RAID

Redundant Array of Independent Disks (RAID) is a storage technology that combines multiple disk drive components into a logical unit. The data is distributed over the drives in one of several ways, called "RAID Levels", but it depends on the redundancy for the required performance.

RAID now covers a wide variety of functions for computer data storage schemes that can divide and replicate data amongst several drives. It's an example of storage virtualization and the array can be accessed by the operating system by just one drive. There are different schemes, each having a balance of the key goals: reliability and availability, performance and capacity. 

The standard schemes are referred to as the "levels". There was originally five, but it continues to have more variations involved.

RAID 0
Has no redundancy, but provides improved performance and additional storage with no fault tolerance. Any failure of drive destroys the array and it is likely for more drives in the array to fail.

RAID 1
Data is written identically to two drives which provides a mirrored set. The request is serviced by the two drives containing the data. At least two disks are required to constitute the array.

RAID 2
All disk spindle rotation is synchronized and data is striped so each sequential bit is on a different drive.

RAID 3
All disk spindle rotation is synchronized and data is striped so each sequential byte is on a different drive. This level is not commonly used.

RAID 4
It is equivalent to RAID 5 except all parity data is stored on a single drive. The arrangement files may be distributed around multiple drives.

RAID 5
Distributes parity along with the data that requires all drives but one to be presented to operate. The array is not destroyed by one single drive failure. If there is failure, any subsequent rads can be calculated from the parity such that the drive failure is masked from the end user. It requires 3 disks

RAID 6
Provides fault tolerance for up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems. This is increasingly import as large capacity drives lengthen the time needed to recover from a failure of a drive.

RAID 10
Often referred to as RAID 1+0 Data are written in strips across primary disks that have been mirrored to the secondary disks.