Redundant Arrays of Independent Disks

Computerworld - A redundant array of independent disks (RAID) is a common system for high-volume data storage at the server level. RAID systems use many small-capacity disk drives to store large amounts of data and to provide increased reliability and redundancy. Such an array appears to the computer as a single logical unit consisting of multiple disk drives.

RAID storage can be done in a number of ways. Some RAID types emphasize performance, others reliability, fault tolerance or error correction. Which type you choose depends on what you're trying to accomplish.

Common to all RAID systems, however - and their real advantage - is the "hot-swapping" ability: You can pull out a defective drive and insert a new one in its place. For most RAID types, data on a failed disk can be rebuilt automatically without the server or the system ever having to be shut down.

RAID isn't the only way to protect large amounts of data, but regular backups and mirroring software are slower and often require shutting down the system if a drive fails.

Even if the disk doesn't crash the server, information technology workers would still need to shut down the servers to replace the drive. RAID instead rebuilds data from the remaining drives using mirrored or parity information, without requiring a shutdown.

The three most common RAID implementations are Levels 0, 3 and 5.

RAID Level 0, data striping, is the most basic model. On a normal hard drive, data is stored on consecutive sectors of the same disk. RAID 0 uses a minimum of two disk drives and divides data into blocks that range from 512 bytes to several megabytes, which are written alternately to the disks. Segment 1 is written to Disk 1, Segment 2 to Disk 2, and so on. When the system reaches the final drive in the array, it writes to the next available segment of Drive 1, and so forth.

Striping the data distributes the I/O load evenly across all the drives. And since drives can be written to or read from simultaneously, performance increases noticeably. But there's no data protection. If a disk fails, data is lost. RAID 0 isn't for mission-critical environments, but it's well suited to applications such as video production and editing or image editing.

RAID Level 3 includes data striping, but it also assigns one drive to store parity information. This provides some fault tolerance and is especially useful in data-intensive or single-user environments for accessing long sequential records. RAID 3 doesn't overlap I/O, and it requires synchronized-spindle drives to prevent performance degradation with short records.