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FScan disk scheduling algorithm - GeeksforGeeks

Last Updated : 15 Jul, 2025

Fixed period SCAN (FSCAN) disk scheduling algorithm mainly focuses on handling high variance in shortest seek time first (SSTF). SCAN algorithm is also proposed to handle above mentioned situation but using SCAN algorithm causes long delay while handling requests which are at extremes of disk. FSCAN algorithm determines how read and write head of disk will move in order to handle issue of handling issue of high variance of SSTF. How it works? FSCAN makes use of two queues, one of queues stores old r/w requests and other queue stores new r/w requests. When old requests are handled then only new requests are processed. Variations of FSCAN algorithm can also consist of N queues which in turn will make response time faster. How it handles issue of "high variance in SSTF" ? FSCAN addresses above mentioned issue by "freezing" queue once scan starts, requests that arrive after scan starts are processed in the next scan. Performance analysis : Citing theoretical analysis, it can be seen that SCAN results in lower average response time than FSCAN and higher average response time than shortest seek time first (SSTF). FSCAN algorithm has nice performance due to high throughput and low average response times. FSCAN removes problem of indefinite postponement. Example : How requests are processed  

FSCAN (or Fair SCAN) is a disk scheduling algorithm that is designed to provide fairness to all processes requesting I/O operations on a disk. It is an improved version of the SCAN algorithm, which suffers from the problem of starvation of requests from certain processes.

1. FSCAN maintains two queues: the active queue and the expired queue. The active queue contains requests that are currently being serviced, while the expired queue contains requests that were received while the disk head was sweeping the disk in the current direction and have not been serviced yet.
2. The algorithm services requests from the active queue until it becomes empty, and then switches to the expired queue, which contains
3. requests that were received while the disk head was moving in the opposite direction. Once the expired queue is empty, the algorithm switches back to the active queue.
4. FSCAN ensures fairness by preventing a single process from dominating the disk access. It does this by imposing a limit on the maximum number of requests that a process can have in the active queue at any given time.
5. FSCAN also reduces the average waiting time for requests by minimizing the maximum waiting time, which is achieved by switching between the active and expired queues at appropriate times.

1. Operating System Concepts by Abraham Silberschatz, Peter B. Galvin, and Greg Gagne
   Modern Operating Systems by Andrew S. Tanenbaum and Herbert Bos
2. Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau
3. Understanding the Linux Kernel by Daniel P. Bovet and Marco Cesati
4. These books provide a comprehensive introduction to operating systems and cover a wide range of topics, including disk scheduling
5. algorithms, process scheduling, memory management, file systems, and more.
    

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