RAID: Redundant Arrays of Inexpensive Disks

By definition:_
RAID is a technology that supports the integrated use of two or more hard-drives in various configurations for the purposes of achieving greater performance, reliability through redundancy, and larger disk volume sizes through aggregation. RAID is also sometimes referred to as “Redundant Arrays of Inexpensive Drives” or “Redundant Arrays of Independent Disks/Drives”. RAID is an umbrella term for computer data storage schemes that divide and replicate data among multiple hard disk drives. RAID’s various designs balance or accentuate two key design goals: increased data reliability and increased I/O (input/output) performance.

There are many level of RAID configurations, the most common ones are RAID 0, RAID 1, RAID 3, RAID 4, RAID 5, RAID 6.

RAID 0:_

Striped set without parity. 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 drive, the data is broken into fragments. The number of fragments is dictated by the number of disks in the drive. 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. When one sector on one of the disks fails, however, the corresponding sector on every other disk is rendered useless because part of the data is now corrupted. RAID 0 does not implement error checking so any error is unrecoverable. More disks in the array means higher bandwidth, but greater risk of data loss.

RAID 1:_

It also named as DATA Mirroring. Mirrored set without parity. Provides fault tolerance from disk errors and single disk failure. 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.

RAID 3:_

Striped set with dedicated parity. This mechanism provides an improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single parity disk is a bottle-neck for writing since every write requires updating the parity data. One minor benefit is the dedicated parity disk allows the parity drive to fail and operation will continue without parity or performance penalty.

RAID 4:_

Identical to RAID 3 but does block-level striping instead of byte-level striping.

RAID 5:_

Striped set with distributed parity. Distributed parity requires all but one drive 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.

RAID 6:_

Striped set with dual 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 because large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. With dual parity, it gives time to rebuild the array without the data being volatile while the failed drive is being recovered.

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