RAID stands for "Redundant Array of Inexpensive/Independent Disks". RAID works by combining two or more hard drives. It can increase performance and/or data redundancy and/or capacity. There are many different RAID types, the most common being RAID 0 (stripe), RAID 1 (mirror) and RAID 5 (stripe with parity).

In a RAID 0 (stripe) array, data is split equally between the number of disks in the array. For example, when a 2 MB file is written to a RAID 0 array with two hard drives, the file is split in to two parts and 1 MB of data is written to each hard drive. This increases capacity and performance, but sharply decreases redundancy since only one of the drives needs to fail for all information to be lost.

Performance = Drive Speed * Number of Drives

Capacity = Drive Size * Number of Drives

Redundancy allows for no drives to fail.

In a RAID 1 (mirror) array, a complete copy is written to each hard drive in the array. Capacity and performance stay the same, but redundancy is increased. As long as one drive works, you will not lose data.

Performance = Drive Speed

Capacity = Drive Size

Redundancy allows for all but one drive to fail.

In a RAID 5 (stripe with parity) array, you need at least three hard drives. Files are split up to all but one of the drives (similar to RAID 0) and a parity bit is written to the last drive. This increases performance, capacity and redundancy. Performance is not as good as RAID 0, but still better than RAID 1. Redundancy is not as good as RAID 1, but is still quite reliable.

Performance = Drive Speed * (Number of Drives - 1) (Theoretical)

Capacity = Drive Size * (Number of Drives - 1)

Redundancy allows for one drive to fail.

There are other RAID levels, but they are not as common.

The main benefit of using a RAID array is data redundancy and/or performance.

Non-commercial (home) users and enthusiasts wanting a very large increase in performance (theoretically multiples of the number of drives used) usually go for RAID 0 since they often are not too worried about drive failure and they enjoy the larger drive capacities that it provides.

Small businesses who only need cheap redundancy usually go for RAID 1. If one drive fails, the drive can be replaced and the mirror array restored.

Larger businesses or those needing file/database performance go for RAID 5. They can afford the price of an extra drive and a more expensive RAID controller. RAID 5 provides high performance with large capacities while still maintaining data redundancy and is the most desirable in a business or enterprise environment.

There are some other options for RAID such as RAID 10 or RAID 01.

RAID 10 features two separate RAID 0 arrays which are then mirrored in a RAID 1 array.

RAID 01 is the opposite, with two separate RAID 1 arrays which are then striped with a RAID 0 array.

There are other non-standard RAID levels, but most are rather uncommon.

Alternatively, there is also drive spanning (aka JBOD), but this provides no extra redundancy or performance. Drives are "glued", if you will, end to end.

The benefits of each RAID level vary. The type of RAID used depend on the job it must fulfill.

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