Advice on our SAN

Hi There, I am a bit of a newbie to the world of SAN's. so please be gentle

Anyway here is what I am trying to do, we have 3 video editors who are using a SAN. we predominantly use Sony Vegas, and deal mostly with mxf 35mb/s files or Decklink Uncompressed files all in HD.

We have a Xyratex 5402E Raid Enclosure loaded with 12 x 500GB SATA Drives, the enclosure has 2 Fiber Channel Controllers, which in turn are both connected to a QLogic 4GB Switch. and then we have 3 XP64 computers connected via FC to the Qlogic switch at 4GB.

The 12 drives are configured currently as 2 banks of RAID 5, so we have 6 drives Raided together as a Raid 5, and the other 6 also raided together as a Raid 5.

Then in windows XP we Raid 0 the 2 partitions back together to create a single shared resource. We are using MetaSAN software on the 3 systems to access the drives giving us file level sharing.

So my problem, It just seems really slow, I can manage get a maximum Read / Write of about 225MB/s Read and 150MB/s Write, which isn't too bad I guess, but I would expect it to be a bit quicker, and also we get pauses and inconsistency's with the data rate. The data rate will just plummet down to about 20MB/s Read and Write for a second then fire back upto full speed, which to be honest is crap if you are trying to edit HD streams.

I think the problem is basically down to Block Size settings in the Raid Configuration, Cache Settings in the Raid software, sector size settings in Windows, and also which Raid partition is accessible through which Fiber Controller on the Xyratex COntroller. and unfortunately this has so may variables I am not sure where to start to try and improve the situation.

So my question to SAN Masters, is HELP !!!!, well, lets say do you have any advice to get my speeds up and more consistent.

Many Thanks for this

Matt