SCAN Disk Scheduling Algorithm

The SCAN disk scheduling algorithm is another type of disk scheduling algorithm. In this algorithm, we move the disk arm into a specific direction (direction can be moved towards large value or the smallest value). Each request is addressed that comes in its path, and when it comes into the end of the disk, then the disk arm will move reverse, and all the requests are addressed that are arriving in its path. Scan disk scheduling algorithm is also called an elevator algorithm because its working is like an elevator.

Advantages of SCAN Disk Scheduling Algorithm

The advantages of SCAN disk scheduling algorithm are:

  1. In SCAN disk scheduling, there is a low variance of response time.
  2. In this algorithm, throughput is high.
  3. Response time is average.
  4. In SCAN disk scheduling, there is no starvation.

Disadvantages of SCAN Disk Scheduling Algorithm

The disadvantages of SCAN disk scheduling algorithm are:

  1. SCAN disk scheduling algorithm takes long waiting time for the cylinders, just visited by the head.
  2. In SCAN disk scheduling, we have to move the disk head to the end of the disk even when we don’t have any request to service.

Example of SCAN Disk Scheduling Algorithm

Consider a disk containing 200 tracks (0-199) and the request queue includes the track number 93, 176, 42, 148, 27, 14, 180, respectively. The current position of read//write head is 55, and direction is towards the larger value. Calculate the total number of cylinders moved by the head using SCAN disk scheduling.

Solution

As mentioned in the following example, the disk contains 200 tracks. So, we take a track line between 0 to 199.

The current position of the read/write head is 55. So, we start at 55, then move the read/write head. When all the requests are addressed, then we calculate a total number of cylinders moved by the head.

SCAN Disk Scheduling Algorithm
Figure: SCAN Disk Scheduling

Total Number of cylinders moved by head = (199-50) + (199-14)

                                                                      = 329

C-SCAN Disk Scheduling Algorithm

C-SCAN stands for Circular-SCAN.  C-SCAN is an enhanced version of SCAN disk scheduling. In the C-SCAN disk scheduling algorithm, the disk head starts to move at one end of the disk and moves towards the other end and service the requests that come in its path and reach another end. After doing this, the direction of the head is reversed. The head reaches the first end without satisfying any request and then it goes back and services the requests which are remaining.

Advantages of C-SCAN Disk Scheduling Algorithm

The advantages of the C-SCAN disk scheduling algorithm are:

  1. C-SCAN offers better uniform waiting time.
  2. It offers a better response time.

 Disadvantages of C-SCAN Disk Scheduling Algorithm

  1. In C-SCAN disk scheduling, there are more seek movements as compared to SCAN disk scheduling.
  2. In C-SCAN disk scheduling, we have to move the disk head to the end of the disk even when we don’t have any request to service.

 Example of C-SCAN Disk Scheduling Algorithm

Consider, a disk contains 200 tracks (0-199) and the request queue contains track number 82, 170, 43, 140, 24, 16,190, respectively.  The current position of R/W head is 50, and the direction is towards the larger value. Calculate the total number of cylinders moved by head using C-SCAN disk scheduling.

Solution

As mentioned in the following example, the disk contains 200 tracks. So, we take a track line between 0 to 199.

The current position of the read/write head is 50. So we start at 50, then move the read/write head based on the direction. Here, we move towards the larger value as given in the question. When all the requests are addressed, then we calculate a total number of cylinders moved by the head.

SCAN Disk Scheduling Algorithm
Figure: C-SCAN Disk Scheduling

                             Total Number of cylinders moved by head = (199-50) + (199-0) + (43-0)

                                                                                                   = 149+199+43

                                                                                                   = 391

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