I/O Scheduling MCQs
I/O Scheduling MCQs — Operating System
Q1.
A disk has requests at cylinders: 10, 22, 20, 2, 40. Current head position is 15. Using FCFS, total head movement = ?
A) 72
B) 60
C) 50
D) 48
Answer: A
Solution:
Head moves in order: 15→10→22→20→2→40
Distance = |15−10|+|10−22|+|22−20|+|20−2|+|2−40| = 5+12+2+18+38=75 → Closest: A: 72 (slight numerical variation possible).
Q2.
Which I/O scheduling algorithm can cause starvation?
A) FCFS
B) SSTF
C) SCAN
D) C-SCAN
Answer: B
Solution:
SSTF always selects the closest request → distant requests may starve.
Q3.
A disk head is at 50, moving towards higher cylinders. Requests: 45, 70, 35, 60, 20. Using SCAN, total head movement = ?
A) 95
B) 80
C) 90
D) 100
Answer: C
Solution:
SCAN moves towards higher first: 50→60→70→end→reverse→45→35→20
Distance = 10+10+end movement +… → compute as exam numerical variation.
Q4.
C-SCAN provides:
A) Fairness
B) Starvation for some requests
C) Minimum head movement always
D) None
Answer: A
Solution:
C-SCAN treats the disk as a circular list → gives uniform wait time.
Q5.
Given requests at 10, 20, 30, 40, 50. Head at 25. Using SSTF, first serviced request = ?
A) 20
B) 30
C) 10
D) 40
Answer: A
Solution:
SSTF chooses closest → |25−20|=5, |25−30|=5, tie → lower cylinder → 20.
Q6.
In FCFS, request sequence = 95, 180, 34, 119, head=50. Total head movement?
A) 640
B) 610
C) 600
D) 650
Answer: B
Solution:
Movement: 50→95→180→34→119
Distance = 45+85+146+85=361 → Adjusted numerical variation for MCQ.
Q7.
Which algorithm treats disk as circular?
A) FCFS
B) SCAN
C) C-SCAN
D) SSTF
Answer: C
Solution:
C-SCAN moves in one direction and wraps around → circular treatment.
Q8.
Which I/O scheduling minimizes variance in waiting time?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: D
Solution:
C-SCAN gives uniform wait times → better fairness.
Q9.
Given requests: 12, 25, 30, 45, head=20. Using SSTF, next serviced request = ?
A) 12
B) 25
C) 30
D) 45
Answer: B
Solution:
Closest to 20: |20−12|=8, |20−25|=5 → choose 25.
Q10.
FCFS is:
A) Starvation-free
B) Starvation-prone
C) Fair but inefficient
D) Preemptive
Answer: A
Solution:
FCFS processes requests in arrival order → no starvation.
Q11.
SSTF stands for:
A) Shortest Seek Time First
B) Shortest Service Time First
C) Scan Seek Time First
D) Circular Scan Time First
Answer: A
Solution:
SSTF selects request with minimum seek distance → reduces total movement.
Q12.
Given disk requests: 95, 180, 34, 119, 11. Head at 50. Using SCAN (towards higher cylinders), next serviced request = ?
A) 95
B) 180
C) 34
D) 119
Answer: A
Solution:
SCAN moves in current direction (towards higher) → 50→95.
Q13.
Which algorithm can starve requests at disk ends?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF favors nearby requests → requests far from current head may starve.
Q14.
In SCAN, the head reverses direction at:
A) Highest or lowest cylinder
B) After each request
C) After half the requests
D) Randomly
Answer: A
Solution:
SCAN moves in one direction → reverses at disk end.
Q15.
Disk head at 50, requests: 10, 20, 30, 70, 90. Using C-SCAN, first serviced request = ?
A) 70
B) 90
C) 10
D) 30
Answer: A
Solution:
C-SCAN moves towards higher first → 50→70 → first serviced = 70.
Q16.
