Question

Consider a magnetic disk that has 1,764 cylinders, 500 sectors per track (on average), 0.07 milliseconds minimum seek time, and 5,376 rpm rotational speed. Assume that the disk heads have a constant speed. Each sector is 512 bytes, and each block is four sectors.

What is the average rotational delay in milliseconds (rounded to no decimal places)?

Answer 1

Average rotational delay is nothing but half the time taken by a disk to complete one revolution.

As per the details given in the question, the disk is rotating at 5376 RPM.
Using the unitary method to find the rotational delay:

• 5376 rotations in 1 minute.
• 5376 rotations $\rightarrow$ 1 minute  
• 5376 rotations $\rightarrow$ 60 seconds
• 1 rotation $\rightarrow$ (60 seconds) / 5376
• 1 rotation $\rightarrow$ (60 x 1000 milliseconds) / 5376
• 1 rotation $\rightarrow$ 11.16 milliseconds

Therefore avg. rotational delay = half the time taken by a disk to complete one revolution
= 1/2 x 11.16 milliseconds = 5.58 = 6 milliseconds (rounded figure)