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
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