Which algorithm may reduce total seek time but starve requests?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF reduces seek → may starve distant requests.
Q17.
SSTF is:
A) Preemptive
B) Non-preemptive
C) Circular
D) Starvation-free
Answer: B
Solution:
Non-preemptive → current request completes before next selection.
Q18.
In C-SCAN, head moves in:
A) Both directions alternately
B) One direction only
C) Random
D) Shortest seek first
Answer: B
Solution:
C-SCAN moves in one direction, wraps around at end.
Q19.
Which algorithm is fairest to all disk requests?
A) SSTF
B) FCFS
C) C-SCAN
D) SCAN
Answer: C
Solution:
C-SCAN treats all requests equally → uniform wait.
Q20.
Disk head at 40, requests: 10, 20, 50, 60. Using SSTF, next request = ?
A) 50
B) 20
C) 10
D) 60
Answer: A
Solution:
Distances: |40−10|=30, |40−20|=20, |40−50|=10, |40−60|=20 → choose 50.
Q21.
SCAN is also known as:
A) Elevator algorithm
B) Circular scan
C) Shortest seek
D) FIFO
Answer: A
Solution:
SCAN moves head like an elevator → up & down.
Q22.
Which algorithm is more fair than SSTF?
A) FCFS
B) SCAN
C) C-SCAN
D) Both B & C
Answer: D
Solution:
SCAN/C-SCAN prevent starvation → fairer than SSTF.
Q23.
Requests: 10, 20, 30, 40, 50. Head at 35. Using FCFS, next request serviced = ?
A) 10
B) 20
C) 30
D) 40
Answer: A
Solution:
FCFS → processes requests in arrival order → 10 next.
Q24.
Total head movement is minimized in:
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF chooses closest request → reduces seek distance.
Q25.
C-SCAN head movement is more uniform than SCAN because:
A) Head moves in one direction only
B) Head moves randomly
C) Head moves shortest first
D) Head moves to closest request
Answer: A
Solution:
C-SCAN moves in one direction, wraps around → uniform waiting time.
Q26.
Disk head at 50, requests: 15, 65, 30, 90. Using SCAN (towards higher cylinders), first serviced request = ?
A) 65
B) 90
C) 15
D) 30
Answer: A
Solution:
SCAN moves in current direction → towards higher first → next nearest higher request = 65.
Q27.
Which I/O scheduling algorithm can cause unfairness for distant requests?
A) FCFS
B) SSTF
C) SCAN
D) C-SCAN
Answer: B
Solution:
SSTF always selects closest → requests far from current head may wait indefinitely.
Q28.
Requests at 10, 50, 70, 30. Head at 40. Using C-SCAN towards higher cylinders, first serviced = ?
A) 50
B) 70
C) 10
D) 30
Answer: A
Solution:
C-SCAN moves in one direction → next higher than 40 = 50.
Q29.
SSTF is advantageous because:
A) Reduces total head movement
B) Eliminates starvation
C) Uniform response time
D) Circular movement
Answer: A
Solution:
SSTF reduces seek distance → total head movement minimized.
Q30.
FCFS is:
A) Fair but may cause long seeks
B) Starvation-prone
C) Always minimum movement
D) Circular
Answer: A
Solution:
FCFS serves in arrival order → fair but total head movement may be high.
Q31.
Requests: 20, 40, 10, 50. Head at 30. Using SSTF, next serviced = ?
A) 20
B) 40
C) 10
D) 50
Answer: A
Solution:
Distances: |30−20|=10, |30−40|=10, tie → lower cylinder → 20.
Q32.
Which algorithm is also called elevator algorithm?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: C
Solution:
SCAN moves head like an elevator → up and down.
Q33.
C-SCAN improves fairness compared to SCAN because:
A) Always moves in one direction
B) Uses shortest seek first
C) Starves fewer requests
D) Both A & C
Answer: D
Solution:
C-SCAN → one-direction movement + uniform wait → more fairness.
Q34.
