Glossary
RAID: Redundant Array of Inexpensive Disks - A system of two or more disks used to attain higher data transfer rates and increased reliability.
Parity: Technique for verifying the validity of stored information by appending an extra bit and checking the sum with an odd or even result required.
Striping: Technique for connecting multiple drives to appear to be one drive by partitoning interleaving stripes across the drives.
Disk Mirroring: Technique in RAID1 where multiple drives appear to be one drive that increases the Read performance by storing duplicate data on the different drives.
RAID
| Type | Method | Description |
|---|---|---|
| RAID 0: | Disk Striping | Data transferred in parallel for concurrent processing without mirroring or parity. Fast to take advantage of SCSI but less reliability than single drives and no redundancy in case of drive failure. |
| RAID 1: | Disk Mirroring | Data transferred to one disk and copied on another with striping and mirroring to insure data availability if a drive fails. The ultimate in redundancy. |
| RAID 2: | Bit Interleaving | Data transferred and striped to multiple disks with error-correction and parity stored on one or more separate disks. Is fast but requires a large number of drives, so it is seldom used. |
| RAID 3: | Byte Interleaving | Data transferred to an even number of disks by striping it in blocks with a parity information on a separate drive, but drives cannot function independently. Not often used with drives, but often used on tape arrays. |
| RAID 4: | Sector Interleaving | Data transferred to individual drives in blocks with parity information updated constantly for the entire block on each drive. |
| RAID 5: | Block Interleaving | Data transferred in blocks to each drive and parity is stored on each drive, but different drives along with the blocks, and allows simultaneous read/write operations, but is not suited for high demand accesses if speed is paramount because the controller must put the data and the parity information on separate drives during a write operation. |
| RAID 6: | Block Striping | Data striped to multiple drives with parity stored on each or on a separate drive with a second completely different parity scheme. Has good read performance and slower write operations, but is not suited for high demand accesses if speed is paramount, but is excellent for mission-critical situations to protect against multiple drive failures. Few vendors offer it and it is expensive, though data remains available with failed drives. |
| RAID 0+1 | Mirrored striping | Data striped across drives and mirrored.Offers the best of each of the two standards in performance, reliability and availability by using the maximum in redundancy, but at a high cost. |
| RAID 5+3 | Data Striped across multiple RAID 3 arrays | Data striped to each array with higher performance than RAID 5 and good availability because the dependability is mixed with the fact that the information exists in more than one place; and able to survive drive failures in each array. |
