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RAID Interfaces

There are two primary interfaces that are used for RAID arrays. Traditionally, all RAID was done on high-end machines and used the SCSI interface. Lately, hardware RAID cards for the ubiquitous IDE/ATA interface have begun to explode in popularity. Therefore, when designing a system that you intend to use RAID, you have a choice of interface to make. Of course, this is a choice that affects the design of the system in a fundamental way. The matter of SCSI vs. IDE/ATA for RAID is similar to the general ongoing debate over the two interfaces, which I have contrasted in this section.

The decision of SCSI vs. IDE is much like that of hardware RAID vs. software RAID--it comes down to cost vs. features. IDE/ATA RAID is much less expensive than SCSI RAID but much more limited in virtually every way: array capacities are smaller since a smaller number of drives can be used; performance is lower in many different ways; support for more complex RAID levels is absent; and advanced features are much less common. And those are just the more important ones. :^)

In practice, most serious RAID implementations still use SCSI; I don't see IDE/ATA replacing SCSI RAID for heavy-duty use on servers or high-end workstations, because it is just too limiting. What IDE/ATA RAID is doing however, is opening up the world of consumer-grade hard disks to RAID, enabling millions who cannot afford the cost of SCSI to enjoy some of the important benefits of RAID economically. If you already have an IDE/ATA system and want to install RAID, you no longer have to face the sometimes daunting task of moving over to SCSI to do it.

Next: Multiple Channels and Throughput Issues

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