Performance Issue on a Xraid (Raid 5) for video digitalization

Question

Hello Everyone
I have a Mac Pro (2x3ghz + 8 Go Ram + Fiber channel  card) directly linked to a Xraid via Fiber (there is no Xsan and no Xserve as controller).
The Xraid is composed of 8 X 750 Go HD and set up as Raid 5.
We are working in video production/editing and it is important for us to a good level of security on our data but also to be able to digitalize rushes directly on the Raid.
We are working essentially in HUD format with Panasonic Cams (HUD) and thus are working with VCR HUD format which is meant to require 100 mb/s data rate.
When trying to capture (Final cut studio 2) directly on the Raid, we have frames drop and when I run a little utility software to test the  writing speed on the Raid it tells me that the rate is around 95-98 Mb/s which probably explains the frames dropping.
My question is : Is this normal to have such a low data rate in writing with the Xraid configuration ?
If yes is there a way to improve it by some means ?
Thanks in advance
Julien

Answer 1

How are your 8 drives configured? That's an unusual, and certainly sub-optimal, configuration.
RAID has three components that typically impact performance. One is the bus architecture (in this case a fiber channel/ATA hybrid), second is disk capacity (speed degrades as the array fills), and the third is the number of drives.
You can't do much about the bus architecture, other than trust that Apple have scaled it appropriately.
It's worth checking the disk space to see if that's impacting performance, but I suspect the drive configuration is going to be the issue.
Since you're running 8 drives I'm guessing you're running two 4-drive arrays.
The optimal performance for any array is achieved with the largest number of drives. Since the XServe RAID controllers are designed for 7 drives each, you're essentially running each array at 4/7ths of its potential.
Of course, you don't gain much by switching all the drives onto a single controller since now you're limited by the single fiber channel link to the host, although whether one link/7 drives is better or worse than 2 links/4 drives each is something that will have to be tested.
Your best solution may be to expand the array to 14 drives to boost the disk throughput.