RAID (redundant array of independent disks, originally redundant array of inexpensive disks) is a storage technology that combines multiple disk drive components into a logical unit. Data is distributed across the drives in one of several ways called "RAID levels", depending on what level of redundancy and performance (via parallel communication) is required.
RAID is an example of storage virtualization and was first defined by David patterson,
RAID is now used as computer data storage schemes that can divide and replicate data among multiple physical drives. The physical drives are said to be in a RAID array,which is accessed by the operating system as one single drive
RAID 0 (block-level striping without parity or mirroring) has no (or zero) redundancy. It provides improved performance and additional storage but no fault tolerance. Hence simple stripe sets are normally referred to as RAID 0. Any drive failure destroys the array, and the likelihood of failure increases with more drives in the array (at a minimum, catastrophic data loss is almost twice as likely compared to single drives without RAID). A single drive failure destroys the entire array because when data is written to a RAID 0 volume, the data is broken into fragments called blocks. RAID 0 does not implement error checking, so any error is uncorrectable. More drives in the array means higher bandwidth, but greater risk of data loss
In RAID 1 (mirroring without parity or striping), data is written identically to two drives, thereby producing a "mirrored set"; at least two drives are required to constitute such an array. While more constituent drives may be employed, many implementations deal with a maximum of only two; of course, it might be possible to use such a limited level 1 RAID itself as a constituent of a level 1 RAID, The array continues to operate as long as at least one drive is functioning. With appropriate operating system support, there can be increased read performance, and only a minimal write performance reduction; implementing RAID 1 with a separate controller for each drive in order to perform simultaneous reads (and writes) is sometimes called multiplexing (or duplexing when there are only two drives).
In RAID 2 (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 drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive.
Typically RAID is used in large file servers, transaction of application servers, where data accessibility is critical, nd fault tolerance is required. Nowadays, RAID is also being used in desktop systems for CAD,
