Why does RAID5 with an odd number of data disks have poor write performance

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http://serverfault.com – According to the comments on my other question, it's more than possible that the reason my MD RAID array is performing poorly is because my RAID5 array has 5 data disks. I've tried searching for information on why this is, but haven't found anything, so I'm looking for more information about why this is, and what sort of impact it can have versus having 4 data disks. (HowTos)