redundant array of inexpensive disks

   Abbreviated RAID. In networking and mission-critical applications, a method of using several hard disk drives (often SCSI or Integrated Drive Electronics [IDE] drives) in an array to provide fault tolerance in the event that one or more than one drive fails. Each level of RAID is designed for a specific use:

   • RAID 0

   Data is striped over one or more drives in drives, but there is no redundant drive.

   RAID 0 provides no fault tolerance because the loss of a hard disk means a complete loss of data. Some classification schemes omit RAID 0 for this reason.

   • RAID 1

   Two hard disks of equal capacity duplicate or mirror each other's contents. One disk continuously and automatically back sup the other disk. This method is also known as disk mirroring or disk duplexing, depending on whether one or two independent hard-disk controllers are used.

   • RAID 2

   Bit-interleaved data is written across several drives, and then parity and error-correction information is written to additional separate drives. The specific number of error-correction drives depends on the allocation algorithm in use.

   • RAID 3

   Bit-interleaved data is written across several drives, but only one parity drive is used. If an error is detected, the data is reread to resolve the problem. The fact that data is reread in the event of an error may add a small performance penalty.

   • RAID 4

   Data is written across drives by sectors rather than at the bit level, and a separate drive is used as a parity drive for error detection. Reads and writes occur independently.

   • RAID 5

   Data is written across sectors, and parity information is added as another sector, just as if it were ordinary data. This level of RAID can provide faster performance as the parity information is written across all the drives, rather than to a single parity drive.

   There is not much difference in speed or quality among these levels. The appropriate level of RAID for any particular installation depends on network usage. RAID levels 1,3, and 5 are available commercially, and levels 3 and 5 are proving popular for networks.

   Several vendors have created their own RAID levels, including 6, 7,10, 11, and 35. Some of these are actually combinations of existing RAID levels, such as Compaq's Level 0+1, which combines RAID levels 0 and 1.

   See also backup; disk striping; disk striping with parity; parity; single large expensive disk.