RAID

10/12/96

RAID (Redundant Array of Inexpensive Disks) is a technique proposed in 1988 for using a number of small, inexpensive disks to emulate a larger, faster, more reliable disk. It uses a device called a RAID controller to scatter data over two or more small, relatively low performance, disks in ways that either enhance speed or reliability of storage. RAID comes in a number of flavors called "Levels". There is no inference that Level 1 (for example) is intermediate between Levels 0 and 2 on any given parameter. The levels are:

Raid Level Name Details

RAID 0 Striping locks all the disks into synchronization and reads and writes portions of data to different disks at the same time thereby yielding very high data transfer rates, but no improvement in access speed. There is no redundancy in the disk usage in RAID 0.

RAID 1 Shadowing/Mirroring writes the same data to several disks that are synchronized but out of phase. This offers a great improvement in access speed and improved read performance , but, at best, no improvement in write data transfer rates.

RAID 2 Error corrected one bit per disk. Offers very high transfer rates with no improvement in access time. Used occasionally in supercomputer environments -- Superseded by RAID 3.

RAID 3 Block (usually Byte) Striping with Dedicated Parity on a separate disk.

RAID 4 Conventional storage with parity information on a separate drive. Any performance improvement comes from the capability to do simultaneous read operations of different files.

RAID 5 Similar to Raid 4 but with parity distributed over the data disks and always on a different drive than the data.

RAID 6 Block Striping with two sets of Distributed Parity.

Raid Level	Name	Details
RAID 0	Striping	locks all the disks into synchronization and reads and writes portions of data to different disks at the same time thereby yielding very high data transfer rates, but no improvement in access speed. There is no redundancy in the disk usage in RAID 0.
RAID 1	Shadowing/Mirroring	writes the same data to several disks that are synchronized but out of phase. This offers a great improvement in access speed and improved read performance , but, at best, no improvement in write data transfer rates.
RAID 2		Error corrected one bit per disk. Offers very high transfer rates with no improvement in access time. Used occasionally in supercomputer environments -- Superseded by RAID 3.
RAID 3		Block (usually Byte) Striping with Dedicated Parity on a separate disk.
RAID 4		Conventional storage with parity information on a separate drive. Any performance improvement comes from the capability to do simultaneous read operations of different files.
RAID 5		Similar to Raid 4 but with parity distributed over the data disks and always on a different drive than the data.
RAID 6		Block Striping with two sets of Distributed Parity.

RAID Levels can be combined (e.g. RAID 0 +1, and RAID 35) resulting in intermediate performance on some parameters. For example a RAID 0 + 1 setup can offer higher data transfer rates than RAID 1 with better access times than RAID 0. RAID performance impacts may be different for reads than for write. RAID may be software or hardware based.

  Performance for an 8GB disk Array
    Disk Required for 8Gb effective storage: 
      R0(2x4Gb); R1(2x8Gb); R2(11x1Gb), R3/4/5(10x2Gb)
    Read Transaction Rate relative to 8Gb non-RAID
      R0/2/3=Same, R1=2x, R4=4x, R5=5x
    Write Transaction Rate relative to 8Gb non-RAID
      R0/1/2/3/4=Same, R5=2x+
    Transfer Rate relative to 8Gb non-RAID
      R0=Same, R1=2x, R2=8x, R3/R4=4x, R5=5X(read)/2x+(write)

RAID is generally implemented using SCSI interfaces but as of early 1999, IDE RAID controllers are starting to appear.