The document discusses three disk scheduling algorithms: SSTF, SCAN, and C-SCAN. SSTF stands for Shortest Seek Time First and services requests based on minimizing seek time. SCAN and C-SCAN are variations that improve on FCFS by serving requests in a single direction around the disk to reduce seek time. C-SCAN additionally ensures uniform wait times for requests.

1. SSTF, SCAN, C-SCAN algorithms
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2. HOME PREVIOUS TOPIC NEXT
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3. Recap
In the last class, you have learnt:
- Disk scheduling its need
– The FCFS disk scheduling algorithm
– Advantages
– Disadvantages
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4. Objectives
On Completion of this period, you would be able to
know :
• SSTF , SCAN, C-SCAN disk scheduling algorithm
• Advantages
• Disadvantages
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5. SSTF Scheduling
• The Shortest-Seek-Time-First Scheduling
– Services the requests at the current head position
– Before moving the head far away
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6. SSTF Scheduling
• Minimal seek time is the criteria for selection of
requests
– Seek time is proportional to the track difference between
requests
– Head is moved to the closest track in the request queue
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7. SSTF Scheduling
• Consider for example an ordered disk queue with
requests involving
– Tracks 98, 183, 37, 122, 14, 124, 65, 67
– Head pointer 53
• The request queue (0-199)
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8. SSTF Scheduling
• If the disk head is initially at cylinder 53,
– it will first move from 53 to 65,
– then to next closest position 67, 37, 14, 98, 122, 124
and 183 as shown in the next slides
• The total head movement of 236 tracks
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9. SSTF Scheduling
Initial head position
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10. SSTF Scheduling
Head movement from 53 to 65th track
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11. SSTF Scheduling
Head movement from 65th to 67th track
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12. SSTF Scheduling
Head movement from 67th to 37th track
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13. SSTF Scheduling
Head movement from 37th track to 14th track
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14. SSTF Scheduling
Head movement 14th track to 98th track
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15. SSTF Scheduling
Head movement from 98th to 122 track
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16. SSTF Scheduling
Head movement from 122 to 124
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17. SSTF Scheduling
Head movement from 124 to 183rd track
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19. SSTF Scheduling
Advantages
• It is substantial improvement over FCFS
• It is a essential form of Shortest Job First
Scheduling
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20. SSTF Scheduling
Disadvantages
• It may cause starvation of some requests
• It is not optimal
• The total head movement can be further reduced
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21. Summary
In this class, you have learnt:
• The SSTF, SCAN,C-SCAN Scheduling
• Advantages
• Disadvantages
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22. Frequently Asked Questions
1. List the advantages and disadvantages of SSTF,SCAN, C-SCAN
algorithms
2. Compare SCAN and C-SCAN scheduling algorithm
3. Explain SSTF scheduling algorithm with the help an example
4. Explain SCAN scheduling algorithm with the help an example
5. Explain C-SCAN scheduling algorithm with the help an example
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23. Quiz
1. The SSTF Scheduling is based on Minimal seek
time criteria for selection of requests [T/F]
TRUE
2. The SSTF Scheduling may cause starvation of some
requests [T/F]
TRUE
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24. Quiz
3. The SCAN Scheduling algorithm is also called
elevator algorithm [T/F]
TRUE
4. The SCAN Scheduling algorithm doesn't provide
uniform wait time
TRUE
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25. Quiz
5. C-SCAN Scheduling provides uniform waiting time
than SCAN Scheduling [T/F]
TRUE
6. C-SCAN Scheduling treats the cylinders as a circular
list that wraps around from the last cylinder to the
first one. [T/F]
TRUE
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26. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
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