Modern operating systems handle multiple input\/output requests at the same time. When several processes request access to data stored on a disk, the system must decide which request should be served first. Without proper scheduling, disk movement increases, access time becomes slower, and overall system performance drops. This is where disk scheduling algorithms help. They optimize the movement of the disk head, reduce waiting time, and improve storage efficiency.<\/p>\n\n\n\n
Read this blog to understand disk scheduling algorithms, their types, advantages, limitations, and real-world applications.<\/p>\n\n\n\n
Quick Answer: <\/strong><\/p>\n\n\n\n Disk scheduling algorithms are techniques used by operating systems to decide the order in which disk I\/O requests are processed. They help reduce seek time, improve disk performance, and manage multiple read\/write requests efficiently. Common disk scheduling algorithms include FCFS, SSTF, SCAN, C-SCAN, LOOK, and C-LOOK.<\/p>\n\n\n\n Disk Scheduling Algorithms in operating systems are scheduling techniques used to determine the sequence in which pending disk I\/O requests are serviced. Since magnetic disks involve mechanical head movement, the operating system aims to minimize seek time, rotational latency, and access delay. These algorithms optimize disk head movement across tracks to improve throughput, reduce response time, prevent starvation, and enhance overall storage subsystem performance under multi-process workloads.<\/p>\n\n\n\n Go beyond learning operating system concepts and build industry-ready software expertise with HCL GUVI\u2019s AI-Powered <\/em>Software Development Course<\/em><\/a>. Designed for graduates and working professionals to master software development and Generative AI, the program helps you become SDE-ready for top product-based companies. Earn 4 certifications from MongoDB, IITM Pravartak, NSDC, and HCL GUVI, and learn through an industry-focused curriculum with MongoDB as the Official Program Partner.<\/em><\/p>\n\n\n\n The First Come First Serve (FCFS) algorithm is a vital concept in operating systems<\/a>. It processes disk requests strictly in the order they arrive in the request queue. It follows a queue-based approach where the earliest request receives service first.<\/p>\n\n\n\n For example:<\/p>\n\n\n\n Request Queue: 98, 183, 37, 122 The head moves:<\/p>\n\n\n\n 53 \u2192 98 \u2192 183 \u2192 37 \u2192 122<\/p>\n\n\n\n Shortest Seek Time First selects the request closest to the current disk head position. Instead of servicing requests in arrival order, it prioritizes minimum head movement.<\/p>\n\n\n\n Example:<\/p>\n\n\n\n Current head: 53<\/p>\n\n\n\n Request Queue:<\/p>\n\n\n\n 98, 183, 37, 122, 14<\/p>\n\n\n\n Distance calculations:<\/p>\n\n\n\n The algorithm services track 37 first.<\/p>\n\n\n\n SCAN moves the disk head in one direction while servicing all requests encountered along its path. Once it reaches the disk boundary, the head reverses direction and services requests in the opposite direction.<\/p>\n\n\n\n Because the movement resembles an elevator, SCAN is commonly called the Elevator Algorithm.<\/p>\n\n\n\n Example:<\/p>\n\n\n\n Head position: 53<\/p>\n\n\n\n Direction: Increasing<\/p>\n\n\n\n Queue:<\/p>\n\n\n\n 14, 37, 65, 98, 122, 183<\/p>\n\n\n\n Movement:<\/p>\n\n\n\n 53 \u2192 65 \u2192 98 \u2192 122 \u2192 183 \u2192 end \u2192 37 \u219214<\/p>\n\n\n\n C-SCAN improves SCAN by servicing requests in only one direction. When the head reaches one end of the disk, it immediately returns to the opposite end without servicing requests during the return movement.<\/p>\n\n\n\n This creates circular servicing behavior.<\/p>\n\n\n\n Example:<\/p>\n\n\n\n Head movement:<\/p>\n\n\n\n 53 \u2192 65 \u2192 98 \u2192122 \u2192183 \u2192 end \u2192 beginning \u219214 \u219237<\/p>\n\n\n\n LOOK improves SCAN by preventing unnecessary movement to the physical end of the disk. Instead of moving to the extreme boundary, the disk head only travels as far as the last request in that direction.<\/p>\n\n\n\n The algorithm “looks” ahead before moving further.<\/p>\n\n\n\n Example:<\/p>\n\n\n\n Queue:<\/p>\n\n\n\n 14,37,65,98,122<\/p>\n\n\n\n Head:<\/p>\n\n\n\n 53<\/p>\n\n\n\n Movement:<\/p>\n\n\n\n 53 \u219265\u219298\u2192122 \u219237\u219214<\/p>\n\n\n\n Unlike SCAN, it does not move to the disk boundary.<\/p>\n\n\n\n C-LOOK combines C-SCAN and LOOK principles. The disk head moves only in one direction and stops at the last request rather than reaching the physical disk boundary.<\/p>\n\n\n\n After servicing the last request, the head jumps directly to the first pending request.<\/p>\n\n\n\n Example:<\/p>\n\n\n\n 53 \u219265\u219298\u2192122 \u219214\u219237<\/p>\n\n\n\n C-LOOK often provides better performance than C-SCAN because it avoids unnecessary travel.<\/p>\n\n\n\n N-Step SCAN divides incoming requests into smaller subqueues of fixed size N. Each queue is processed independently using the SCAN algorithm.