reduced Seek Time: By servicing requests in the order they appear in one direction, SCAN reduces the seek time compared to FCFS.
  2. Better Throughput: The algorithm ensures that all requests are processed efficiently, leading to higher throughput.
  3. Fairness: SCAN provides a level of fairness by servicing requests in the order they are encountered, minimizing the risk of starvation for any particular request.

Disadvantages of SCAN Scheduling

  1. Long Wait Times for Edge Requests: Requests at the extreme ends of the disk may experience longer wait times as they are only serviced once the disk arm reaches them.
  2. Complexity: The implementation of SCAN is more complex than simpler algorithms like FCFS, as it requires maintaining the direction of the disk arm and sorting requests based on their position.

Variations of SCAN Scheduling

  1. Circular SCAN (C-SCAN): This variation treats the disk as a circular buffer. After reaching the end in one direction, the disk arm quickly returns to the beginning and starts servicing requests again. This ensures a more uniform wait time for requests.
  2. LOOK and C-LOOK: In these variations, the disk arm only goes as far as the last request in each direction, rather than going all the way to the end of the disk. This can further reduce seek times.

Use Cases for SCAN Scheduling

  1. Operating Systems: Commonly used in operating systems to manage disk I/O requests efficiently.
  2. Database Systems: Suitable for database systems that require efficient and predictable disk access patterns.
  3. High-Performance Computing: Useful in high-performance computing environments where disk I/O performance is critical.

Sources for Further Reading