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Re: Disk performance
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Re: Disk performance


  • Subject: Re: Disk performance
  • From: Sam Vaughan <email@hidden>
  • Date: Sat, 3 Nov 2007 09:20:16 +1100

On 02/11/2007, at 7:21 PM, Anton Altaparmakov wrote:

Yes you did. You want "skip=" not "seek=". Using seek will seek inside /dev/null not inside /dev/rdisk0s2 thus all your reads are from the same place. Try it again with skip instead of seek and you should find the same results as the other test with individual partitions.

Thanks, Anton. Sure enough it's staring me in the face in the man page!

seek=n Seek n blocks from the beginning of the *output* before copying.

Skip and iseek are in units of the block size specified, in this case 1m, so by changing it from 'g' to 'k' I get what I was looking for:

$ for i in `jot 10 0 900`; do
> echo -n offset "${i}GB: "
> sudo dd if=/dev/rdisk0s2 of=/dev/null bs=1m count=1k skip=${i} k 2>&1 | \
> perl -ne 'if (/\((\d+)/) {printf "%.1fMB/s\n", $1 / (1 << 20);}'
> done
offset 0GB: 82.4MB/s
offset 100GB: 84.3MB/s
offset 200GB: 81.2MB/s
offset 300GB: 78.9MB/s
offset 400GB: 74.6MB/s
offset 500GB: 71.0MB/s
offset 600GB: 66.7MB/s
offset 700GB: 61.6MB/s
offset 800GB: 54.6MB/s
offset 900GB: 46.7MB/s
$


Nice results.

Given that the innermost GB I can read at offset 930GB comes in at about 43MB/s, and circumference is proportional to radius, I guess these results mean that the innermost track is about 3.5/2 * 43/83 = 0.9 inches out from the centre of the disk; i.e. just over half-way. Seems a fair way out...

Then again I don't get 10/7.2 * 83 = 115MB/s from the first track of the 10k Raptor, so there's obviously more to it than my gross simplification.

Thanks again Anton,

Sam

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References: 
 >Disk performance (From: Sam Vaughan <email@hidden>)
 >Re: Disk performance (From: Anton Altaparmakov <email@hidden>)

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