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.
I have never heard of any sort of "odd/even" performance impact of a RAID5/6 array, and like you, I can't find anything useful from a quick web search.
There are potential issues with the number of disks and write performance in a RAID5/6 array, but they're related to "more disks == slower writes", because (depending on the implementation) the RAID system may want to read off all the data disks in the stripe in order to recalculate parity (so for a 6-disk RAID5, it would involve 4 reads -- one for each of the unchanged data blocks in the stripe -- and two writes -- one for the changed block, and one for the parity block). A good implementation will instead read the changed data block and the parity block, recalculate the parity, and write to the changed data block and the parity block, meaning two reads and two writes, regardless of the number of disks in the set.
It's the reverse. The issue is when the number of drives is even with RAID 5. With RAID 5, the ideal number of drives is one more than a power of 2, so 5 drives is one of the optimum sizes. This allows the implementation to make both the block size and the stripe size powers of two.
With five drives, the stripe size (amount of user data that must be written to the RAID array as a unit) will be four times the block size (amount of user data that must be written to a drive as a unit). The block size must be multiple of 512 bytes (or 4KB on newer drives), frequently it must be a power of two equal to or greater than the drive's native block size. So with five drives, the stripe size must be 2KB or more (16KB or more on 4KB drives).
As a general rule, the performance boost of adding an additional spinddle will exceed the performance cost of having a sub-optimal drive count. So an array with six drives will still typically outperform an array with five drives. On typical RAID 5 performance graphs, 3 drives and 4 drives will be right near each other, with 4 slightly on top. Then a bit up will be 5 drives and 6 drives right near each other with 6 slightly on top.