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RAID Performance Issues
RAID was originally developed as a way of protecting data by providing fault tolerance; that's the reason for the "R" at the front of the acronym. Today, while matters of reliability, availability and fault tolerance continue to be essential to many of those who use RAID, performance issues are being given about as much attention. There are in fact whole classes of implementers who build RAID arrays solely for performance considerations, with no redundancy or data protection at all. Even those who do employ redundancy obviously care about getting the most from their array hardware.
The key to performance increases under RAID is parallelism. The ability to access multiple disks simultaneously allows for data to be written to or read from a RAID array faster than would be possible with a single drive. In fact, RAID is in some ways responsible for the demise of esoteric high-performance hard disk designs, such as drives with multiple actuators. A multiple-drive array essentially has "multiple actuators" without requiring special engineering; it's a win-win solution for both manufacturers (which hate low-volume specialty products) and consumers (which hate the price tags that come with low-volume specialty products).
There's no possible way to discuss every factor that affects RAID performance in a separate section like this one--and there really isn't any point in doing so anyway. As you read about RAID levels, and RAID implementation and configuration, many issues that are related to performance will come up. In this section I want to explore some of the fundamentals though, the basic concepts that impact overall array performance. One of my goals is to try to define better what exactly is meant by performance in a RAID context. Most people who know something about RAID would say "RAID improves performance", but some types improve it better than others, and in different ways than others. Understanding this will help you differentiate between the different RAID levels on a performance basis.