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Last modified: Wednesday, June 07, 2006 

(rād) 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.

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.
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    For pages about RAID . Also check out the following links!

    Related Links

    Description of RAID levels 
    This is the first page in a series of pages describing the features provided by each RAID level. The pages are hosted by Advanced Computers & Network Corporation (AC&NC;).

    EnterpriseNetworkingPlanet provides practical advice and news for running and managing an enterprise network. In-depth articles and news cover topics such as network management, network monitoring, servers, communications, Internet telephony, operating systems, and much more.

    Open Networks Today 
    Networking news moves at a fast pace, and Open Networks Today lets you keep up with it. Open Networks Today offers its readers the ability to control how news is presented through customizing content filters, discussions, and news feed links. provides easy-to-understand help for small-network builders. The site contains how-to information for setting up and debugging home-office and small-business networks. Users can also find extensive troubleshooting information, tips on getting applications to work through firewalls, product reviews on network hardware and software, and more.

    Digi-Data Corporation
    Home page of Digi-Data Corporation, manufacturer of RAID controller products. Their site contains links to product information and a description of RAID and related resources.

    Using RAID subsystems
    Short FAQ covering questions about RAID subsystems.

    Yahoo!'s RAID page
    Yahoo!'s directory of RAID.

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    HP Business Store: RAID Drives - Purchase RAID Drives online. Also find HP Compaq notebooks and desktops, HP printers, servers, handhelds, and storage solutions.

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    Aberdeen: RAID Drives - Online retailer of computer hard drives including IDE, SCSI and removable drives.

    related categories

    Data Storage

    related terms

    data mirroring

    disk drive

    disk mirroring

    disk striping

    fault tolerance



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