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RAID - redundant array of independent disks

Originally, the term RAID stood for "redundant array of inexpensive disks," but now it usually refers to a "redundant array of independent disks." While older storage devices used only one disk drive to store data, RAID storage uses multiple disks in order to provide fault tolerance, to improve overall performance, and to increase storage capacity in a system.

How RAID Works

With RAID technology, data can be mirrored on one or more other disks in the same array, so that if one disk fails, the data is preserved. Thanks to a technique known as "striping," RAID also offers the option of reading or writing to more than one disk at the same time in order to improve performance. In this arrangement, sequential data is broken into segments which are sent to the various disks in the array, speeding up throughput. Also, because a RAID array uses multiple disks that appear to be a single device, it can often provide more storage capacity than a single disk.

RAID Levels

RAID devices use many different architectures, depending on the desired balance between performance and fault tolerance. These architectures are called "levels." Standard RAID levels include the following:

Level 0: striped disk array without fault tolerance
Level 1: mirroring and duplexing
Level 2: error-correcting coding
Level 3: bit-interleaved parity
Level 4: dedicated parity drive
Level 5: block interleaved distributed parity
Level 6: independent data disks with double parity
Level 10: a stripe of mirrors

Some devices use more than one level in a hybrid or nested arrangement, and some vendors also offer non-standard proprietary RAID levels.

RAID History and Alternatives

Before RAID devices became popular, most systems used a single drive to store data. This arrangement is sometimes referred to as a single large expensive disk or SLED. However, SLEDs have some drawbacks. First, they can create I/O bottlenecks because the data cannot be read from the disk quickly enough to keep up with the other components in a system, particularly the processor. Second, if a SLED fails, all the data is lost unless it has been recently backed up onto another disk or tape.

In 1987, three University of California, Berkeley, researchers -- David Patterson, Garth A. Gibson, and Randy Katz -- first defined the term RAID in a paper titled A Case for Redundant Arrays of Inexpensive Disks (RAID). They theorized that spreading data across multiple drives could improve system performance, lower costs and reduce power consumption while avoiding the potential reliability problems inherent in using inexpensive, and less reliable, disks. The paper also described the five original RAID levels.

Today, RAID technology is nearly ubiquitous among enterprise storage devices and is also found in many high-capacity consumer storage devices. However, some non-RAID storage options do exist. One alternative is JBOD, short for "Just a Bunch of Drives." JBOD architecture utilizes multiple disks, but each disk in the device is addressed separately. JBOD provides increased storage capacity versus a single disk, but doesn't offer the same fault tolerance and performance benefits as RAID devices.

Another RAID alternative is concatenation or spanning. This is the practice of combining multiple disk drives so that they appear to be a single drive. Spanning increases the storage capacity of a drive; however, as with JBOD, spanning does not provide reliability or speed benefits.

RAID should not be confused with data backup. Although some RAID levels do provide redundancy, experts advise utilizing a separate storage system for backup and disaster recovery purposes.

Implementing RAID

In order to set up a RAID array, you'll need a group of disk drives and either a software or a hardware controller. Software RAID runs directly on a server, utilizing server resources. As a result, it may cause some applications to run more slowly. Most server operating systems include some built-in RAID management capabilities.

You can also set up your own RAID array by adding a RAID controller to a server or a desktop PC. The RAID controller runs essentially the same software, but it uses its own processor instead of the system's CPU. Some less expensive "fake RAID" controllers provide RAID management software but don't have a separate processor.

Alternatively, you can purchase a pre-built RAID array from a storage vendor. These appliances generally include two RAID controllers and a group of disks in their own housing.

Using a RAID array is usually no different than using any other kind of primary storage. The RAID management will be handled by the hardware or software controller and is generally invisible to the end user.

RAID Technology and Standards

The Storage Networking Industry Association has established the Common RAID Disk Data Format (DDF) specification. In an effort to promote interoperability among different RAID vendors, it defines how data should be distributed across the disks in a RAID device.

Another industry group called the RAID Advisory Board worked during the 1990s to promote RAID technology, but the group is no longer active.

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