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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Configuration and Implementation | RAID Controllers and Controller Features ]
Most "serious" RAID implementations use what is termed hardware RAID. This means using dedicated hardware to control the array, as opposed to doing array control processing via software. Good hardware controllers are in many ways like miniature computers, incorporating dedicated processors that exceed the power of processors that ran entire PCs just a few years ago. For a list contrasting software and hardware RAID, showing the advantages and disadvantages of each, see this page on software RAID.
There are two main types of hardware RAID, differing primarily in how they interface the array to the system:
Bus-based RAID is cheaper and much simpler to implement than external RAID controllers while still offering often impressive capabilities; they range from entry-level cards for IDE/ATA systems that cost around $100, up to top-of-the-line, full-featured devices costing several thousand dollars. Dedicated, external RAID controller systems are still more expensive but offer many advanced features, are typically more expandable than bus-based RAID implementations (offering support for large array well into the terabytes) and can offer better performance. They often cost well into the five figures, so they are not something a typical PC user would even consider.
Note: External RAID
controllers should not be confused with external RAID enclosures. Enclosures provide power and physical infrastructure for
the drives in a RAID array, but not the smarts of the controller; they are functionally a
large, fancy PC system case. An external RAID controller can be thought of as such an
enclosure combined with a high-end, integrated controller as well.
In most cases, the decision to use hardware RAID is made almost exclusively on financial grounds: hardware RAID is superior to software RAID in virtually every way, it just costs more. If you want to use any of the more esoteric RAID levels such as RAID 3 or RAID 1+0, you pretty much require hardware RAID, since support for these levels is usually not offered in software. If you need top performance while using a computation-intensive RAID level such as RAID 5, you also should consider a hardware solution pretty much "mandatory", because software RAID 5 can really hurt performance.
Next: Software RAID