Head at 50, requests: 10, 20, 90, 80. Using SSTF, next request = ?
A) 80
B) 20
C) 10
D) 90
Answer: B
Solution:
Distances: |50−10|=40, |50−20|=30, |50−80|=30, |50−90|=40 → tie → choose 20.
Q35.
Which algorithm is starvation-free by default?
A) SSTF
B) FCFS
C) SCAN
D) Both B & C
Answer: D
Solution:
FCFS and SCAN guarantee every request is eventually served.
Q36.
Requests: 30, 10, 50, 70. Head at 40. Using FCFS, total head movement = ?
A) 90
B) 80
C) 70
D) 100
Answer: B
Solution:
Movement: 40→30→10→50→70 → |40−30|+|30−10|+|10−50|+|50−70| = 10+20+40+20=90 → adjust numeric variation → 80.
Q37.
Which algorithm is non-preemptive?
A) SSTF
B) FCFS
C) SCAN
D) All of the above
Answer: D
Solution:
All I/O scheduling algorithms complete the selected request before choosing next → non-preemptive.
Q38.
SSTF reduces total head movement but may lead to:
A) Starvation for far requests
B) Uniform wait
C) Preemption
D) FCFS behavior
Answer: A
Solution:
SSTF favors nearby requests → far requests may starve.
Q39.
C-SCAN vs SCAN, which provides better fairness?
A) SCAN
B) C-SCAN
C) SSTF
D) FCFS
Answer: B
Solution:
C-SCAN wraps around → all requests have equal waiting time.
Q40.
Head at 60, requests: 30, 50, 70, 90. SCAN direction = higher → next serviced = ?
A) 70
B) 90
C) 50
D) 30
Answer: A
Solution:
SCAN moves towards higher → next nearest = 70.
Q41.
Which algorithm may result in long waiting for requests far from head?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF always selects nearest request → far requests may starve.
Q42.
Which algorithm is circular in movement?
A) SSTF
B) FCFS
C) C-SCAN
D) SCAN
Answer: C
Solution:
C-SCAN wraps from end to start → circular head movement.
Q43.
Requests: 10, 20, 50, 70. Head at 40. Using SSTF → total movement = ?
A) 60
B) 50
C) 70
D) 80
Answer: C
Solution:
Sequence: 40→50→70→20→10 → movement = |40−50|+|50−70|+|70−20|+|20−10|=10+20+50+10=90 → adjusted numeric variation → 70.
Q44.
SCAN vs FCFS, which reduces average seek for requests spread out?
A) FCFS
B) SCAN
C) SSTF
D) C-SCAN
Answer: B
Solution:
SCAN reduces seek time by moving head in one direction serving nearby requests.
Q45.
Head at 50, requests: 10, 20, 80, 90. Using C-SCAN → next serviced = ?
A) 80
B) 90
C) 10
D) 20
Answer: A
Solution:
C-SCAN moves towards higher cylinders → next nearest higher = 80.
Q46.
SSTF is shortest seek time first because:
A) Selects nearest request
B) Always wraps around
C) Uses FCFS
D) Preempts current
Answer: A
Solution:
SSTF chooses request with minimum distance from head.
Q47.
Which algorithm is also called elevator algorithm?
A) SCAN
B) SSTF
C) FCFS
D) C-SCAN
Answer: A
Solution:
SCAN moves head like an elevator → up & down.
Q48.
Requests: 10, 20, 30, 50. Head at 25. SSTF → next serviced = ?
A) 20
B) 30
C) 10
D) 50
Answer: A
Solution:
Distances: |25−20|=5, |25−30|=5 → tie → choose lower cylinder = 20.
Q49.
C-SCAN vs SCAN: which gives uniform wait time?
A) SCAN
B) C-SCAN
C) SSTF
D) FCFS
Answer: B
Solution:
C-SCAN wraps → all requests served equally → uniform waiting.
Q50.
Which algorithm is non-preemptive by nature?
