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RAID Classification



  RAID Classification
Disk arrays seems to have been invented indepedently by a variety of groups. One specifically, the Computer Architecture group at the University of California, Berkeley invented the term Redundant Arrays of Inexpensive Disk  (RAID).

The original RAID classification described five levels of RAID (RAID 1 through 5). To these have been added RAID-0 (data striping), RAID-1 Enhanced (data stripe mirroring) and Orthogonal RAID-5 (which includes extra redundancy of components such as disk adapters). RAID-0 is not a pure RAID type, since it does not provide any redundancy.

Different designs of arrays perform optimally in different environments. The two main environments are those where high transfer rates are very important, and those where a high I/O rate is needed, that is, applications requesting short length random records.
RAID Level    Description
  
RAID-0    Block Interleave Data Striping w/o Parity
  
RAID-1    Disk Mirroring / Duplexing
  
RAID-1 (Enhanced)    Data Stripe Mirroring
  
RAID-2    Bit Interleave Data Striping with Hamming Code
  
RAID-3    Bit Interleave Data Striping with Parity Disk
  
RAID-4    Block Interleave Data Striping with one Parity Disk
  
RAID-5    Block Interleave Data Striping with Skewed Parity
  
Orthogonal RAID-5    RAID-5 with additional redundancy (such as disk adapters)


RAID Performance Characteristics

RAID Level Capacity Large Transfers High I/O Rate Data Avail.
Single Disk Fixed (100%) Good Good Note 1
RAID-0 Excellent Very Good Very Good Poor Note 2
RAID-1 Moderate (50%) Good Good Good
RAID-2 Very Good Good Poor Good
RAID-3 Very Good Very Good Poor Good
RAID-4 Very Good Very Good Poor Good
RAID-5 Very Good Very Good Good Good
Orthogonal RAID-5 Very Good Very Good Good Very Good


Note 1 : The MTBF for single disks can range from 10.000 to 800.000 hours.
Note 2 : Availibility = MBTF of one disk divided by the number of disks in the array.

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