Question

Are problems or shortanswer questions. Name and describe three disk scheduling algorithms.

Short answer

FCFS processes requests in order received, SSTF chooses the nearest request, and SCAN services requests in one sweep before reversing direction.

Step-by-step solution

Understand Disk Scheduling Algorithms

Disk scheduling algorithms are used to decide the order in which disk I/O requests will be serviced. Different algorithms have varying ways of choosing which requests to process first to optimize performance.

First-Come, First-Served (FCFS)

This is the simplest disk scheduling algorithm. Requests are handled in the order they arrive in the queue. It can be inefficient because it does not consider the current position of the read/write head or the distances to requested sectors.

Shortest Seek Time First (SSTF)

SSTF selects the request with the shortest seek time from the current head position. This reduces the total movement of the read/write head but may lead to starvation of some requests located further away.

Elevator Algorithm (SCAN)

The SCAN algorithm moves the disk arm toward the end of the disk, servicing requests in one direction until it reaches the end, then reverses direction. This method is also known as the Elevator Algorithm because of its back-and-forth movement, similar to an elevator.

Key concepts

First-Come, First-Served (FCFS)

When we talk about the First-Come, First-Served (FCFS) disk scheduling algorithm, we are referring to the most straightforward approach to handling disk I/O requests. Imagine a line at a grocery store where each customer is served in the order they arrive. FCFS works under a similar principle. The requests are queued up and taken care of one by one, based purely on arrival time. This simplicity can be beneficial in terms of implementation ease and predictability. However, it comes at a cost.

The drawback of FCFS is that it does not take into account the location of the read/write head. This might result in long waiting times, especially for requests situated at long distances from each other, causing what is known as the "convoy effect." A lengthy task can hold up the process, delaying subsequent requests even if they require minimal head movement. Efficient FIFO mechanisms would thus need to address this weakness to improve response time.

Shortest Seek Time First (SSTF)

The Shortest Seek Time First (SSTF) algorithm works by selecting the I/O request that requires the least movement from the current position of the disk head. This decision-making process makes SSTF efficient in minimizing the mechanical seek time of the disk head, thereby improving overall throughput.

By choosing the nearest requests, SSTF effectively reduces total seek time when compared to FCFS. However, there's an inherent risk. Requests may experience starvation if they are consistently far from the current head position. Imagine waiting for a bus that stops for passengers closer along its route; those far out may have to wait indefinitely if nearer requests keep arising. Clever implementation of SSTF may include future-read algorithms to counteract such starvation scenarios.

Elevator Algorithm (SCAN)

The Elevator Algorithm, or SCAN, functions much like the operation of an elevator in a building. The disk arm moves in one direction and processes all requests until it reaches the end of that direction. Once it hits the end, it reverses and proceeds in the opposite direction, picking up requests on its way back.

This back-and-forth motion ensures that no requests are left unserviced for an inconveniently long time. SCAN aims to systematically cover all requests, thereby providing a more balanced queue management. While avoiding starvation issues apparent in SSTF, SCAN might still suffer from long wait times as requests coming just after the arm has passed their direction must wait for the entire return journey. Some variations like the Circular SCAN (C-SCAN) can be employed to address this, always providing a comprehensive method for handling I/O requests effectively.