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RAID (Redundant Array of Independent Disk)Technology


l A natural solution is a large array of small independent disks acting as a single higher-performance logical disk. A concept called data striping is used, which utilizes parallelism to improve disk performance.
l Data striping distributes data transparently over multiple disks to make them appear as a single large, fast disk. 

    Different raid organizations were defined based on different
    combinations of the two factors of granularity of data
    interleaving (striping) and pattern used to compute redundant
    information.
l Raid level 0 has no redundant data and hence has the best
     write performance.
l Raid level 1 uses mirrored disks.
l Raid level 2 uses memory-style redundancy by using
    Hamming codes, which contain parity bits for distinct
    overlapping subsets of components. Level 2 includes both
    error detection and correction.
l Raid level 3 uses a single parity disk relying on the disk
    controller to figure out which disk has failed.
l Raid Levels 4 and 5 use block-level data striping, with
    level 5 distributing data and parity information across all
    disks.
l Raid level 6 applies the so-called P + Q redundancy
    scheme using Reed-Soloman codes to protect against up
     to two disk failures by using just two redundant disks.
Use of RAID Technology 
Different raid organizations are being used under
different situations
l Raid level 1 (mirrored disks)is the easiest for rebuild of a disk from other disks– It is used for critical applications like l
   Raid level 2 uses memory-style redundancy by using Hamming codes, which contain parity bits for distinct overlapping subsets of components. Level 2 includes both error detection and correction.
l Raid level 3 ( single parity disks relying on the disk controller to figure out which disk has failed) and level 5 (block-level data striping) are preferred for Large volume storage, with level 3 giving higher transfer rates.
l Most popular uses of the RAID technology currently are: Level 0 (with striping), Level 1 (with mirroring) and Level 5 with an extra drive for parity.
l Design Decisions for RAID include – level of RAID, number of disks, choice of parity schemes, and grouping of disks for block-level striping.  

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