ITPEC FE Morning April 2018 Question 12
What is the average rotational latency of a hard disk drive in milliseconds when its rotation speed is 2000 rotations per minute?
Explanation
15 ms — one rotation takes 30 ms, and on average the data is half a rotation away.
- •One rotation: 60,000 ÷ 2,000 = 30 ms
- •Average rotational latency: 30 ÷ 2 = 15 ms
After the head reaches the track, the wanted sector may be just arriving (no wait) or have just passed (a full rotation), so the average wait is half a rotation.
Answer (b)
Why not others:
- •(d) 30 — the time of one full rotation, the worst-case wait rather than the average
- •(a) 10 and (c) 25 — neither is a whole or half rotation at 2,000 rpm; 10 ms is a third of a rotation and 25 ms five sixths
Key rule: Average rotational latency is half a rotation: 60,000 ÷ R ÷ 2 ms, where R is the speed in rotations per minute.
Computer Component · AI-written, may contain errors
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