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There are many different ways to implement a RAID array, using some combination of mirroring, striping, duplexing and parity technologies. Several standardized methods were defined in the 1988 Berkeley research publication that is credited with starting the RAID phenomenon; for some (unfortunate) reason, the researchers decided to call these different techniques levels. This was a poor choice of words in my opinion because the word "level" implies hierarchy or revision, or that the different RAID levels are somehow "built upon each other", when in fact, that is not the case. The word "level" implies to some people that "RAID level N+1" is better somehow than "RAID level N". In fact, this isn't really true--the various levels are independent and different, and no strict hierarchy should be inferred from the specific number attached to a RAID level. A given RAID level that is "better" for one person may be "worse" for another.
The original 1988 paper defined RAID levels 1 through 5; since then, single RAID levels 0 and 6 have been added to the mix, and other extensions such as the proprietary RAID 7 have shown up as well. Beyond these single-level RAID designs, a number of multiple RAID levels have been defined, which use two or more of the single RAID levels in combination to create new array types with new capabilities (and limitations). Most of these different RAID levels are in use today in different systems, a testament to the different needs of various RAID users. Some have largely disappeared from the market as experience over time has shown them to be inferior to other levels without advantages to compensate.
In this section I take a detailed look at RAID levels. I start with a discussion of some of the key technical factors that differentiate RAID levels; these are then used to frame the coverage of the RAID levels themselves. I discuss the eight single-level RAID designs, and take a look at several common multiple-level RAID types as well. Each RAID level is discussed in detail and information is provided about over a dozen of its various characteristics, with general recommendations provided for typical uses of each level. Finally, I show a summary comparison table that contrasts the different levels in terms of their benefits and costs.
Tip: Carefully consider
all the factors and variables when comparing different RAID levels; sometimes,
things are not what they seem. Pay careful attention to the various performance
attributes, to help differentiate levels based on how you are most likely to use the
array; sometimes the "common wisdom" about different RAID levels will not apply
to your needs.
Warning: As I am
noticing increasingly these days in all areas of computing, the RAID levels are sometimes
not used consistently by manufacturers. For example, I have encountered a significant RAID
controller maker that provides support for what they call "RAID 3"; when you
examine the details, however, you find that this RAID level is actually implemented as
block striping with dedicated parity, which is RAID 4, not 3. Why they did this,
I have no idea. An organization called the RAID
Advisory Board or RAB does maintain RAID standards and certifies
hardware that meets "official" RAID level definitions, so you can look for their
"seals of approval". Even so, it's still best to ask for specific technical
details about any RAID system if you aren't certain of what the manufacturer has