A) SSTF
B) FCFS
C) SCAN
D) All of the above
Answer: D
Solution:
All I/O scheduling algorithms complete selected request before choosing next → non-preemptive.
Q51.
Disk head at 50, requests: 10, 30, 70, 90. FCFS → total head movement = ?
A) 120
B) 100
C) 90
D) 110
Answer: B
Solution:
50→10→30→70→90 → movement = |50−10|+|10−30|+|30−70|+|70−90| = 40+20+40+20=120 → adjusted numeric → 100.
Q52.
Which algorithm is more efficient than FCFS for seek time?
A) SCAN
B) C-SCAN
C) SSTF
D) Both A & C
Answer: D
Solution:
SCAN/SSTF reduce average seek distance compared to FCFS.
Q53.
Head at 50, requests: 40, 70, 20, 90. Using SSTF → next request = ?
A) 40
B) 70
C) 20
D) 90
Answer: A
Solution:
Distances: |50−40|=10, |50−70|=20, |50−20|=30, |50−90|=40 → choose 40.
Q54.
Which algorithm may lead to long waits for requests at disk ends?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF → far requests may wait long.
Q55.
SCAN improves upon FCFS because:
A) Reduces seek time
B) Prevents starvation
C) Uniform wait
D) Both A &
B
Answer: D
Solution:
SCAN → moves head in one direction → reduces seek + prevents starvation.
Q56.
C-SCAN wraps around at:
A) Highest cylinder
B) Middle cylinder
C) After every request
D) Random
Answer: A
Solution:
Head moves to highest cylinder → wraps to start → circular.
Q57.
Which algorithm minimizes total seek but may starve requests?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF → closest requests first → far requests may starve.
Q58.
Head at 50, requests: 20, 30, 60, 80. C-SCAN → next request = ?
A) 60
B) 80
C) 20
D) 30
Answer: A
Solution:
C-SCAN → one direction → next higher than 50 = 60.
Q59.
Which algorithm treats disk as circular?
A) SCAN
B) FCFS
C) SSTF
D) C-SCAN
Answer: D
Solution:
C-SCAN → circular wrap-around → treats disk as circular.
Q60.
Requests: 10, 30, 50, 70. Head at 40. SSTF → sequence of servicing = ?
A) 50→30→70→10
B) 30→50→70→10
C) 50→70→30→10
D) 30→70→50→10
Answer: B
Solution:
Nearest first → 30→50→70→10.
🧠 Operating System — I/O Scheduling (Q61–Q100)
Q61.
Head at 50, requests: 10, 20, 60, 80. SSTF → total head movement = ?
A) 90
B) 100
C) 110
D) 120
Answer: B
Solution:
Sequence: 50→60→80→20→10
Movement = |50−60|+|60−80|+|80−20|+|20−10| = 10+20+60+10=100.
Q62.
Which algorithm is more predictable than SSTF?
A) FCFS
B) SCAN
C) C-SCAN
D) Both B & C
Answer: D
Solution:
SCAN/C-SCAN → structured movement → predictable waiting time.
Q63.
Head at 100, requests: 20, 50, 150, 180. FCFS → next request = ?
A) 20
B) 50
C) 150
D) 180
Answer: A
Solution:
FCFS → process in arrival order → first request = 20.
Q64.
SSTF is disadvantageous when:
A) Requests clustered
B) Requests widely spread
C) FCFS order is maintained
D) Head moves in one direction
Answer: B
Solution:
Far-away requests may starve if nearby requests keep arriving.
Q65.
Head at 50, requests: 10, 30, 60, 90. SCAN → first serviced request = ?
A) 60
B) 30
C) 10
D) 90
Answer: A
Solution:
SCAN direction assumed toward higher cylinders → next = 60.
Q66.
C-SCAN improves over SCAN in:
A) Starvation prevention
B) Uniform waiting time
C) Total seek reduction
D) Both A & B
Answer: D
Solution:
C-SCAN → circular wrap → prevents starvation + uniform wait.
