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Home » Operating Systems

Various Disk Scheduling Algorithms | Operating System



In this article, we are going to learn about the various disk scheduling algorithms along with its pros and cons in the operating system.
Submitted by Mahak Jain, on November 07, 2018

Let us compare various disk scheduling algorithms:

1. FCFS

In FCFS, the requests are addressed in the sequence they come in the disk queue.

Pros:

  • Simple
  • Not complex
  • Easy to implement
  • No starvation
  • Low overhead
  • No indefinite delay
  • Low overhead

Cons:

  • No preemption possible
  • Low throughput
  • Best services may not be delivered

2. SSTF

In SSTF (Shortest Seek Time First), execution of requests having the shortest seek time takes place first. So, in advance, the seek time of every request is calculated in the queue. They are then scheduled according to their seek time as it is calculated. As a result, the request closer to the disk arm will be executed first. SSTF is certainly better over FCFS because it reduces the average response time and improves the throughput of the system.

Pros:

  • The average time taken for response is reduced
  • Many processes can be processed
  • An increase in throughput

Cons:

  • Starvation
  • Different time is taken for different responses
  • Overhead

3. SCAN

In SCAN algorithm the disk arm travels into a specific way and facilities the requests approaching in its path and after reaching the end of the disk, it converses its direction and again services the request incoming in its path. So, this algorithm works like an elevator and hence also recognized as elevator algorithm. As a result, the requests at the midrange are serviced more and those incoming behind the disk arm will have to wait.

Pros:

  • High throughput
  • Almost similar response times

Cons:

  • Long waiting times

4. CSCAN

In the SCAN algorithm, the disk arm over scans the way that has been scanned, after reversing its path. So, it might be likely that numerous requests are to come at the other end or there may be zero or few requests pending at the scanned area.

These conditions are circumvented in the C-SCAN algorithm in which the disk arm as an alternative of reversing its route goes to the other end of the disk and starts checking the requests from there. So, the disk arm transfers in a circular fashion and this algorithm is also like to SCAN algorithm and hence it is known as C-SCAN (Circular SCAN).

Pros:

  • Provides additional invariable wait time related to SCAN algorithm

5. LOOK

It is like to the SCAN disk scheduling algorithm but the change that the disk arm in spite of going to the end of the disk goes only to the last demand to be checked in front of the head and then backs its direction from there only. Therefore, it avoids the additional postponement which arose due to pointless traversal to the last of the disk.

6. CLOOK

As LOOK is very much similar to SCAN algorithm, in the same way, CLOOK is alike to CSCAN disk scheduling algorithm. In the CLOOK, the disk arm in spite of working to the termination goes only to the last application to be examined in front of the head and then from there moves to the other end’s last request. Thus, it also stops the additional delay which happened due to needless traversal to the last of the disk.






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