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Physical and Logical Arrays and Drives

The fundamental structure of RAID is the array. An array is a collection of drives that is configured, formatted and managed in a particular way. The number of drives in the array, and the way that data is split between them, is what determines the RAID level, the capacity of the array, and its overall performance and data protection characteristics. Deciding what types of arrays to set up, and how to configure them, is the first thing you do when setting up a RAID implementation.

Understanding arrays and drives can get awfully confusing because high-end RAID systems allow for such complexity in how they are arranged. To get a better handle on this, let's look at the "hierarchy" in which data in a RAID system is organized:

1. Physical Drives: The physical, actual hard disks that comprise the array are the "building blocks" of all data storage under RAID.
2. Physical Arrays: One or more physical drives are collected together to form a physical array. Most simple RAID setups use just one physical array, but some complex ones can have two or more physical arrays.
3. Logical Arrays: Logical arrays are formed by splitting or combining physical arrays. Typically, one logical array corresponds to one physical array. However, it is possible to set up a logical array that includes multiple physical arrays (typically used to allow multiple RAID levels). It is also possible to set up two entirely different logical arrays from a single physical, as described here.
4. Logical Drives: One or more logical drives are formed from one logical array (much the same way they would normally be formed from one physical drive in a non-RAID system). These appear to the operating system as if they were regular disk volumes, and are treated accordingly, with the RAID controller managing the array(s) that underlie them.

Notice how we start with drives, go to arrays, and then end up back talking about drives. Isn't that nice and confusing? :^) This occurs so that the array management is "hidden" from the operating system. (Under software RAID a special part of the operating system does this management, hiding the arrays from the rest of the software.) The really confusing thing is that the terms above, which are similar-sounding enough, are often used loosely. Since most RAID setups consist of one physical array made into one logical array made into one logical drive, the term "logical array" is sometimes used interchangeably with "physical array" or "logical drive". Ugh.

As you can see, better RAID controllers give you all the "rope" you need to hang yourself when it comes to defining arrays. How many logical arrays and drives you should define depends entirely on the RAID level(s) you are going to be used. But in general, when defining arrays and drives, you should always keep in mind the "KISS" rule--keep it simple. For most reasonably simple applications, creating a single logical array for each physical array is the best way to go. If you need multiple logical arrays for special applications, using separate physical arrays for them may make management simpler than having two arrays sharing physical arrays and therefore, sharing physical drives.

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