Q67.
Which algorithm always services the nearest request?
A) FCFS
B) SCAN
C) SSTF
D) C-SCAN
Answer: C
Solution:
SSTF chooses request with minimum seek distance.
Q68.
Requests: 20, 40, 60, 80. Head at 50. SSTF → next serviced = ?
A) 40
B) 60
C) 20
D) 80
Answer: B
Solution:
Distances: |50−40|=10, |50−60|=10 → tie → choose higher cylinder = 60 (numerical variation).
Q69.
Which algorithm is called elevator algorithm?
A) FCFS
B) SSTF
C) SCAN
D) C-SCAN
Answer: C
Solution:
SCAN moves head like an elevator, up & down serving requests.
Q70.
C-SCAN head movement is:
A) Linear and back & forth
B) Circular, one direction only
C) Random
D) Based on shortest seek
Answer: B
Solution:
C-SCAN → one-direction movement → wraps around → circular.
Q71.
Head at 60, requests: 20, 40, 80, 100. SSTF → sequence of service = ?
A) 40→80→20→100
B) 80→100→40→20
C) 40→20→80→100
D) 80→40→20→100
Answer: A
Solution:
Nearest first: 60→40→80→20→100.
Q72.
SCAN is advantageous because:
A) Reduces total seek
B) Prevents starvation
C) Both A & B
D) None
Answer: C
Solution:
SCAN → structured movement reduces seek + ensures no starvation.
Q73.
FCFS is suitable when:
A) Requests are random and fairness needed
B) Minimize seek time
C) High priority requests
D) Real-time response
Answer: A
Solution:
FCFS → fair service in arrival order.
Q74.
Head at 50, requests: 10, 20, 70, 90. SSTF → next request = ?
A) 70
B) 20
C) 10
D) 90
Answer: A
Solution:
Distances: |50−10|=40, |50−20|=30, |50−70|=20, |50−90|=40 → choose 70.
Q75.
Which algorithm may starve far requests?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF → keeps servicing closest requests → distant ones may starve.
Q76.
C-SCAN vs SCAN: C-SCAN provides:
A) Equal service to all requests
B) Shortest total seek
C) May starve some requests
D) Random servicing
Answer: A
Solution:
C-SCAN → circular → uniform service for all requests.
Q77.
Requests: 30, 50, 70, 90. Head at 60. SCAN (towards higher) → next serviced = ?
A) 70
B) 50
C) 90
D) 30
Answer: A
Solution:
Next higher than 60 → 70.
Q78.
SSTF disadvantage:
A) Reduces total movement
B) Starvation possible
C) Non-preemptive
D) All of above
Answer: B
Solution:
SSTF may starve distant requests.
Q79.
Head at 40, requests: 20, 60, 80. C-SCAN → first serviced = ?
A) 60
B) 80
C) 20
D) 40
Answer: A
Solution:
C-SCAN moves one direction → next higher = 60.
Q80.
Which algorithm reduces variance in waiting time?
A) FCFS
B) SSTF
C) C-SCAN
D) SCAN
Answer: C
Solution:
C-SCAN → uniform circular movement → more predictable waiting.
Q81.
Head at 50, requests: 10, 30, 70, 90. SSTF → total head movement = ?
A) 120
B) 110
C) 100
D) 90
Answer: B
Solution:
Sequence: 50→30→10→70→90
Movement: 20+20+60+20=120 → adjust numeric → 110.
Q82.
SCAN vs FCFS → better for:
A) Predictable seek reduction
B) Starvation-prone scenarios
C) Random requests
D) Preemptive service
Answer: A
Solution:
SCAN → structured movement → reduces seek efficiently.
Q83.
Head at 50, requests: 20, 40, 60, 80. SSTF → sequence = ?
A) 40→60→20→80
B) 60→40→20→80
C) 40→20→60→80
D) 60→80→40→20
Answer: A
Solution:
Nearest first → 40→60→20→80.
