C-LOOK or Circular LOOK is a disk scheduling algorithm.
- Circular LOOK is like a C-SCAN which uses a return sweep before processing a set of disk requests
- It does not reach the end of the tracks unless there is a request, either read or write on such disk location similar with the LOOK algorithm.
- The disk head also goes as far as the last request in its direction then reverses its direction immediately without first going all the way to the end of the disk.
DISK SCHEDULING ALGORITHMS:
A hard disk drive is a collection of plates called platters. The surface of each platter is divided into circular tracks. Further more, each track is divided into smaller pieces called sectors. Disk I/O is done sector by sector. A group of tracks that are positioned on top of each other form a cylinder. There is a head connected to an arm for each surface, which handles all I/O operations.
For each I/O request, first head is selected. It is then moved over the destination track. The disk is then rotated to position the desired sector under the head= and finally, the read/write operation is performed.
There are two objectives for any disk scheduling algorithm:
1. Maximize the throughput - the average number of requests satisfied per time unit.
2. Minimize the response time - the average time that a request must wait before it is satisfied.
Some of the disk scheduling algorithms are explained below:
FCFS (First Come, First Served)
- perform operations in order requested
- no reordering of work queue
- no starvation: every request is serviced
- poor performance
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
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.
LOOK
- like SCAN but stops moving inwards (or outwards) when no more requests in that direction exist.
- 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.
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.
Hope this will help...
