4 Disk RAID 5 Performance

We just setup a RAID 5 using 4 Ultra 3 SCSI 18GB drives on a Mylex Acceleraid 250 Ultra2 RAID controller in an older 2U Intel server. I set the controller to 80MB/s transfer speed. Looking at the array manager in Windows I can see that the drive interfaces and controller really are running at 80MB/s like they should. However, performance seems slower than I expected from a setup like this. I have a hunch that the backplane is a bottleneck. In looking at the array manager I can see the ESG-SHV SCA HSBP M10 backplane as SCSI ID: 6, but it shows the transfer speed to be ASYNC and the bus width to be 8 bit(narrow). To my understanding that would limit it to 5MB/s and explain the slow speed. The only problem with that explanation is that the enclosure is built for operation with Ultra2 drives and even has a built in Ultra2 Adaptec controller (not in use currently) on the L440GX+ server board. It would make no sense to put a backplane in a server which would cripple its speed. The enclosure is the Intel ISP2150. Technical specifications can be found here: http://www.intel.com/support/motherboards/server/2150/isp2150/sb/cs-013775.htm. I realize that term "slow" is relative. I am comparing my experience with this RAID to that of a single 36GB Raptor. File transfers seem to be at least 4 times slower than the single Raptor, but perhaps my expectations are flawed. Thanks.
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  1. The only thing I can think of is that the Backplane is an Ultra2 Backplane. Acording to Wikipeida (http://en.wikipedia.org/wiki/SCSI) the system should clock in at about 40MB/s. You may want to benchmark it.
  2. The backplane by itself shouldn't slow down everything. If the backplane is ASYNC, then the controller has to talk to it at 5MB/sec, but that doesn't slow down the drives.

    Many tape autoloaders are like this - the tape drive is 80MB/sec or 160MB/sec speed, while the changer robotics are ASYNC. That doesn't slow the tape drive down.

    The drives are only 18GB, which are fairly old and not the same speed as drives of today. I'd benchmark a single drive and see what it comes out to be, then benchmark the RAID 5 setup. The Mylex card isn't the fastest RAID card out there, either, so that could be a source of the problem.
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