Disk Scheduling (Simulation)

FIFO / FCFS

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(98-53)+(183-98)+(183-37)+(122-37)+(122-14)+(124-14)+(124-65)+(67-65) = total head movement of 640 cylinders

SSTF (Shorter Seek Time First)

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(65-53)+(67-65)+(67-37)+(37-14)+(98-14)+(122-98)+(124-122)+(183-124) = total head movement of 236 cylinders

SCAN

like an elevator scans to one end until the edge and scans to the other end again in reverse
2 directions

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(53-37)+(37-14)+(65-14)+(67-65)+(98-67)+(122-98)+(124-122)+(183-124) = total head movement of 208 cylinders (not counting the head movement to 0?)

CSCAN

scans until the edge and goes back to the start in case any were missed like a type writer
1 direction

FSCAN

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Extra info:

Transfer rate
- Rate at which data flow between drive and computer
Positioning time (random-access time) consists of
- Seek time
  - Time to find the desired cylinder/track
- Rotational Delay
  - Time to find the desired sector

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Practice:

Suppose there is a disk with 100 cylinders numbered 0 to 99. The current position of the head is at 15 and is moving upwards. The following I/O request arrived: 5, 0, 25, 15, 2, 85, 12 and 60. Calculate the number of disk arm / head movements that are required if the disk arm scheduling algorithm used is: i. SSTF (Shortest Seek Time first) ii. FIFO iii. Scan

Answer: