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
