[gentoo-amd64] Is my RAID performance bad possibly due to starting sector value?

[Mark Knecht <markknecht@gmail.com>]

Hi,
   Does anyone know of info on how the starting sector number might
impact RAID performance under Gentoo? The drives are WD-500G RE3
drives shown here:

http://www.amazon.com/Western-Digital-WD5002ABYS-3-5-inch-Enterprise/dp/B001EMZPD0/ref=cm_cr_pr_product_top

   These are NOT 4k sector sized drives.

   Specifically I'm a 5-drive RAID6 for about 1.45TB of storage. My
benchmarking seems abysmal at around 40MB/S using dd copying large
files. It's higher, around 80MB/S if the file being transferred is
coming from an SSD, but even 80MB/S seems slow to me. I see a LOT of
wait time in top. And my 'large file' copies might not be large enough
as the machine has 24GB of DRAM and I've only been copying 21GB so
it's possible some of that is cached.

   Then I looked again at how I partitioned the drives originally and
see the starting sector of sector 3 as 8594775. I started wondering if
something like 4K block sizes at the file system level might be
getting munged across 16k chunk sizes in the RAID. Maybe the blocks
are being torn apart in bad ways for performance? That led me down a
bunch of rabbit holes and I haven't found any light yet.

   Looking for some thoughtful ideas from those more experienced in this area.

Cheers,
Mark