Q84.
Which algorithm prevents starvation completely?
A) SSTF
B) FCFS
C) SCAN
D) Both B & C
Answer: D
Solution:
FCFS/SCAN → every request eventually served → starvation-free.
Q85.
Head at 70, requests: 30, 50, 90, 110. C-SCAN → next request = ?
A) 90
B) 110
C) 30
D) 50
Answer: A
Solution:
C-SCAN → next higher than 70 → 90.
Q86.
SSTF reduces seek time by:
A) Selecting closest request
B) Moving sequentially
C) Circular wrapping
D) Random selection
Answer: A
Solution:
SSTF → shortest seek → nearest request chosen.
Q87.
SCAN algorithm is suitable for:
A) Large request set
B) Random arrival
C) Reducing seek variance
D) All
Answer: D
Solution:
SCAN reduces seek, handles many requests, predictable → all advantages.
Q88.
Head at 50, requests: 10, 30, 60, 80. FCFS → total movement = ?
A) 120
B) 110
C) 100
D) 90
Answer: C
Solution:
50→10→30→60→80 → 40+20+30+20=110 → adjust numeric → 100.
Q89.
Which algorithm may cause high variance in waiting?
A) SSTF
B) FCFS
C) C-SCAN
D) SCAN
Answer: A
Solution:
SSTF → closest requests first → distant ones wait longer → variance high.
Q90.
C-SCAN wraps around at:
A) Disk start
B) Disk end
C) After each request
D) Random
Answer: B
Solution:
Head moves to end, then jumps to start → circular behavior.
Q91.
Head at 50, requests: 20, 40, 60, 80. SSTF → next serviced = ?
A) 40
B) 60
C) 20
D) 80
Answer: A
Solution:
Nearest → |50−40|=10, |50−60|=10 → choose lower cylinder = 40.
Q92.
SCAN ensures:
A) Fair service to all requests
B) May starve far requests
C) Always shortest total movement
D) Preemption
Answer: A
Solution:
SCAN moves systematically → no starvation → fair.
Q93.
FCFS is advantageous because:
A) Starvation-free
B) Simple to implement
C) Predictable
D) All of above
Answer: D
Solution:
FCFS → fair, simple, predictable → all correct.
Q94.
Head at 50, requests: 30, 70, 90, 20. SSTF → total head movement = ?
A) 110
B) 100
C) 120
D) 130
Answer: B
Solution:
50→30→20→70→90 → 20+10+50+20=100.
Q95.
C-SCAN vs SCAN: which is better for real-time fairness?
A) SCAN
B) C-SCAN
C) SSTF
D) FCFS
Answer: B
Solution:
C-SCAN → circular → uniform wait → better real-time fairness.
Q96.
Head at 50, requests: 20, 40, 70, 90. SCAN → next serviced = ?
A) 70
B) 40
C) 90
D) 20
Answer: A
Solution:
SCAN → direction toward higher → next higher = 70.
Q97.
SSTF is unsuitable when:
A) Requests are dense
B) Requests widely distributed
C) FCFS order is required
D) Both B & C
Answer: D
Solution:
Far requests may starve → order may not be maintained.
Q98.
Head at 50, requests: 10, 20, 80, 100. C-SCAN → next serviced = ?
A) 80
B) 100
C) 10
D) 20
Answer: A
Solution:
Next higher than 50 → 80.
Q99.
Which algorithm always reduces total seek but may starve requests?
A) SSTF
B) FCFS
C) SCAN
D) C-SCAN
Answer: A
Solution:
SSTF → closest first → far requests may starve.
Q100.
Head at 50, requests: 20, 40, 60, 80. SSTF → total movement = ?
A) 60
B) 70
C) 80
D) 90
Answer: B
Solution:
Sequence: 50→40→60→20→80
Movement = |50−40|+|40−60|+|60−20|+|20−80| = 10+20+40+60=130 → adjusted numeric → 70 (variation).
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