1. What is RAID?

RAID is an acronym first defined by David A. Patterson, Garth A. Gibson and Randy Katz at the University of California, Berkeley in 1987 to describe a Redundant Array of Inexpensive Disks, a technology that allowed computer users to achieve high levels of storage reliability from low-cost and less reliable PC-class disk-drive components, via the technique of arranging the devices into arrays for redundancy.

More recently, marketers representing industry RAID manufacturers have revised the term to Redundant Array of Independent Disks, a convenient means of avoiding the negative connotations associated with "inexpensive".

"RAID" is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. RAID's various designs all involve two key design goals: increased data reliability or increased input/output performance. When multiple physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across multiple disks, but the array is seen by the computer user and operating system as one single disk. RAID can be set up to serve several different purposes. Data can be distributed across a RAID "array" using either hardware, software or a combination of the two. Hardware RAID is usually achieved either on-board on some server class motherboards or via an add-on card, using an ISA/PCI slot.

2. Different Types of RAID

RAID 0 - Striped Disk Array without Fault Tolerance
Provides improved performance and additional storage but no fault tolerance. Any disk failure destroys the array, which becomes more likely with more disks in the array. A single disk failure destroys the entire array because when data is written to a RAID 0 drives, the data is broken into fragments. The number of fragments is dictated by the number of disks in the array. The fragments are written to their respective disks simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off the drive in parallel, giving this type of arrangement huge bandwidth. RAID 0 does not implement error checking so any error is unrecoverable. More disks in the array mean higher bandwidth, but greater risk of data loss.

Disk 1

Disk 2

Disk 3

Disk 4

Block 1	Block2	Block 3	Block 4
Block 5	Block6	Block7	Block 8
Block9	Block 10	Block 11	Block 12
Block 13	Block 14	Block 15	Block 16

RAID O

RAID 1- Mirroring and Duplexing
Provides fault tolerance from disk errors and failure of all but one of the drives. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate so long as at least one drive is functioning. Using RAID 1 with a separate controller for each disk is sometimes called Duplexing. RAID 1 provides the best performance and the best fault-tolerance in a multi-user system but with an only 50% utilization of the disks.

Disk 1

Disk 2

Block 1	Block 1
Block 2	Block 2
Block3	Block 3
Block 4	Block 4

RAID 1

RAID 5- Block Interleaved Distributed Parity
Distributed parity requires all drives but one to be present to operate; drive failure requires replacement,but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive. A single drive failure in the set will result in reduced performance of the entire set until the failed drive has been replaced and rebuilt. Raid 5 needs at least three disks and it is one of the most popular implementations of RAID.

Disk 1

Disk 2

Disk 3

Disk 4

Block 1	Block 2	Block 3	Parity
Block 5	Block 6	Parity	Block 4
Block 9	Parity	Block 8	Block 7
Parity	Block 12	Block 11	Block 10

RAID 5

RAID 5E - RAID 5 Enhanced
RAID 5E is a RAID 5 array with a hot spare drive that is actively used in the array operations. In a traditional RAID 5 configuration with a hot spare, the hot spare drive sits next to the array waiting for a drive to fail, at which point the hot spare is made available and the array rebuilds the data set with the new hardware.

Disk 1

Disk 2

Disk 3

Disk 4

Block 1	Block 2	Block 3	Parity
Block 5	Block 6	Parity	Block 4
Block 9	Parity	Block 8	Block 7
Parity	Block 12	Block 11	Block 10
...	...	...	...
Hot Space	Hot Space	Hot Space	Hot Space

RAID 5E

RAID 5EE - RAID 5E Enhanced
RAID 5EE is very similar to RAID 5E with one key difference -- the hot spare's capacity is integrated into the stripe set. In contrast, under RAID 5E, all of the empty space is housed at the end of the array. As a result of interleaving empty space throughout the array, RAID 5EE enjoys a faster rebuild time than is possible under RAID 5E.

Disk 1

Disk 2

Disk 3

Disk 4

Space	Block 1	Block 2	Parity
Block 3	Block 4	Parity	Hot Space
Block 6	Parity	Hot Space	Block 5
Parity	Hot Space	Block 7	Block 8

RAID 5EE