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An application loads $100$ libraries at startup. Loading each library requires exactly one disk access. The seek time of the disk to a random location is given as $10$ ms. Rotational speed of disk is $6000$ rpm. If all $100$ libraries are loaded from random locations on the disk, how long does it take to load all libraries? (The time to transfer data from the disk block once the head has been positioned at the start of the block may be neglected.) 

  1. $0.50 \ s$
  2. $1.50 \ s$
  3. $1.25 \ s$
  4. $1.00 \ s$
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$\text{Disk access time} =\text{Seek time}+\text{Rotational latency}+\text{Transfer time}$ (given that transfer time is neglected)
$\text{Seek time}=10 \text{ ms}$
$\text{Rotational speed}=6000 \text{ rpm}$

  • $60\;s \to 6000 \text{ rotations}$
  • $1 \text{ rotation}\to 60/6000 \;s$
  • $\text{Rotational latency} =1/2 \times 60/6000 \;s =5 \text{ ms}$

Total time to transfer one library $=10+5=15 \text{ ms}$
$\therefore$ Total time to transfer $100$ libraries $=100 \times 15 \text{ ms}=1.5 s$

Correct Answer: $B$

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