RAID - New RAID Classification

New RAID Classification

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

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

The original "Berkeley" RAID classifications are still kept as an important historical reference point and also to recognize that RAID levels 0–6 successfully define all known data mapping and protection schemes for disk-based storage systems. Unfortunately, the original classification caused some confusion due to the assumption that higher RAID levels imply higher redundancy and performance; this confusion has been exploited by RAID system manufacturers, and it has given birth to the products with such names as RAID-7, RAID-10, RAID-30, RAID-S, etc. Consequently, the new classification describes the data availability characteristics of a RAID system, leaving the details of its implementation to system manufacturers.

  • Failure-resistant disk systems (FRDS) (meets a minimum of criteria 1–6)
    1. Protection against data loss and loss of access to data due to 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 (FTDS) (meets a minimum of criteria 1–15)
    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
  • Disaster-tolerant disk systems (DTDS) (meets a minimum of criteria 1–21)
    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 data and loss of access to data due to multiple drive failures
    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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