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RAID Concepts


Short form for Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks. An advanced technology that simultaneous uses two or more hard disk drives to achieve greater levels of performance, reliability, and larger disk size.

Organization of multiple disks into a large, high performance logical disk. Disk arrays stripe data across multiple disks and access them in parallel to achieve: Higher data transfer rates on large data accesses and Higher I/O rates on small data accesses.

One major disadvantage due to Redundancy is that whenever there is a write operation, there is a change of data. This change also, has to be reflected in the disks storing redundant information.

Need for RAID

  • Array of multiple disks accessed in parallel for greater throughput than a single disk.
  • Redundant data on multiple disks provides fault tolerance.
  • Maximize the number of disks being accessed in parallel.
  • Minimize the amount of disk space being used for redundant data.
  • Data Striping for improved performance.
  • Redundancy for improved availability.

RAID Types and Description





Min/Max # of disks





Striped set without parity or Striping. Provides improved performance and additional storage but no fault tolerance. Any disk failure destroys the array, which becomes more likely with more disks in the array. A single disk failure destroys the entire array because when data is written to a RAID 0 drives, the data is broken into fragments.

2 / 32


Mirrored set without parity or Mirroring. Provides fault tolerance from disk errors and failure of all but one of the drives.


2 / 32


Striped set with dedicated parity or bit interleaved parity or byte level parity. provides an improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single parity disk is a bottle-neck for writing since every write requires updating the parity data.

3 / 32


Block level parity. Identical to RAID 3, but does block-level striping instead of byte-level striping.


3 /32


Striped set with distributed parity or interleave parity. Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure.

3 /32


Striped set with dual distributed parity. Provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high availability systems.

4 /32



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