Standard levels
RAID 0
RAID 0 comprises striping (but no parity or mirroring). This level provides no data redundancy nor fault tolerance, but improves performance through parallelism of read and write operations across multiple drives. RAID 0 has no error detection mechanism, so the failure of one disk causes the loss of all data on the array.
RAID 1
RAID 1 comprises mirroring (without parity or striping). Data is written identically to two (or more) drives, thereby producing a "mirrored set". The read request is serviced by either of the two drives containing the requested data. This can improve performance if data is read from the disk with the least seek latency and rotational latency. Conversely, write performance can be degraded because both drives must be updated; thus the write performance is determined by the slower of the two drives. The array continues to operate as long as at least one drive is functioning.
RAID 2
RAID 2 comprises 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. This level is of historical significance only. Although it was used on some early machines (e.g. the Thinking Machines CM-2), it is not used by any current commercially available systems.
RAID 3
RAID 3 comprises 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 drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive. Although implementations exist, RAID 3 is not commonly used in practice.
RAID 4
RAID 4 comprises block-level striping with dedicated parity. Parity data is stored on a single dedicated drive.
RAID 4 was previously used primarily by NetApp, but has now been largely replaced by an implementation of RAID 6 (RAID-DP).
RAID 5
RAID 5 comprises block-level striping with distributed parity. Unlike in RAID 4, parity information is distributed among the drives. It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.
RAID 6
RAID 6 comprises block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, as large-capacity drives take longer to restore. As with RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced.
