Disk scheduling is one of the most calculation-heavy corners of Operating Systems, and also one of the most predictable. Almost every question hands you a queue of cylinder requests and a starting head position, then asks for total head movement, service order, or how many times the head reverses. The scoring skill is mechanical: apply FCFS, SSTF or SCAN carefully and add the distances without arithmetic slips. GATE, UGC NET and PSU papers all draw from this same well.
Below are 12 solved MCQs from KnowledgeGate's published question bank, most of them real previous-year questions with the exam and year noted. Attempt each before checking the answer; the explanation stays short on purpose. The full theory sits in the disk scheduling learn module.
A note on the deep links: where a question is a GATE PYQ carried inside the GATE course, we link its exact solved page. The questions from other recruitment papers live in the same module's practice sets, so for those the module link above is your entry point.
Basics and FCFS versus SSTF
Q1. Moving the read/write head of a disk in or out to position it on the correct track is known as _____. (DSSSB 2021)
(a) waiting time
(b) access time
(c) seek time
(d) latency time
Answer: (c) seek time.
Seek time is the interval spent moving the head to the target track, and it is usually the largest component of disk access time. Rotational latency is the separate wait for the right sector to spin under the head. Because seek dominates, every scheduling algorithm is really trying to cut total head movement.
Q2. Which of the following disk strategies is likely to give the best throughput? *(GATE 1999, see the solved page)*
(a) Farthest cylinder next
(b) Nearest cylinder next
(c) First come first served
(d) Elevator algorithm
Answer: (b) Nearest cylinder next.
Nearest cylinder next is another name for Shortest Seek Time First, which always serves the closest pending request. By minimising head movement per step, it maximises the number of requests served per unit time. The trade-off is possible starvation of far-away requests, but for raw throughput SSTF wins here.
Q3. An operating system loads and executes a single sequential user process at a time using FCFS disk head scheduling. If FCFS is replaced by SSTF, claimed to give 50% better benchmarks, what improvement in I/O performance of user programs is expected? *(GATE 2004, see the solved page)*
(a) 50%
(b) 40%
(c) 25%
(d) 0%
Answer: (d) 0%.
Disk scheduling only helps when a queue of competing requests can be reordered. With a single sequential process there is never more than one outstanding request, so there is nothing to reorder. SSTF and FCFS behave identically, and the improvement is zero.
Q4. Five requests (P, 155), (Q, 85), (R, 110), (S, 30), (T, 115) sit in the queue with the head at cylinder 100 under SSTF. Which statement is FALSE? *(GATE 2020, see the solved page)*
(a) T is serviced before P
(b) Q is serviced after S, but before T
(c) The head reverses its direction of movement between servicing of Q and P
(d) R is serviced before P
Answer: (b) Q is serviced after S, but before T.
Working SSTF from 100 gives the service order R, T, Q, S, P, since the nearest pending request is picked each time. In that order Q actually comes before S and after T, so the claim that Q is serviced after S and before T is wrong. That makes (b) the false statement.
Q5. A disk system has 100 cylinders. Requests arrive for 4, 34, 10, 7, 19, 73, 2, 15, 6, 20 with the head at cylinder 50. If moving one cylinder takes 1 ms and SSTF is used, how long to satisfy all requests? *(GATE 2009, see the solved page)*
(a) 95 ms
(b) 119 ms
(c) 233 ms
(d) 276 ms
Answer: (b) 119 ms.
SSTF from 50 serves the requests in the order 50, 34, 20, 19, 15, 10, 7, 6, 4, 2, 73, always taking the nearest first. Adding the step distances 16, 14, 1, 4, 5, 3, 1, 2, 2 and the final jump of 71 gives 119 cylinders of movement. At 1 ms per cylinder that is 119 ms.
SCAN, C-SCAN and LOOK
Q6. On a disk with 1000 cylinders (0 to 999), the last serviced request was at track 345 and the head is moving toward track 0. Using SCAN, how many tracks must the arm move to serve the FIFO queue 123, 874, 692, 475, 105, 376? (UGC NET 2012)
(a) 2013
(b) 1219
(c) 1967
(d) 1507
Answer: (b) 1219.
Under SCAN the head continues in its current direction to the end of the disk, so it travels from 345 down to cylinder 0, covering 345 tracks. It then reverses and sweeps up to the farthest pending request at 874, covering another 874 tracks. The total is 345 plus 874, which is 1219 tracks.
