Intro To RAID

Although initially known as Redundant Array of Inexpensive Disks the name changed to Redundant Array of Independent Disks, RAID is a storage technology developed by a team at the University of California in the 1980s to allow an increase in capacity and reliability. The primary idea behind raid was redundancy which allows for disk failure without data loss it has evolved far beyond its initial scope.

Over the years and as data has become ever more critical the evolution of Raid has continued as a vital means of providing sustainable access to data for businesses and as the technology develops and becomes less expensive to home users too. A major advantage to the traditional Raid technology is the ability to function seamlessly even though a disk may be developing bad sectors or even with the failure of an entire disk in the array. Raid storage has also developed to provide an ever increasing demand for speed and storage capacity by deviations to the theory of traditional raid.

Raid types:

There are 6 common raid types referred to as levels (0,1,2,3,4 and 5) and also a similar in appearance technology to Raid known as JBOD as well as combinations of these Raid levels such as level 10 (raid 0+1) and level 6 (raid 5 + 1).

Level 0

Containing 2 or more disks speed is the primary advantage. Raid 0 operates by splitting the data over a set number of disks within the array, known as striping. Data is split into blocks of a certain size set by the user with each block written across disks within the array.

The main disadvantage of Raid 0 is the fact that it is not a True raid being that is has no fault tolerance through disk redundancy and as such should never be used for critical data storage. It is commonly used in graphic design and video editing or where read/write speed is of most importance.

Level 1

2 or more disks make up a raid 1 with the advantage being data safety. Known as mirroring, the data is duplicated across the disks to allow for the possibility of drive failure. If one of the disks does fail the data is still accessible. Raid 1 has no performance advantage as with 0 but it does offer a relatively low cost redundant storage solution.

Level 2

Raid 2 is a raid level which is no longer used due to modern hard disks containing ECC (Error Checking and Correction). Raid 2 uses Hamming algorithm (named after its inventor Richard Hamming) for use on disks which do not have built in error detection. Further information on raid 2 is beyond the scope of this article.

Level 3

Raid 3 is another seldom used raid level which due to its nature is quite specialised in its use, it uses byte level striping and a dedicated parity disk within the array and is suited to sequential read/write operation. Further information on raid 3 is beyond the scope of this article.

Level 4

Like level 3, level 4 raids are often overlooked in favour of a raid 5 solution. Level 4 uses block level striping with a dedicated parity disk. It offers no advantage over raid 5 and does not support multiple simultaneous write operations. Further information on raid 4 is beyond the scope of this article.

Level 5

Made up of 3 or more disks, raid 5 uses block level striping with distributed parity and is one of the most popular raid levels for general business use and as the cost of storage reduces even for home use.

A major advantage of raid 5 is its read/write data transaction rate and high efficiency due to its low ratio of parity disks to data disks. A disadvantage of level 5 is its relatively high cost compared with a raid 1 for example and its complex controller design, it is though regarded as the most versatile raid level. Raid 5 arrays often require a dedicated hardware controller to achieve performance results and although some operating systems support for software raid 5 this comes at a significant costs to performance.

JBOD

Consisting of 2 or more disks, Jbod is the most common non Raid drive architectures but is considered as a storage solution due to its simplicity. Known a Just a Bunch Of Disks, it is just that. Each disks operates as add on storage to the previous disk with no redundancy Jbod should never be considered for critical data storage.

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