11 October 2009

Standard RAID levels

RAID stands for Redundant Array of Inexpensive Disks, because these techniques were originally applied to replace en expensive reliable disk with multiple cheap unreliable disks.

  • RAID 0: not one of the original RAID levels as it does not increase reliability
    • striping: write evenly across two disks (or partitions)
    • concatenation: continue on another disk when the first is full
      • often not considered RAID0
  • RAID 1: mirroring, write all data to multiple disks
  • RAID 0+1, RAID 1+0: stripe and mirror, mirror and stripe
  • RAID 2: write all data evenly to multiple disks, but use dedicated disk for parity checks. Distribute data at the bit level. Obsolete, because individual disks now use bit level parity checking..
  • RAID 3: Same as RAID 2, but distribute data at the byte level. Rarely used.
  • RAID 4: Same as RAID 2, but distribute data at the block (stripe) level. Rarely used.
  • RAID 5: Same as RAID 4, but no dedicated parity disk. Commonly used.
  • RAID 6: Same as RAID 5, but uses two parity blocks.
    • Not one of the original RAID levels. 
Plenty of vendors use other, non standard numbers and variant