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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 Levels | Technical Factors Differentiating RAID Levels ]
Multiple Array Considerations
When reading about the various RAID levels you will quickly come to the (correct) conclusion that choosing a level is largely a matter of tradeoffs and compromises. This is especially true when it comes to dealing with performance considerations. If you have to serve two very different applications, you may not be able to easily find a single RAID level that satisfies all of your requirements. If that is the case, you may be better off creating two separate arrays, using different RAID levels for each.
Let's suppose that you are setting up an enterprise server for a business of 100 people; you can afford a reasonable amount of hardware but you do not have an unlimited budget. The primary application is a business system that requires fast performance and some degree of fault tolerance. However, you also have a design and engineering department, dealing with huge files, and they need "speed, speed and more speed"; and an accounting department that insists upon a mirrored setup for their data because they believe that provides the highest integrity. You could just set up a RAID 5 array and make everyone use it. It will provide fault tolerance and good performance for most uses, but the engineering and accounting folks probably won't be too thrilled. Instead, you could set up both a RAID 5 array and either a RAID 0 or RAID 1 array to satisfy at least one of the "specialty" groups. You could also set up three arrays--RAID 5 for general use, RAID 0 for the design group and RAID 1 for accounting--but realize that you are then getting into more of an administrative headache.
An excellent way to consider doing multiple arrays is to use different machines. This not only spreads the load around, it removes the machine containing the arrays as a single point of failure, improving the overall fault tolerance of the organization. So to continue the example above, perhaps the best solution is to fit a main server with a larger RAID 5 array for general use and a smaller RAID 1 array for accounting, and configure a second, departmental server for design and engineering that runs RAID 0. The RAID 0 array can be set up with an inexpensive controller or even software RAID, to avoid duplicating the controller costs. Just make sure it gets completely backed up every night! :^)
If you do set up two (or more) arrays on the same machine, you will have to decide exactly how to do this. Most better controllers will support multiple arrays without too much trouble. Some will let you set up more than one logical array based on a single physical array, but I personally recommend using different drives for each array. This makes things conceptually simpler, and you also may be able to avoid degrading both arrays in the event of a drive failure.
Next: Single RAID Levels