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I/O throutput estimation

Hi

Why when testing disk on Linux by using `dd`, throutput is so different between I/O load to disk vs I/O load to partition on that disk.
Why ?

Code:

I/O write (sdb1) - 538MB dd if=/dev/zero of=/dev/sdb1 bs=104857600 count=10

I/O write (sdb)  - 138MB dd if=/dev/zero of=/dev/sdb bs=104857600 count=10

I/O read  (sdb1) - 1.1GB dd if=/dev/zero of=/dev/sdb1 bs=104857600 count=10

I/O read  (sdb)  - 154MB dd if=/dev/zero of=/dev/sdb bs=104857600 count=10

Partitions would be formatted with a file system that imposes overhead or the nature of partition management imposes the overhead.
Raw disk interaction - I presume - operates on a different set of parameters, which are probably not achievable in many cases.

Quote:

Originally Posted by drManhattan (Post 4974722)

Code:

I/O write (sdb1) - 538MB dd if=/dev/zero of=/dev/sdb1 bs=104857600 count=10

I/O write (sdb)  - 138MB dd if=/dev/zero of=/dev/sdb bs=104857600 count=10

I/O read  (sdb1) - 1.1GB dd if=/dev/zero of=/dev/sdb1 bs=104857600 count=10

I/O read  (sdb)  - 154MB dd if=/dev/zero of=/dev/sdb bs=104857600 count=10

Could you explain what the above table means? The speed figures, are those supposed to be MB/second or GB/second as reported by dd, or did you measure the speed some other way?

I see no difference between your read and write commands. To read, I think you would use something like this:

Code:

dd if=/dev/sdb of=/dev/null bs=104857600 count=10

edit: totally misread that.

If those listed are the actual commands then:

dd if=/dev/zero of=/dev/sdb1 bs=104857600 count=10

Is only creating a 100mb file. You really want to use something 2x the size of your ram to avoid cache.