A simple replacement of one drive by a group of drives will not increase reliability since the life of the entire system will depend on any of this drives. In fact, reliability (mean time before failure or MTBF) will decrease with increasing number of drives since the probability for one of them to fail will grow. This is why a certain level of redundancy is needed in the design of drive array to increase reliability of the entire storage system. RAID is an assembly of disk drives, known as disk array, that operates as one storage unite. In general, the drives could be any storage system with random data access, such as magnetic hard drives, optical storage, magnetic tapes, etc. When the speed (data transfer rate) is an issue, the fastest SCSI hard drives are typically used.

RAID Functions

Redundant Array of Independent Disks (RAID) technology serves the following functions:

Immediate availability of data and, depending on the RAID level,recovery of lost data.
Redundancy of data at a chosen level.

Depending on the level of RAID that you are using, this method of data storage provides the data redundancy needed for a highly secure system, with additional benefit of faster retrieval of data through multiple channel access. If one or a few disk drives fail, they can be normally exchanged without interruption of normal system operation. Thus, disk arrays can ensure that no data is lost if one disk drive in the array fails. The array includes drives, controllers, enclosure, power supplies, fans, cables, etc. and software. Each array is addressed by the host computer as one drive. There are several types of RAID configuration, called levels, which control the ways of organizing data on the drives and organizing the flow of data to and from the host computer.

In 1993, the Raid Advisory Board (RAB) established the RAID Level Conformance program, which closely followed initial classification found in early UC Berkley work.

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