<\/p>\n\n\n\n This prevents newly arriving requests from continuously affecting the scheduling order.<\/p>\n\n\n\n FSCAN uses two separate request queues:<\/p>\n\n\n\n While one queue undergoes processing using SCAN, newly arriving requests enter the second queue.<\/p>\n\n\n\n After completion, queue roles are swapped.<\/p>\n\n\n\n Disk scheduling algorithms are an important part of operating system performance because they determine how disk I\/O requests are serviced efficiently. From FCFS and SSTF to SCAN and C-LOOK, each algorithm follows a different strategy to reduce seek time and improve throughput. Understanding their working principles, trade-offs, and real-world applications helps build a stronger foundation in operating systems and storage management concepts.<\/p>\n\n\n\n Disk arm stickiness occurs when the disk head repeatedly services nearby requests and remains concentrated around a specific region of the disk, delaying distant requests.<\/p>\n\n<\/div>\n<\/div>\n N-Step SCAN and FSCAN are used to manage large request queues efficiently and prevent newly arriving requests from continuously interfering with ongoing scheduling operations.<\/p>\n\n<\/div>\n<\/div>\n Disk scheduling has less impact on SSDs because SSDs do not use mechanical disk heads. However, operating systems still apply scheduling techniques for queue management and workload optimization.<\/p>\n\n<\/div>\n<\/div>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":" Modern operating systems handle multiple input\/output requests at the same time. When several processes request access to data stored on a disk, the system must decide which request should be served first. Without proper scheduling, disk movement increases, access time becomes slower, and overall system performance drops. This is where disk scheduling algorithms help. 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What are Disk Scheduling Algorithms in OS?<\/strong><\/h2>\n\n\n\n
Key Characteristics of Disk Scheduling Algorithms<\/strong><\/h2>\n\n\n\n
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Goals of Disk Scheduling Algorithms<\/strong><\/h2>\n\n\n\n
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Types of Disk Scheduling Algorithms<\/strong><\/h2>\n\n\n\n
1. First Come First Serve (FCFS) Disk Scheduling Algorithm<\/strong><\/h3>\n\n\n\n
Working:<\/strong><\/h4>\n\n\n\n
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Initial Head Position: 53<\/p>\n\n\n\nAdvantages:<\/strong><\/h4>\n\n\n\n
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Limitations:<\/strong><\/h4>\n\n\n\n
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2. Shortest Seek Time First (SSTF) Algorithm<\/strong><\/h3>\n\n\n\n
Working:<\/strong><\/h4>\n\n\n\n
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Advantages:<\/strong><\/h4>\n\n\n\n
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Limitations:<\/strong><\/h4>\n\n\n\n
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3. SCAN Disk Scheduling Algorithm<\/strong><\/h3>\n\n\n\n
Working:<\/strong><\/h4>\n\n\n\n
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Advantages:<\/strong><\/h4>\n\n\n\n
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Limitations:<\/strong><\/h4>\n\n\n\n
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4. Circular SCAN (C-SCAN)<\/strong><\/h3>\n\n\n\n
Working:<\/strong><\/h4>\n\n\n\n
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Advantages:<\/strong><\/h4>\n\n\n\n
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Limitations:<\/strong><\/h4>\n\n\n\n
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5. LOOK Disk Scheduling Algorithm<\/strong><\/h3>\n\n\n\n
Working:<\/strong><\/h4>\n\n\n\n
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Advantages:<\/strong><\/h4>\n\n\n\n
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Limitations:<\/strong><\/h4>\n\n\n\n
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6. Circular LOOK (C-LOOK)<\/strong><\/h3>\n\n\n\n
Working:<\/strong><\/h4>\n\n\n\n
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Advantages:<\/strong><\/h4>\n\n\n\n
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Limitations:<\/strong><\/h4>\n\n\n\n
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7. N-Step SCAN Algorithm<\/strong><\/h3>\n\n\n\n
Advantages:<\/strong><\/h4>\n\n\n\n
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8. FSCAN Algorithm<\/strong><\/h3>\n\n\n\n
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Advantages:<\/strong><\/h4>\n\n\n\n
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Real-World Applications of Disk Scheduling Algorithms<\/strong><\/h2>\n\n\n\n
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Conclusion<\/strong><\/h2>\n\n\n\n
FAQs<\/strong><\/h2>\n\n\n
What is disk arm stickiness in disk scheduling?<\/strong><\/h3>\n
Why are N-Step SCAN and FSCAN used?<\/strong><\/h3>\n
Does disk scheduling matter in SSDs?<\/strong><\/h3>\n