--- title: "Disk scheduling algorithms with numerical" subject: "Operating System" chapter: 6 key: "semester_6/operating_system:disk-scheduling-algorithms-numerical" source: https://www.bctengineer.com/q/semester_6/operating_system/disk-scheduling-algorithms-numerical site: BCT Engineering --- # Disk scheduling algorithms with numerical Operating System · Chapter 6: I/O Management and Disk Scheduling - Asked in: year not given - Video: [7.8- All Disk Scheduling Algorithm In Operating System | FCFS | SSTF | SCAN | CSCAN | LOOK | CLOOK](https://www.youtube.com/watch?v=uEM7LfP_0mw) — TutorialsSpace- Er. Deepak Garg - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/operating_system/disk-scheduling-algorithms-numerical - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-6 ## Question What is disk scheduling? Explain the disk scheduling algorithms with numerical. ## Short answer Disk scheduling orders pending requests to minimize seek time. For given head 53 moving to 0, queue 98,183,37,122,14,124,65,67: total head movements FCFS 640, SSTF 236, SCAN 236, LOOK 208, C-SCAN 386, C-LOOK 326 cylinders. ## Answer **Disk scheduling** is the method the operating system uses to decide the order in which pending disk I/O requests are served. Its aim is to reduce the total seek time (arm movement), since seek time dominates disk access time (seek + rotational latency + transfer time), and so to improve throughput and fairness. #### Algorithms 1. **FCFS:** serve requests in arrival order. Fair and simple, but the arm may swing wildly across the disk. 2. **SSTF:** serve the request nearest to the head. Low seek time, but far-away requests may starve. 3. **SCAN (elevator):** the arm moves in one direction serving requests, reaches the end of the disk, reverses and serves requests on the way back. No starvation. 4. **C-SCAN:** serve requests in one direction only; on reaching the end, return to the other end without serving, then sweep again. Gives a more uniform waiting time. 5. **LOOK / C-LOOK:** like SCAN / C-SCAN, but the arm goes only as far as the last request in each direction. #### Numerical Disk has cylinders 0–199, head at 53 and moving towards 0, queue = 98, 183, 37, 122, 14, 124, 65, 67. **FCFS:** 53 → 98 → 183 → 37 → 122 → 14 → 124 → 65 → 67 45 + 85 + 146 + 85 + 108 + 110 + 59 + 2 = 640 **SSTF:** 53 → 65 → 67 → 37 → 14 → 98 → 122 → 124 → 183 12 + 2 + 30 + 23 + 84 + 24 + 2 + 59 = 236 **SCAN:** 53 → 37 → 14 → 0 → 65 → 67 → 98 → 122 → 124 → 183 16 + 23 + 14 + 65 + 2 + 31 + 24 + 2 + 59 = 236 **LOOK:** 53 → 37 → 14 → 65 → 67 → 98 → 122 → 124 → 183 16 + 23 + 51 + 2 + 31 + 24 + 2 + 59 = 208 **C-SCAN:** 53 → 37 → 14 → 0 → 199 → 183 → 124 → 122 → 98 → 67 → 65 16 + 23 + 14 + 199 + 16 + 59 + 2 + 24 + 31 + 2 = 386 (return jump 0 → 199 counted) **C-LOOK:** 53 → 37 → 14 → 183 → 124 → 122 → 98 → 67 → 65 16 + 23 + 169 + 59 + 2 + 24 + 31 + 2 = 326 (return jump 14 → 183 counted) | Algorithm | Total head movement | |---|---| | FCFS | 640 | | SSTF | 236 | | SCAN | 236 | | LOOK | 208 | | C-SCAN | 386 | | C-LOOK | 326 | **Answer:** FCFS has the most movement (640) and LOOK the least (208). SSTF and SCAN both give 236. Under heavy load, SCAN/C-SCAN are preferred because they avoid starvation. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does C-SCAN differ from SCAN? A. It serves in both directions B. It serves in one direction only and returns to the start without serving C. It turns back at the last request D. It serves the nearest request first Answer: B. C-SCAN gives more uniform waiting time by serving only in one direction. (Key point: C-SCAN idea) 2. SCAN gives 236 and LOOK 208. Why does LOOK move less? A. It turns at the last request (14) instead of going to cylinder 0 B. It skips cylinder 37 C. It goes to 199 first D. It uses FIFO order Answer: A. LOOK saves the trip to 0 and back, 28 cylinders. (Key point: LOOK vs SCAN) 3. Among FCFS, SSTF, SCAN, LOOK, C-SCAN here, which is best and worst? A. SSTF best, C-SCAN worst B. FCFS best, LOOK worst C. LOOK (208) best, FCFS (640) worst D. SCAN best, SSTF worst Answer: C. Totals: FCFS 640, SSTF 236, SCAN 236, LOOK 208, C-SCAN 386. (Key point: Best and worst) 4. How does C-LOOK differ from C-SCAN? A. It goes to both disk ends B. It serves on the return jump C. It uses nearest-first selection D. The arm goes only as far as the last request in each direction before jumping back Answer: D. C-LOOK jumps from 14 to 183, not through 0 and 199, giving 326 against 386. (Key point: C-LOOK) ## Related in Operating System - [Scheduling algorithms with numerical](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Ascheduling-algorithms-numerical) — Operating System, chapter 2 - [Disk scheduling with 100 cylinders](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-100-cyl-fcfs-sstf-look-cscan-scan) — Operating System, chapter 6, asked 2× - [Disk scheduling, 1000 cylinders, head at 345](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-1000-cyl-fifo-sstf-scan) — Operating System, chapter 6 - [Disk scheduling: FCFS, SSTF, SCAN, LOOK, C-SCAN](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-200-cyl-head-moving-60) — Operating System, chapter 6 - [Scheduling algorithms in interactive systems](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Ainteractive-scheduling-algorithms) — Operating System, chapter 2 - [Disk scheduling: FCFS, SSF, SCAN at 53](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adisk-200-cyl-fcfs-ssf-scan-53) — Operating System, chapter 6, asked 3× ## Related in other subjects - [Preemptive priority scheduling (4, 10, 5 ms)](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Asched-priority-4-10-5-p4) — Embedded System, chapter 6, asked 2× - [Priority and SJF scheduling with P4 and P5](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Asched-priority-and-sjf-5-process) — Embedded System, chapter 6, asked 2× - [Round robin scheduling (5, 8, 7 ms, q=2)](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Asched-rr-5-8-7-q2) — Embedded System, chapter 6