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Redundant Arrays of Inexpensive Disks (RAID)

Many higher-end systems, especially servers, now employ a technology called redundant arrays of inexpensive disks, or RAID. This concept allows for great improvements in both reliability and performance. The idea is to store data on multiple disk drives running in parallel. The primary motivation in many cases is reliability. From a performance standpoint, most RAID levels improve performance by allowing multiple accesses to happen simultaneously, and also by using algorithms that reduce seek time and latency by taking advantage of having multiple drives at their disposal. The exact performance impact depends entirely on the level of RAID used; some improve read performance at the expense of write performance, for example.

I have written an entire section on RAID that covers its issues, levels and implementation in some detail, including a discussion of its impact on storage subsystem performance. Once used almost exclusively in a corporate setting for large, expensive machines, a new crop of inexpensive RAID controllers and hard disks is bringing RAID into the "mainstream", and small RAID arrays are now commonly seen in the machines of "power users".

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