cengizhan cengizhan - 27 days ago 13
C Question

Why is buffered I/O faster than unbuffered I/O

While reading this, I found a reasonable answer, which says:


Case 1: Directly Writing to File On Disk

100 times x 1 ms = 100 ms



I understood that. Next,


Case 3: Buffering in Memory before Writing to File on Disk

(100 times x 0.5 ms) + 1 ms = 51 ms



I didn't understand the 1 ms. What is the difference in between writing 100 data to disk and writing 1 data to disk? Why do both of them cost 1 ms?

Answer

The disc access (transferring data to disk) does not happen byte-by-byte, it happens in blocks. So, we cannot conclude if that the time taken for writing 1 byte of data is 1 ms, then x bytes of data will take x ms. It is not a linear relation.

The amount of data written to the disk at a time depends on block size. For example, if a disc access cost you 1ms, and the block size is 512 bytes, then a write of size between 1 to 512 bytes will cost you same, 1 ms only.

So, coming back to the eqation, if you have , say 16 bytes of data to be written in each opeartion for 20 iteration, then,

  • for direct write case

time = (20 iteration * 1 ms) == 20 ms.

  • for buffered access

time = (20 iteration * 0.5 ms (bufferring time)) + 1 ms (to write all at once) = 10 + 1 == 11 ms.

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