Disk Scheduling Algorithms

work queue: list of disk operations requested

1. FCFS (First Come, First Served)
   • perform operations in order requested
   • no reordering of work queue
   • no starvation: every request is serviced
   • poor performance

2. SSTF (Shortest Seek Time First)
   • after a request, go to the closest request in the work queue, regardless of direction
   • reduces total seek time compared to FCFS
   • Disadvantages
     • starvation is possible; stay in one area of the disk if very busy
     • switching directions slows things down

3. SCAN
   • go from the outside to the inside servicing requests and then back from the outside to the inside servicing requests.
   • repeats this over and over.
   • reduces variance compared to SSTF.

4. LOOK
   • like SCAN but stops moving inwards (or outwards) when no more requests in that direction exist.

5. C-SCAN (circular scan)
   • moves inwards servicing requests until it reaches the innermost cylinder; then jumps to the outside cylinder of the disk without servicing any requests.
   • repeats this over and over.
   • variant: service requests from inside to outside, and then skip back to the innermost cylinder.

6. C-LOOK
   • moves inwards servicing requests until there are no more requests in that direction, then it jumps to the outermost outstanding requests.
   • repeast this over and over.
   • variant: service requests from inside to outside, then skip back to the innermost request.