Disk Scheduling

Disk scheduling is the technique utilized by computer operating systems to determine the sequence in which block I/O operations are dispatched to storage volumes. This process, also known as I/O scheduling, orchestrates the order of disk operations to optimize performance and resource utilization.

Important Terms related to Disk Scheduling Algorithms

Seek Time: As we know, the data may be stored on various blocks of disk. To access these data according to the request, the disk arm moves and finds the required block. The time taken by the arm in doing this search is known as "Seek Time".
Rotational Latency: The required data block needs to move at a particular position from where the read/write head can fetch the data. So, the time taken in this movement is known as "Rotational Latency". This rotational time should be as less as possible so, the algorithm that will take less time to rotate will be considered a better algorithm.
Transfer Time: When a request is made from the user side, it takes some time to fetch these data and provide them as output. This time taken is known as "Transfer Time".
Disk Access Time: It is defined as the total time taken by all the above processes. Disk access time = (seek time + rotational latency time + transfer time)
Disk Response Time: The disk processes one request at a single time. So, the other requests wait in a queue to finish the ongoing process of request. The average of this waiting time is called "Disk Response Time".

Disk Scheduling Algorithms

Visualise and compare disk scheduling techniques interactively

FCFS - First Come First Serve

In this algorithm, incoming requests are added to the queue's end, and the next number in the queue is served next. However, this approach doesn't yield optimal results. To calculate the number of head movements, one simply counts the tracks traversed from one request to the next.

SSTF - Shortest Seek Time First

The Shortest Seek Time First (SSTF) algorithm prioritizes disk I/O requests that demand the least arm movement from the current position, irrespective of direction. This strategy minimizes overall seek time compared to FCFS, enabling the head to advance to the nearest track in the service queue.

SCAN - Elevator Algorithm

This algorithm involves the disk arm moving in a single direction until reaching the end, servicing all requests along the way, then reversing direction to satisfy requests in the opposite path. This behavior is akin to an elevator's operation, hence its nickname, the elevator algorithm.

C-SCAN - Circular SCAN

In the C-SCAN algorithm, the disk arm travels in one direction, servicing requests until it reaches the last cylinder. Then, without servicing any requests, it jumps to the last cylinder in the opposite direction. Subsequently, it reverses direction and begins servicing the remaining requests.

LOOK - Modified SCAN

Similar to the SCAN algorithm, this method halts the disk arm's movement inward or outward when no further requests exist in that direction. Unlike SCAN, it mitigates the overhead by avoiding unnecessary movement of the disk arm in a direction where no requests are present.

C-LOOK - Circular LOOK

The C-Look Algorithm shares similarities with the C-SCAN algorithm. Here, the disk arm travels outward, servicing requests until reaching the highest requested cylinder. It then jumps to the lowest requested cylinder without servicing any requests before resuming outward movement to fulfill the remaining requests.

LIFO - Last In First Out

In the Last In, First Out (LIFO) algorithm, the most recent jobs are prioritized for service over existing ones. This means that the job that was most recently entered or added to the queue is the first to be serviced, followed by the others in the same order.

RSS - Random Seek Scheduling

The Random Seek Scheduling algorithm, as its name implies, is inherently random in nature. It finds utility in scenarios where scheduling encompasses random elements like processing time, due dates, weights, and unpredictable machine breakdowns. This algorithm is commonly employed in analysis and simulation tasks due to its adaptability to such stochastic environments.