Most Common RAID (Redundant Array of Independent Disks)
RAID 5
Is disk striping with parity, and it is one of the most common forms of RAID use today. It operates similarly to disk striping, as in RAID 0. The parity information is spread across all of the disks in the array instead of being limited to a single disk, as in RAID 3. Most implementations require a minimum of three disks and support a maximum of 32.
RAID 0
Is disk striping. It uses multiple drives and maps them together as a single physical drive. This is done primarily for performance, not for fault tolerance. If any drive in a RAID 0 array fails, the entire logical drive becomes unusable.
RAID 1
Is disk mirroring. Disk mirroring provides 100 percent redundancy because everything is stored on two disks. If one disk fails, another disk continues to operate. The failed disk can be replaced, and the RAID 1 array can be regenerated. This system offers the advantage of 100 percent data redundancy at the expense of doubling the storage requirements. Each drive keeps an exact copy of all information, which reduces the effective storage capability to 50 percent of the overall rated storage. Some implementations of disk mirroring are called disk duplexing (duplexing is a less commonly used term). The difference between mirroring and duplexing is one more controller card. With mirroring, one controller card writes sequentially to each disk. With duplexing, the same data is written to both disks simultaneously. Disk duplexing has much faster write performance than disk mirroring. Many hardware implementations of RAID 1 are actually duplexing, but they are still generally referred to as mirrors.
RAID 3
Is disk striping with a parity disk. RAID 3 arrays implement fault tolerance by using striping (RAID 0) in conjunction with a separate disk that stores parity information. Parity information is a value based on the value of the data stored in each disk location. This system ensures that the data can be recovered in the event of a failure. The process of generating parity information uses the arithmetic value of the binary data. This process allows any single disk in the array to fail while the system continues to operate. The failed disk is removed, a new disk is installed, and the new drive is then regenerated using the parity information. RAID 3 is common in older systems, and it's supported by most Unix systems.
