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RAID is short for Redundant Array of Independent Disks and is used for fault tolerance and performance.

There are a number of different RAID levels including:

RAID 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.

Capacity Calculation: S * N

RAID 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.

Capacity Calculation: (S * N)/2

RAID 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is commonly implemented and also preferred by Profitability.net.

Capacity Calculation: S * (N-1)

RAID 10 –- A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.

Capacity Calculation: (S * N)/2

RAID 50 -- It is a combination of RAID levels that utilizes multiple RAID 5 sets striped in a single array. In a RAID 0+5 array, a single hard drive failure can occur in each of the RAID 5 sides without any loss of data on the entire array. If, however more than one disk is lost in any of the RAID 5 arrays all the data in the array is lost. As the number of hard drives increase in an array, so does the possibility of a single hard drive failure. Although there is an increased write performance in RAID 0+5, once a hard drive fails and reconstruction takes place, there is a noticeable decrease in performance, data/program access will be slower, and transfer speeds on the array will be effected.

Capacity Calculation: NS * RA

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*S = Size of Drive, N = Number of Drives, NS = The "width" of the RAID 5 dimension in the Array, RA = Number of RAIDS arrays*
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