review  various Disk scheduling algorithms

FCFS

the simplest form of disk scheduling is ,of course, the first-come,first-served algorithm. This algorithm is inrinsically fair,but it generally does not provide the fastest service.

 

SSTF

It seems reasonable to service all the requests close to the current head position before moving the head far away to service other request .This assumption is the basis for the shortest-seek-time-first(SSTF) algorithm. The SSTF algorithm selects the request with the minimum seek time from the current head position Since seek time increases with the number fo cylinders traversed by the head SSTF chooses the pending request closest to the current head position.

 

SCAN

In the SCAN algorithm, the disk arm starts at one end of the disk and moves toward the other end, servicing requests as it reaches each cylinder, until it gets to the other end of the disk . At the other end, the direction of head movement is reversed, and servicing continues. The head continuously scans back and forth across the disk. The SCAN algorithm is sometimes called the  elevator algorithm, since the requests going up and then reversing to service requests the other way.

And there are two kinds of scan algorithm: C-SCAN and SCAN

 

LOOK

as we described them,both SCAN and C-SCAN move the disk arm across the full width of the disk . In practice, neither algorithm is often implemented this way. More commonly, the arm goes only as far as the final request in each direction. Then, it reverses direction immediately, without going all the way to the end of the disk.