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In computer storage, a Redundant Array of Independent Drives , also known as Redundant Array of Inexpensive Drives (RAID) is an term for data storage schemes that divide and/or replicate data among multiple hard drives. They offer, depending on the scheme, increased data reliabilityand/or I/O performance.

Fundamentally, RAID combines multiple hard disks into a single logical unit. There are two ways this can be done: in hardware and in software. Hardware combines the drives into a logical unit in dedicated hardware which then presents the drives as a single drive to the operating system. Software does this within the operating system and presents the drives as a single drive to the users of the system.[1]

RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.

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: EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems. [2]
  • Reference

    1.http://en.wikipedia.org/wiki/RAID

    2.http://www.webopedia.com/TERM/R/RAID.html

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