Understanding Disk Fault Tolerance

Understanding Disk Fault Tolerance

Fault tolerance refers to the capability of a computer or network to continue to function when some component fails. Disk fault tolerance refers to methods of storing data on the disk in such as way as to create redundancy of the data, so that it can be retrieved or recreated if a disk fails. Fault tolerance is not a substitute for backing up your data, but should be used in conjunction with a regular backup schedule that includes offsite storage. Generally, a fault tolerance solution will enable you to get up and running again more quickly than if you have to restore from backup, but backups are another line of defense in case the entire computer fails or is destroyed by fire, flood, etc.

There are several different ways to achieve disk fault tolerance. The most common implementation is known as RAID, or Redundant Array of Independent (or Inexpensive) Disks. Multiple disks can be configured in a number of different ways to create a fault-tolerant array. Data can simply be mirrored from one disk to another, or parity information can be stored that will enable the regeneration of lost data. RAID can be implemented either as a hardware or software solution. There are many different “levels” of RAID: 0, 1, 2, 3, 4, 5, 6, 7, 10, 0+1, and 53 are the most common. Some of these can be implemented only via the hardware. For more information about the different levels of RAID, see the RAID.edu Web site at www.acnc.com/04_01_00.html.

Windows Server 2003 has built-in support for three levels of software-implemented RAID:

  • level 0 (disk striping, no parity)
  • level 1 (disk mirroring)
  • level 5 (striping with parity)

The biggest advantage of hardware RAID is performance; disk access is faster because you don’t have the operating system overhead (the RAID disks appear as one to the operating system). The big advantage of software RAID is cost; you don’t have to buy extra expensive RAID controllers or other additional hardware to use it.

Source: Managing and Maintaining a Windows Server 2003 Environment Study Guide

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