Disappointing RAID10 Performance

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I was hoping to get better performance with RAID10 than from the raw disks, but that's turned out to not be the case.  Experimenting with the readahead buffer I get these bandwidths with the following command:
# time dd if={somelarge}.iso of=/dev/null bs={readahead size}

/dev/sd?
 1024 71.3 MB/s
 2048 71.2 MB/s
 4096 77.7 MB/s
 8192 69.4 MB/s
 16384 76.6 MB/s

/dev/md2
 1024  67.1
 2048  69.1
 4096  75.7
 8192  64.9
 16384 69.0

Using RAID10offset2 on 2 WD 2TB drives, and always the same input file.

Why would RAID10 performance be -poorer-? 
If it weren't for mirroring, this wouldn't be worth it.


 


      
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