The Scan (also known as the Elevator) disk scheduling algorithm is a popular method for scheduling disk requests in computer systems. It is a modification of the First-Come, First-Served (FCFS) disk scheduling algorithm and aims to reduce the total seek time by moving the disk head in one direction until the end of the disk, and then reversing the direction of the disk head to service the remaining disk requests.
In the Scan disk scheduling algorithm, the disk head starts at one end of the disk and moves towards the other end, servicing disk requests as it encounters them. When the disk head reaches the end of the disk, it reverses direction and services the remaining disk requests in the opposite direction.
To implement the Scan disk scheduling algorithm, we first need to keep track of the current position of the disk head. This can be done using a simple variable that stores the current position of the disk head on the disk.
Next, we need to keep track of the list of disk requests. This can be done using a data structure such as an array or a linked list. Each entry in the list should include the disk request's cylinder number.
Once we have our list of disk requests, we can begin the process of selecting the disk request to be serviced next. This can be done using a loop that iterates over the list of disk requests and selects the next disk request in the list based on the direction of the disk head.
After the disk request has been serviced, we need to update the current position of the disk head and the list of disk requests. The current position of the disk head should be updated to the cylinder number of the disk request that was just serviced, and the disk request should be removed from the list of disk requests.
The process of selecting and servicing disk requests then repeats until all disk requests have been serviced. At this point, the total seek time for all disk requests can be calculated and compared to other disk scheduling algorithms to determine the best approach for a given set of disk requests.
The Scan disk scheduling algorithm has several advantages over other disk scheduling algorithms. For example, it is more efficient than the FCFS disk scheduling algorithm, as it reduces the total seek time by moving the disk head in one direction and servicing disk requests along the way. Additionally, it is a fair and predictable algorithm, as disk requests are serviced based on the order in which they are encountered.
However, the Scan disk scheduling algorithm also has some disadvantages. For example, it may not be well-suited for systems with a large number of disk requests, as the time required to seek the disk can become quite large. Additionally, it may not be the best choice for systems with heavy disk I/O, as it may not be able to keep up with the high number of disk requests.
In conclusion, the Scan (Elevator) disk scheduling algorithm is a popular method for scheduling disk requests in computer systems. It is more efficient than the FCFS disk scheduling algorithm and provides a fair and predictable approach to disk scheduling. However, it may not be the best choice for systems with a large number of disk requests or heavy disk I/O, as it may not be able to keep up with the high number of disk requests. When choosing a disk scheduling algorithm, it's important to consider the specific needs and requirements of your system, as well as the characteristics of your workload, to determine the best approach for your specific situation.
