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RAID Systems (page 4)

   New RAID classification

In 1996, RAB introduced an improved classification of the RAID systems. It divides RAID into three types:

  • Failure-resistant disk systems (that protect against data loss due to disk failure)
  • Failure-tolerant disk systems (that protect against loss of data access due to failure of any single component)
  • Disaster-tolerant disk systems (that consist of two or more independent zones, either of which provides access to stored data).

The original "Berkley" RAID classification is still kept as an important historical reference point and it also to recognize that RAID Levels 0-6 successfully define all known data mapping and protection schemes for disk. Unfortunately, the original classification caused some confusion due to assumption that higher RAID level implies higher redundancy and performance.  These confusion was exploited by RAID system manufacturers, and gave birth to the products with such names as RAID-7, RAID-10, RAID-30, RAID-S, etc.). The new system describes the data availability characteristics of the RAID system rather then the details of its implementation.

The next list provides criteria for all three classes of RAID:

Failure-resistant disk systems (meets criteria 1 to 6 minimum):

  1. Protection against data loss and loss of access to data due to disk drive failure
  2. Reconstruction of failed drive content to a replacement drive
  3. Protection against data loss due to a "write hole"
  4. Protection against data loss due to host and host I/O bus failure
  5. Protection against data loss due to replaceable unit failure
  6. Replaceable unit monitoring and failure indication

Failure-tolerant disk systems (meets criteria 1 to 15 minimum):

  1. Disk automatic swap and hot swap
  2. Protection against data loss due to cache failure
  3. Protection against data loss due to external power failure
  4. Protection against data loss due to a temperature out of operating range
  5. Replaceable unit and environmental failure warning
  6. Protection against loss of access to data due to device channel failure
  7. Protection against loss of access to data due to controller module failure
  8. Protection against loss of access to data due to cache failure
  9. Protection against loss of access to data due to power supply failure

Failure-tolerant disk systems (meets criteria 1 to 15 minimum):

  1. Protection against loss of access to data due to host and host I/O bus failure
  2. Protection against loss of access to data due to external power failure
  3. Protection against loss of access to data due to component replacement
  4. Protection against loss of of data and loss of access to data due to multiple disk failure
  5. Protection against loss of access to data due to zone failure
  6. Long-distance protection against loss of data due to zone failure

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