Minimizing Seek Time Across Competing Read Requests
A mechanical disk's read/write head physically moves across platters to reach requested data, and that movement (seek time) is by far the slowest part of a disk operation — disk scheduling algorithms exist specifically to minimize total head movement across a queue of pending requests, rather than serving them in arbitrary arrival order.
FCFS serves requests in arrival order — simple, but can produce a lot of unnecessary back-and-forth head movement if requests arrive in a scattered order. SSTF (Shortest Seek Time First) always services whichever pending request is physically closest to the head's current position, minimizing immediate seek distance but risking starvation of far-away requests if closer ones keep arriving. SCAN moves the head steadily in one direction, servicing requests as it passes them, then reverses at the end — like an elevator, avoiding SSTF's starvation risk while still keeping movement efficient. C-SCAN (Circular SCAN) only services requests in one direction, then jumps back to the start without servicing on the return trip, giving more uniform wait times across the disk. LOOK is SCAN's more practical refinement — reversing direction as soon as no further requests remain ahead, rather than always sweeping all the way to the physical end of the disk.
Comparing total head movement (the sum of absolute differences between consecutive serviced cylinders) across all five algorithms on an identical request queue is exactly how operating systems courses demonstrate the real efficiency gap between naive and elevator-style scheduling.