SCAN-DISK SCHEDULING
SCAN (Elevator) algorithm
In SCAN disk scheduling algorithm, head starts from one end of the disk and moves towards the other end, servicing requests in between one by one and reach the other end. Then the direction of the head is reversed and the process continues as head continuously scan back and forth to access the disk. So, this algorithm works as an elevator and hence also known as the elevator algorithm. As a result, the requests at the midrange are serviced more and those arriving behind the disk arm will have to wait. Suppose process P5 of size 7MB comes. But this process cannot be accommodated in spite of available free space because of contiguous allocation (as spanning is not allowed). Hence, 7MB becomes part of External Fragmentation.
Advantages of SCAN (Elevator) algorithm
This algorithm is simple and easy to understand. SCAN algorithm have no starvation. This algorithm is better than FCFS Scheduling algorithm .
DISADVANTAGES
More complex algorithm to implement. This algorithm is not fair because it cause long waiting time for the cylinders just visited by the head. It causes the head to move till the end of the disk in this way the requests arriving ahead of the arm position would get immediate service but some other requests that arrive behind the arm position will have to wait for the request to complete.