DMA Q2

Question

The MegaGiga hard disk rotates at $10000$ rpm ($6$ ms/rot) with a seek time given by = $1+0.001$tmsec, where $t$ is the number of tracks the arm seeks. Assume a block size of $512$ bytes, $1024$ sector/track, $8192$ tracks, and $4$ platters. The disk has a $16$ MB track buffer. The disk controller can $\textsf{DMA}$ read or write data between memory and the disk device at a rate of $100 \textsf{ MB/sec}$.

Estimate the worst case delay to read $512$ bytes from this disk.

1. $25$ms
2. $15.2$ms
3. $14.9$ms
4. $26.3$ms

Answer 1

best match is b . 15.2  we have to calculate the worst case time delay and worst case will happen when everything will be maximum,.

so it will be max seektime + max rotaional latency + maximum transfer time

transfer time here include both preparation time and then dma transfer time .

maximum seek time will be when i have to cross all the tracks = 1+0.001*8192= 9.192ms

maximum rotational latency = total time to take one round only. as in gneral case we divide with 2 . that is because on an average the disk has to move half. 2 is for the average, but here we have not to consider the average case . so it will be time to take one round . which will be equal to 6 ms.

now the transfer will be like.  first the data will be buffered in the buffer and when 512 bytes wll be ready dma will be called to transfer it to the memory .

so disk transfer rate will be .

one round = 6 ms

512*1024 bytes = 6ms

512 bytes = 0.058 ms.

now the data is available in the buffer now time taken by the dma

512 bytes / 100MB = negligible .

total time will be 9.192+6+0.058 = 15.25 ms option b .

Comments

well explained..thanks

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"the disk has a 16mB track buffer " what does it mean ?

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means it has a buffer .so first during preparation time data will be stored there. what i think they wanted to frame was is . that if the buffer cannot be non sequentially accessed . then in worst case the data will be at the end of the buffer and the dma first have to transffer whole data before it.

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if it is the right answer and it matches . then choose as the answer.