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RAID!

4-Drive RAID inside  a 8600
Dual Channel SCSI Configuration

Why Dual Channel?
As shown in my extensive tests of SCSI controllers and Raid software, you can get a bit more performance with a dual channel SCSI setup (either two SCSI cards or a dual channel SCSI card). Instead of one SCSI channel with 4 drives, this configuration has two drives per channel with two channels. Since Raid writes to each drive in series (a simplifed explanation) you get higher performance by having two channels working in parallel essentially. You won't see double the performance, but speed will be higher, especially peak transfer rates and with larger numbers of drives in the array. (For comparisons on how much difference a 2nd card/channel makes, see the performance deltas shown in part 3 of my SCSI Raid tests.)

The downside is cost, as adding a 2nd SCSI card adds expense and dual channel cards (often the only option if you don't have two free PCI slots) can be more expensive than even two single channel SCSI cards. With Ultra2 SCSI cards appearing on the market now [see this review of 3 Mac Ultra2 SCSI Cards], watch for reduced pricing/closeouts on std Ultra SCSI cards. Remember the drives are usually the limiting factor, so an Ultra2 SCSI may not provide more real world performance if your drives are not also Ultra2 SCSI then you won't see any real gains. If you're running a large multi-drive array with modern drives however, the faster Ultra2 SCSI card would be preferred. [Note that Ultra2 SCSI drives don't provide on-drive termination, they use a terminator block at the end of the Ultra2 SCSI (twisted pair) cable.]

What's Different in the Dual-Channel Configuration:
As mentioned previously, in the 3 PCI slot 8600 I used a Adaptec 3940UW Dual Channel card, since I knew I wanted a dual channel setup and had only one free PCI slot. Here are the differences in setting up the 4 drive array for dual channel operation:

  • Replace the supplied 4 drive cable with a pair of cables, one for each of the 3940's two internal wide SCSI connectors. Each cable should be a standard 68-pin wide scsi type with three total connectors, one for the controller connector and one for each of the two drives.

  • In the single channel/cable setup, the last drive (lower right) was terminated. In the dual channel setup, there are now two cables, so we must enable termination on the bottom left drive as well, since it is at the end of the cable on that channel. No other drive jumpers need changing.

Illustration of Drive Jumper Settings - Dual Channel Configuration:

Dual Channel Config

Take care in routing the cables - make sure they do not contact the fan on the 8600 side cover. The completed dual channel setup should look like the photo below.

Dual Channel Configuration
Final Cabled Dual Channel Array

Note that I did not have to reconfigure the Raid volumes for this change. SoftRaid 2.0 was used on the single channel setup and rebooting with the dual channel setup mounted the volume without problems.


If you arrived here without reading the start of this article on how to assemble the kit, check the first page of this article.

For more articles on disk and SCSI controller upgrades (or IDE or Firewire), see my Storage Articles page.

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