Q4 6 marks) Suppose that a 2MiB file consisting of 512-byte logical blocks is st
ID: 3593156 • Letter: Q
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Q4 6 marks) Suppose that a 2MiB file consisting of 512-byte logical blocks is stored on a disk drive with the following characteristics: Parameter Rotational rate T? Value 12,000 RPM 6ms 1000 avg seek Average # sectors/track Surfaces Sector size 512 bytes (a) Best case: Estimate the optimal time (in ms) required to read the file over all possible mappings of logical blocks to disk sectors. Justify your answer. (b) Random case: Estimate the time (in ms) required to read the file if blocks are mapped randomly to disk sectors. Justify your answer.Explanation / Answer
Answer 4:
Based on given information, the time for one revolution is (1000 × 60) / 12000 RPM = 5 ms.
So that,average rotational latency = 3ms.
2 MB file needs 4000 logical blocks.
4a) In the best case, the 4000 blocks on same cylinder in contiguous sectors.
Once the head rotates to correct position, it will take 4 full rotations because given that 1000 sectors per rotations.
So the total time =(averageseek time) + (average rotational latency) + (transfer time [4 full rotations])
=>6ms + 3ms + 4*5 ms
=>6+3+20 ms
= 29ms.
4b) For each block, it need to move the head, and wait for sector to come under the head.
Hence,
4000 × ((average seek time) + (average rotational latency) + (transfer time [1/1000]
= 4000 × (6ms + 3ms + 0.006ms) = 36024 ms = 36 seconds!
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