Advantages and Disadvantages of various Disk scheduling algorithms

Prerequisite – Disk Scheduling Algorithms

1. First Come First Serve (FCFS) :
Advantages –

  1. First Come First Serve algorithm has a very simple logic, it executes the process requests one by one in the sequence they arrive.
  2. Thus, First Come First Serve is very simple and easy to understand and implement.
  3. In FCFS eventually, each and every process gets a chance to execute, so no starvation occur.

Disadvantages –

  1. This scheduling algorithm is nonpreemptive, which means the process can’t be stopped in middle of execution and will run it’s full course.
  2. FCFS being a nonpreemptive scheduling algorithm, the short processes which are at the back of the queue have to wait for the long process at the front to finish
  3. The throughput of FCFS is not very efficient.
  4. FCFS is implemented on small systems only where input-output efficiency is not of utmost importance.

2. Shortest Seek Time First (SSTF) :
Advantages –

  1. The total seek time is reduced compared to First Come First Serve.
  2. SSTF improves and increases throughput.
  3. Less average waiting time and response time in SSTF.

Disadvantages –



  1. In SSTF there is an overhead of finding out the closest request.
  2. Starvation may occur for requests far from head.
  3. In SSTF high variance is present in response time and waiting time.
  4. Frequent switching of the Head’s direction slows the algorithm.

3. SCAN :
Advantages –

  1. Scan scheduling algorithm is simple and easy to understand and implement.
  2. Starvation is avoided in SCAN algorithm.
  3. Low variance Occurs in waiting time and response time.

Disadvantages –

  1. Long waiting time occurs for the cylinders which are just visited by the head.
  2. In SCAN the head moves till the end of the disk despite the absence of requests to be serviced.

4. C-SCAN :
Advantages –

  1. C-SCAN Algorithm is the successor and the improved version of the SCAN scheduling Algorithm.
  2. The Head move from one end to the other of the disk while serving all the requests in between.
  3. The waiting time for the cylinders which were just visited by the head is reduced in C-SCAN compared to the SCAN Algorithm.
  4. Uniform waiting time is provided.
  5. Better response time is provided.

Disadvantages –

  1. More seek movements are caused in C-SCAN compared to SCAN Algorithm.
  2. In C-SCAN even if there are no requests left to be serviced the Head will still travel to the end of the disk unlike SCAN algorithm.

5. LOOK :
Advantages –

  1. If there are no requests left to be services the Head will not move to the end of the disk unlike SCAN algorithm.
  2. Better performance is provided compared to SCAN Algorithm.
  3. Starvation is avoided in LOOK scheduling algorithm.
  4. Low variance is provided in waiting time and response time.

Disadvantages –

  1. Overhead of finding the end requests is present.
  2. Cylinders which are just visited by Head have to wait for long time.

6. C-LOOK :
Advantages –

  1. In C-LOOK the head does not have to move till the end of the disk if there are no requests to be serviced.
  2. There is less waiting time for the cylinders which are just visited by the head in C-LOOK.
  3. C-LOOK provides better performance when compared to LOOK Algorithm.
  4. Starvation is avoided in C-LOOK.
  5. Low variance is provided in waiting time and response time.

Disadvantages –

  1. In C-LOOK an overhead of finding the end requests is present.

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