Using SANs and NAS

Appendix B. RAID Levels

Table B-1 contains a brief description of each RAID level. A more detailed description of each level follows.

Table B-1.  RAID definitions

Level	Description
RAID	A disk array in which part of the physical storage capacity stores redundant information about user data stored on the remainder of the storage capacity. The redundant information enables regeneration of user data in the event that one of the array’s member disks or the access data path to it fails.
Level 0	Disk striping without data protection. (Since the “R” in RAID means redundant, this isn’t technically RAID.)
Level 1	Mirroring. All data is replicated on a number of separate disks.
Level 2	Data is protected by Hamming code. Uses extra drives to detect 2-bit errors and correct 1-bit errors on the fly. Interleaves by bit or block.
Level 3	Each virtual disk block is distributed across all array members but one, with parity check information stored on a separate disk.
Level 4	Data blocks are distributed as with disk striping. Parity check is stored in one disk.
Level 5	Data blocks are distributed as with disk striping. Parity check data is distributed across all members of the array.
Level 6	Like RAID 5 but with additional independently computed check data.

RAID 0

The RAID hierarchy begins with RAID 0—or the striping of several physical disks into one larger virtual disk. RAID can then be combined with RAID 1 in two ways, referred to as RAID 0+1 or RAID 1+0, depending on ...