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Old 06-19-2013, 07:37 AM   #1
drManhattan
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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

Last edited by drManhattan; 06-19-2013 at 07:46 AM.
 
Old 06-19-2013, 10:02 AM   #2
thedaver
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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.
 
Old 06-19-2013, 03:24 PM   #3
Beryllos
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Quote:
Originally Posted by drManhattan View Post
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
 
Old 06-20-2013, 03:08 PM   #4
szboardstretcher
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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.

Last edited by szboardstretcher; 06-20-2013 at 03:14 PM.
 
  


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