RAID

[Guest Michael Yardley]

râd) Short for Redundant Array of Independent (or Inexpensive) Disks,

a category of disk drives that employ two or more drives in

combination for fault tolerance and performance. RAID disk drives are

used frequently on servers but aren't generally necessary for personal

computers. RAID allows you to store the same data redundantly (in

multiple paces) in a balanced ay to improve overall performance.

There are number of different RAID levels:

 

Level 0 -- Striped Disk Array without Fault Tolerance: Provides data

striping (spreading out blocks of each file across multiple disk

drives) but no redundancy. This improves performance but does not

deliver fault tolerance. If one drive fails then all data in the array

is lost.

Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1

provides twice the read transaction rate of single disks and the same

write transaction rate as single disks.

Level 2 -- Error-Correcting Coding: Not a typical implementation and

rarely used, Level 2 stripes data at the bit level rather than the

block level.

Level 3 -- Bit-Interleaved Parity: Provides byte-level striping with a

dedicated parity disk. Level 3, which cannot service simultaneous

multiple requests, also is rarely used.

Level 4 -- Dedicated Parity Drive: A commonly used implementation of

RAID, Level 4 provides block-level striping (like Level 0) with a

parity disk. If a data disk fails, the parity data is used to create a

replacement disk. A disadvantage to Level 4 is that the parity disk

can create write bottlenecks.

Level 5 -- Block Interleaved Distributed Parity: Provides data

striping at the byte level and also stripe error correction

information. This results in excellent performance and good fault

tolerance. Level 5 is one of the most popular implementations of

RAID.

Level 6 -- Independent Data Disks with Double Parity: Provides block-

level striping with parity data distributed across all disks.

Level 0+1 -- A Mirror of Stripes: Not one of the original RAID levels,

two RAID 0 stripes are created, and a RAID 1 mirror is created over

them. Used for both replicating and sharing data among disks.

Level 10 -- A Stripe of Mirrors: Not one of the original RAID levels,

multiple RAID 1 mirrors are created, and a RAID 0 stripe is created

over these.

Level 7: A trademark of Storage Computer Corporation that adds caching

to Levels 3 or 4.

RAID S: (also called Parity RAID) EMC Corporation's proprietary

striped parity RAID system used in its Symmetrix storage systems.

See "Storage Strategies Made Simp