A Quick Abstract:

1. SCAN Disk Scheduling Algorithm:

Working:

The disk arm moves in one direction (e.g., from the outermost to the innermost track).

Services requests in the direction of movement.

When reaching the end, reverses direction.

Advantages:

Efficient for systems with high request density.

Reduces the likelihood of starvation.

Disadvantages:

May lead to increased seek times for requests at the end tracks.

2. C-SCAN Disk Scheduling Algorithm:

Working:

Similar to SCAN but only services requests in one direction.

When reaching the end, the arm "wraps around" to the beginning.

Advantages:

Reduces the impact of seeking from the last track to the first track.

Provides a more uniform wait time for requests.

Disadvantages:

May still result in increased seek times for requests near the end tracks.

3. C-LOOK Disk Scheduling Algorithm:

Working:

Similar to C-SCAN but does not service requests at the empty area during the return sweep.

The arm moves to the last request and then reverses direction.

Advantages:

More efficient in terms of seek time compared to C-SCAN.

Reduces the potential wait time for requests near the end tracks.

Disadvantages:

May still lead to increased seek times for certain requests.

4. Example Scenario:

Consider disk requests for sectors 30, 25, 35, 20, and 40, and the disk head initially positioned at sector 30.

SCAN:

Input:

Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}

Initial head position = 50

Direction = left (We are moving from right to left)

Output:

Total number of seek operations = 226

Seek Sequence is

41,34,11,0,60,79,92,114,176

C-SCAN:

Input:  

Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}

Initial head position = 50

Direction = right(We are moving from left to right)

Output:

Initial position of head: 50

Total number of seek operations = 389

Seek Sequence: 60, 79, 92, 114, 176, 199, 0, 11, 34, 41

C-LOOK:

Input:

Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}

Initial head position = 50

Direction = right (Moving from left to right)

Output:

Initial position of head: 50

Total number of seek operations = 321

Seek Sequence is 60, 79 , 92 , 114 , 176, 11 , 34, 41

5. Conclusion:

SCAN, C-SCAN, and C-LOOK are disk scheduling algorithms designed to optimize seek times and enhance overall disk performance. Each algorithm has its strengths and weaknesses, and the choice depends on the specific characteristics of the system and the workload.

Notes From The Slides:

SCAN

• The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.
• SCAN algorithm Sometimes called the elevator algorithm
• Illustration shows total head movement of 236 cylinders
• But note that if requests are uniformly dense, largest density at other end of disk and those wait the longest

SCAN (Cont.)

C-SCAN

• Provides a more uniform wait time than SCAN
• The head moves from one end of the disk to the other, servicing requests as it goes

• When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip

• Treats the cylinders as a circular list that wraps around from the last cylinder to the first one
• Total number of cylinders?

C-SCAN (Cont.)

C-LOOK

• LOOK a version of SCAN, C-LOOK a version of C-SCAN
• Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk
• Total number of cylinders?

C-LOOK (Cont.)

SCAN

• The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.
• SCAN algorithm Sometimes called the elevator algorithm
• Illustration shows total head movement of 208 cylinders
• But note that if requests are uniformly dense, largest density at other end of disk and those wait the longest