Arco DupliDisk-PCI: RAID for IDE Users

The popularity of RAID among SCSI users is easily justified by the data security RAID offers. There have been several IDE-based RAID systems offered over the years, but most have disappeared due to lack of interest, poor marketing, or inflexible setup requirements. Arco Computer Products Inc. intends to change IDE’s RAID reputation with the DupliDisk-PCI, a single-board inexpensive RAID alternative that works with most operating systems.

DupliDisk-PCI is a single-slot PCI board that is short enough to fit into any available PCI connector. An ISA-based system is also available from Arco. The DupliDisk-PCI supports RAID 1 only, which is disk mirroring. Data written to one drive is automatically written to a second. In case of a single drive failing, the second (mirror) copy contains an exact image of the failed drive, and data security is maintained. Because it works on IDE, EIDE, ATA, and UDMA drives, DupliDisk-PCI has that system’s four drive limit to contend with. Through front and read connectors on the board, two chains of two drives are attached. The two drives on the primary cable are mirrored to the two drives on the secondary, with the mirroring taking place automatically through parallel write instructions. The DupliDisk-PCI is not a replacement for existing IDE controllers, but fits between the drives and the existing controller.

Setup of the DupliDisk-PCI is simple: insert it into an available slot (which is used only to supply power and setup configuration). Arco includes two special cables that must be used for the run from DupliDisk-PCI to standard controller, while the old IDE cables run from the DupliDisk-PCI to the drives. There’s no change to the CMOS, which sees the drives as usual. Software supplied with the DupliDisk-PCI is installed under DOS (it won’t work under Windows) which sets up the mirroring, then reboot. Mirror drives must be the same size or larger than the originals. You can mirror existing drives by adding new drives to the mirror chain. The software included with the system (again DOS only) allows duplication of the disks manually, as well as disabling of the RAID levels to regain all your disk space. No special drivers are required for the system, so it should work with any operating system.

We set up the DupliDisk-PCI in a Pentium Pro 150MHz running Unixware 7 on a single 6.4GB ATA drive. We plugged a second 8.1GB ATA drive in as the mirror, and set up the system under DOS. After the mirroring was established, we rebooted and the system behaved exactly as before. To test the mirroring, we unplugged the main drive and after a set of status beeps from the DupliDisk-PCI, our system kept running. A set of three LEDs on the back panel of the DupliDisk-PCI show the current condition of both chains of drives, as well as the condition of the system as a whole.

RAID 1 is a pure mirror RAID level with nothing fancy like striping or error correction. In many ways, though, RAID 1 is the purest RAID level as it always maintains a true mirror of whatever is on your disks. Unlike SCSI based RAID systems, the DupliDisk-PCI doesn’t seem to use parallel reads from the two drives to speed read operations. Most RAID 1 SCSI systems can make use of this feature to enhance read performance. Comparing the DupliDisk-PCI with our two ATA drives against a similar SCSI RAID controller (DPT’s SmartRAID IV) shows the slower speed of the ATA system, but the difference wasn’t as pronounced as we expected. Bear in mind too that the DPT board costs many times the DupliDisk-PCI’s $225 list price, and that large capacity IDE and ATA drives are still a fraction of the cost of SCSI drives. This all makes for an inexpensive RAID subsystem.

If you run IDE drives and want data security, DupliDisk-PCI offers an inexpensive method of achieving this goal with inexpensive IDE drives. As such, the whole system must be judged a complete success. The ability to install into existing systems and set up mirroring at any time makes the DupliDisk-PCI a logical upgrade for existing machines.