Standard levels
A number of standard schemes have evolved which are referred to as levels. There were five RAID levels originally conceived, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary). RAID levels and their associated data formats are standardized by the Storage Networking Industry Association (SNIA) in the Common RAID Disk Drive Format (DDF) standard
RAID 0
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. Any drive failure destroys the array, and the likelihood of failure increases with more drives in the array.
RAID 1
In RAID 1 (mirroring without parity or striping), data are written identically to two drives, thereby producing a "mirrored set"; the read request is serviced by either of the two drives containing the requested data, whichever one involves least seek time plus rotational latency. Similarly, a write request updates the stripes of both drives. The write performance depends on the slower of the two writes (i.e. the one that involves larger seek time and rotational latency). 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. The array continues to operate as long as at least one drive is functioning.
RAID 2
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.[5] This theoretical RAID level is not used in practice.
RAID 3
In RAID 3 (byte-level striping with dedicated parity), all disk spindle rotation is synchronized, and data are striped so each sequential byte is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive.[5] Although implementations exist,[9] RAID 3 is not commonly used in practice.
RAID 4
RAID 4 (block-level striping with dedicated parity) is equivalent to RAID 5 (see below) except that all parity data are stored on a single drive. In this arrangement files may be distributed between multiple drives. Each drive operates independently, allowing I/O requests to be performed in parallel.
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 (block-level striping with distributed parity) distributes parity along with the data and requires all drives but one to be present to operate; 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. RAID 5 requires at least three disks
RAID 6
RAID 6 (block-level striping with double distributed parity) provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems. This becomes increasingly important as large-capacity drives lengthen the time needed to recover from the failure of a single drive. Like RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced and the associated data rebuilt.
RAID 10
In RAID 10, often referred to as RAID 1+0 (mirroring and striping), data is written in stripes across primary disks that have been mirrored to the secondary disks.
