FCFS Scheduling

The simplest form of disk scheduling is the first-come,first-served algorithm.

This algorithm is intrinsically fair, but is does not provide the fastest service.

A disk queue with requests for I/O to blocks on cylinders in that order.

 

SSTF Scheduling

This assumption is the basis for the shortest-seek-time-first algorithm. The SSTF algorithm selects the request with the minimum seek time from the current head position. Since seek time increases with the number of cylinders traversed by the head, SSTF chooses the pending request closest to the current head position.

SSTF scheduling is essentially a form of shortest-job-first scheduling; and like SJF scheduling, it may cause starvation of some requests.

 

SCAN Scheduling

The SCAN algorithm is sometimes called the elevator algorithm, since the disk arm behaves just like an elevator in a building, first servicing all the requests going up and then reversing to service requests the other way.

In 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, untill it gets to the other end of the disk.

Disk arm will reverses direction when the head reaches one end.

 

S-SCAN Scheduling

Circular SCAN scheduling is a variant of SCAN designed to provide a more uniform wait time. Like SCAN, C-SCAN moves the head from one end of the disk to other,s servicing requests along the way. When the head reaches the other end, however, it immediately returns to the beginning of the disk without serbicing any requests on the return trip.

 

LOOK Scheduling

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.