RAID, an acronym for redundant array of independent disks, is a technology that provides increased storage reliability through redundancy, combining multiple relatively low-cost, less-reliable disk drives components into a logical unit where all drives in the array are interdependent. This concept was first defined by David A. Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 as redundant array of inexpensive disks.[1] Marketers representing industry RAID manufacturers later reinvented the term to describe a redundant array of independent disks as a means of dissociating a low-cost expectation from RAID technology.[2]
RAID is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple disk drives. The schemes or architectures are named by the word RAID followed by a number (e.g., RAID 0, RAID 1). The various designs of RAID systems involve two key goals: increase data reliability and increase input/output performance. When multiple physical disks are set up to use RAID technology, they are said to be in a RAID array.[3] This array distributes data across multiple disks, but the array is addressed by the operating system as one single disk. RAID can be set up to serve several different purposes.
A number of standard schemes have evolved which are referred to as levels. There were five RAID levels originally conceived, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary).
- RAID0 (block-level striping without parity or mirroring) provides improved performance and additional storage but no redundancy or fault tolerance (making it not true RAID, according to the acronym’s definition). However, because of the similarities to RAID (especially the need for a controller to distribute data across multiple disks), simple stripe sets are normally referred to as RAID 0. Any disk failure destroys the array, and the likelihood of failure increases with more disks in the array (at a minimum, catastrophic data loss is twice as likely compared to single drives without RAID). A single disk failure destroys the entire array because when data is written to a RAID 0 volume, the data is broken into fragments called blocks. The number of blocks is dictated by the stripe size, which is a configuration parameter of the array. The blocks 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, increasing bandwidth. RAID 0 does not implement error checking, so any error is uncorrectable. More disks in the array means higher bandwidth, but greater risk of data loss.
- In RAID1 (mirroring without parity or striping), data is written identically to multiple disks (a “mirrored set”). Although many implementations create sets of 2 disks, sets may contain 3 or more disks. Array provides fault tolerance from disk errors or failures and continues to operate as long as at least one drive in the mirrored set is functioning. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, as well as a very small performance reduction when writing. Using RAID 1 with a separate controller for each disk is sometimes called duplexing.
- In RAID2 (bit-level striping with dedicated Hamming-code parity), all disk spindle rotation is synchronized, and data is striped such that each sequential bit is on a different disk. Hamming-code parity is calculated across corresponding bits on disks and stored on one or more parity disks. Extremely high data transfer rates are possible.
- In RAID3 (byte-level striping with dedicated parity), all disk spindle rotation is synchronized, and data is striped such that each sequential byte is on a different disk. Parity is calculated across corresponding bytes on disks and stored on a dedicated parity disk. Very high data transfer rates are possible.
- RAID4 (block-level striping with dedicated parity) is identical to RAID 5 (see below), but confines all parity data to a single disk, which can create a performance bottleneck. In this setup, files can be distributed between multiple disks. Each disk operates independently which allows I/O requests to be performed in parallel, though data transfer speeds can suffer due to the type of parity. The error detection is achieved through dedicated parity and is stored in a separate, single disk unit.
- RAID5 (block-level striping with distributed parity) distributes parity along with the data and 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.
- RAID6 (block-level striping with double distributed parity) provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems. This becomes increasingly important as large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single-parity RAID levels are as vulnerable to data loss as a RAID 0 array until the failed drive is replaced and its data rebuilt; the larger the drive, the longer the rebuild will take. Double parity gives time to rebuild the array without the data being at risk if a single additional drive fails before the rebuild is complete.
The following table provides an overview of the most important parameters of standard RAID levels. Space efficiency is given as an equation in terms of the number of drives, n, which results in a value between 0 and 1, representing the fraction of the sum of the drives’ capacities that is available for use. For example, if three drives are arranged in RAID 3, this gives a space efficiency of 1-(1/3) = 0.66. If their individual capacities are 250GB each, for a total of 750GB over the three, the usable capacity under RAID 3 for data storage is 500GB.
| Level | Description | Minimum # of disks | Space Efficiency | Fault Tolerance | Read Benefit | Write Benefit | Image |
| RAID 0 | Block-level striping without parity or mirroring. | 2 | 1 | 0 (none) | nX | nX | |
| RAID 1 | Mirroring without parity or striping. | 2 | 1/n | n-1 disks | nX | 1X | |
| RAID 2 | Bit-level striping with dedicated Hamming-code parity. | 3 | 1 – 1/n ⋅ log2(n-1) | 1 disk when the corrupt disk is found by the ( ) recover-record code. | |||
| RAID 3 | Byte-level striping with dedicated parity. | 3 | 1 – 1/n | 1 disk | |||
| RAID 4 | Block-level striping with dedicated parity. | 3 | 1 – 1/n | 1 disk | |||
| RAID 5 | Block-level striping with distributed parity. | 3 | 1 – 1/n | 1 disk | (n-1)X | variable | |
| RAID 6 | Block-level striping with double distributed parity. | 4 | 1 – 2/n | 2 disks |
IHNC - "THINKS AHEAD OF ALL TAKES U AHEAD OF ALL 
