11.3 Disk Scheduling 365 In FCFS, we process the requests in the order they arrive, without regard to the current position of the heads. Therefore, under a FCFS algo-rithm, the heads move from cylinder 26 (its current position) to cylinder 49. After the request for cylinder 49 is satisfied (that is, the information is read or written), the heads move from 49 to 91. After processing the request at 91, the heads move to cylinder 22. Processing continues like this, in the order that the requests were received. Note that at one point the heads move from cylinder 91 all the way back to cylinder 22, during which they pass over several cylinders whose requests are currently pending. Shortest-Seek-Time-First Disk Scheduling The shortest-seek-time-first (SSTF) disk-scheduling algorithm moves the heads the minimum amount it can to satisfy any pending request. This approach could potentially result in the heads changing directions after each request is satisfied. Let’s process our hypothetical situation using this algorithm. From our
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