Seek-time and head-movement numericals
Q7. A disk has 200 tracks (0 to 199). The head is servicing track 120 and the previous request was track 90. Pending requests in arrival order are 30, 70, 115, 130, 110, 80, 20, 25. How many times does the head change direction under SSTF and FCFS? *(GATE 2004, see the solved page)*
(a) 2 and 3
(b) 3 and 3
(c) 3 and 4
(d) 4 and 4
Answer: (c) 3 and 4.
Under SSTF the head repeatedly chases the nearest request, and tracing that path from 120 reverses direction three times. FCFS instead follows the fixed arrival order 30, 70, 115, 130, 110, 80, 20, 25, which zig-zags and reverses four times. So the counts are 3 and 4 respectively.
Q8. A disk head follows SSTF and starts at track 180, with 2048 tracks total and the head free to start anywhere. What is the maximum number of requests such that the head changes direction after servicing every request? *(GATE 2007, see the solved page)*
(a) 9
(b) 10
(c) 11
(d) 12
Answer: (c) 11.
To force a reversal after every service, requests are placed so their distances from the current position form a strictly increasing pattern of alternating sides, roughly doubling each time. With 2048 tracks the largest such alternating chain you can fit holds 11 requests. So the maximum cardinality is 11.
Q9. A disk head follows SSTF starting at track 180. Which request set causes the head to change direction after servicing every request, given that a tie does not change direction and all requests arrive before servicing? *(GATE 2007, see the solved page)*
(a) 11, 139, 170, 178, 181, 184, 201, 265
(b) 10, 138, 170, 178, 181, 185, 201, 265
(c) 10, 139, 169, 178, 181, 184, 201, 265
(d) 10, 138, 170, 178, 181, 185, 200, 265
Answer: (b) 10, 138, 170, 178, 181, 185, 201, 265.
For every service to flip the direction, the nearest request must alternate sides of the head with no equidistant tie ever occurring. Only set (b) keeps the successive nearest requests strictly alternating above and below without a tie that would let the head keep going. The other sets contain a spacing that breaks the alternation.
Q10. With the disk head initially at 32, find the number of disk moves required under FCFS if the queue is 98, 37, 14, 124, 65, 67. (UGC NET 2013)
(a) 310
(b) 324
(c) 320
(d) 321
Answer: (d) 321.
FCFS serves requests strictly in arrival order, so the head goes 32 to 98, then 37, 14, 124, 65 and 67. The step distances are 66, 61, 23, 110, 59 and 2. Adding them gives 321 moves.
Q11. A disk queue holds requests for cylinders 98, 183, 37, 122, 14, 124, 65, 67 and the head starts at 53. Under FCFS, what is the total number of head movements? (BPSC 2024)
(a) 640
(b) 620
(c) 630
(d) More than one of the above
Answer: (a) 640.
FCFS follows the queue as given, so the head moves 53 to 98, then 183, 37, 122, 14, 124, 65 and 67. The distances are 45, 85, 146, 85, 108, 110, 59 and 2. Their sum is 640.
Q12. For the queue 98, 183, 37, 122, 14, 124, 65, 67 with the head at 53 under SSTF, arrange the head positions A. 67, B. 37, C. 65, D. 98, E. 14 in order of traversal. (UGC NET 2026)
(a) A, B, C, D, E
(b) C, A, E, B, D
(c) C, A, B, E, D
(d) A, C, D, B, E
Answer: (c) C, A, B, E, D.
SSTF from 53 picks 65 first (distance 12), which is C, then 67 (distance 2), which is A. Next comes 37, which is B, then 14, which is E, and finally 98, which is D. So the ordering of these labelled positions is C, A, B, E, D.
Where these 12 fit in your preparation
The set mirrors how the topic is examined: the conceptual basics and FCFS versus SSTF questions (Q1 to Q5), the single SCAN sweep that tests end-of-disk behaviour (Q6), and the head-movement and direction-change numericals that dominate every paper (Q7 to Q12). If your totals come out wrong, the culprit is almost always a distance added twice or a missed reversal, so lay out the service order in a single line before summing.
For the theory and more worked sweeps, use the disk scheduling learn module and place the topic in context with our Operating Systems for GATE breakdown. GATE aspirants get the full OS sequence inside GATE Guidance by Sanchit Sir; NET aspirants can start from the NET CS category page. Solve each queue by hand once, then re-solve from memory a week later.
