Disk Scheduling Algorithms

Disk scheduling algorithms are used by operating systems to determine the order in which disk I/O requests are serviced by the disk drive. These algorithms aim to minimize disk access time and maximize disk throughput by optimizing the movement of the disk’s read/write head. Several disk scheduling algorithms have been developed over the years, each with its characteristics and trade-offs. Some common disk scheduling algorithms include:

  1. FCFS Scheduling Algorithm
  2. SSTF Scheduling Algorithm
  3. SCAN Scheduling Algorithm
  4. C-SCAN Scheduling Algorithm
  5. LOOK Scheduling Algorithm
  6. C-LOOK Scheduling Algorithm

In the below PDF we discuss about Disk Scheduling  in detail in simple language, Hope this will help in better understanding.

Operating System

Importance of Disk Scheduling Algorithms:

Efficient disk scheduling is vital for improving system performance and reducing latency. By optimizing the order in which data requests are serviced, disk scheduling algorithms minimize seek time, rotational latency, and overall disk access time. This directly translates into enhanced throughput and responsiveness, crucial in today’s demanding computing environments.

Types of Disk Scheduling Algorithms:

  1. FCFS disk scheduling algorithm: FCFS (First-Come, First-Served) is the simplest disk scheduling algorithm, servicing requests in the order they arrive. While easy to implement, FCFS can lead to poor performance, especially in scenarios with a mix of short and long requests, as it doesn’t consider the location of data on the disk.
  2. SSTF disk scheduling algorithm: SSTF (Shortest Seek Time First) selects the request with the shortest seek time from the current head position. This algorithm aims to minimize seek time and is more efficient than FCFS in reducing overall access time. However, it may result in starvation for requests located far from the current head position.
  3. SCAN disk scheduling algorithm: The SCAN algorithm, also known as the elevator algorithm, works by servicing requests in a specific direction (e.g., towards the outer tracks) until reaching the end of the disk, then reverses direction. This prevents starvation of requests at one end of the disk but may lead to increased average response time for requests located at the extremes.
  4. C-SCAN (Circular SCAN) disk scheduling algorithm: C-SCAN is a variant of the SCAN algorithm that restricts movement to one direction only, servicing requests in a circular fashion. When the head reaches the end of the disk, it jumps to the opposite end without servicing any requests in between. While this reduces variance in response time, it may lead to increased waiting time for requests located close to the head.
  5. LOOK disk scheduling algorithm: LOOK is similar to SCAN but lacks the “circular” behavior. Instead of moving to the end of the disk, LOOK reverses direction when there are no pending requests in the current direction. This reduces unnecessary head movement compared to SCAN, resulting in slightly better performance.
  6. C-LOOK (Circular LOOK) disk scheduling algorithm: C-LOOK combines the circular behavior of C-SCAN with the efficiency of LOOK. It services requests in a circular manner, like C-SCAN, but reverses direction when there are no pending requests in the current direction, similar to LOOK. This balances response time and throughput effectively.


Disk scheduling algorithms play a crucial role in optimizing data access efficiency and system performance. While each algorithm has its advantages and limitations, selecting the most appropriate one depends on the specific workload characteristics and system requirements. By understanding the principles behind these algorithms, system administrators and developers can make informed decisions to ensure optimal disk performance and responsiveness in their computing environments.

Related Question

Disk scheduling is a technique used by operating systems to determine the order in which disk I/O requests are serviced by the disk drive.

Disk scheduling algorithms optimize the efficiency of disk access by minimizing seek time and rotational latency, thus improving overall system performance.

Seek time is the time taken by the disk arm to move the read/write heads to the track containing the desired data.

Rotational latency is the time taken for the desired sector of the disk to rotate under the disk head after the head has been positioned over the correct track.

Common disk scheduling algorithms include FCFS (First-Come, First-Served), SSTF (Shortest Seek Time First), SCAN (Elevator), C-SCAN (Circular SCAN), LOOK, and C-LOOK.


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