RAID

RAID may refer to any of the following:

1. Short for redundant array of independent disks, RAID is an assortment of hard drives connected and set up in ways to help protect or speed up the performance of a computer's disk storage. RAID is commonly used on servers and high-performance computers. The picture of the Drobo is a good example of a device using RAID technology. RAID uses several techniques used in RAID, as explained below.

Spanning and software striping

Splitting information and writing it across multiple physical disk drives. RAID 0 utilizes this technique.

Mirroring

Duplication of data from one disk drive to another.

Duplexing

Duplicates the disk drive and the disk controller.

Deferred

Data is cached in cache memory and writes to the hard drive as the disk drive becomes available.

Hot swapping

Failed disk drives can be replaced, and data can be placed back onto the disk drive while the remainder of the system operates.

Hot sparing

Disk drive is automatically initialized into the array when another fails.

Spindle synchronization

Synchronization of the rotation of all disk drives in the array allows information to be written all at once.

Versions of RAID

RAID 0

Software stripping and block interleave (minimum 2 drives). Data is written to each drive in succession, each block going to the next available drive (striping) for a faster operation and less overload chance. The volume can be much larger than any single drive. Since no redundancy is provided, the failure of a single drive brings the system down. RAID 0 is the fastest and most efficient array type but offers no fault tolerance.

RAID 1

Disk mirroring and duplexing ( minimum 2 drives ). Drives are used in pairs, and all data is written identically to both drives. Each drive can be duplexed by connecting it to its own interface controller. The failure of one drive does not bring down the system. Instead, the other drive continues to operate. Of course, two drives are now used for the equivalent storage capacity of one drive. There is no performance gain at this level. The array of choices for performance-critical, fault-tolerant environments. Also, RAID 1 is the only choice for fault tolerance if no more than two drives are desired.

RAID 2

Data striping and bit interleave. Data is written across each drive in succession, one bit at a time. Checksum data is recorded in a separate drive. RAID 2 is very slow for disk writes and is seldom used today since ECC (error-correcting code) is embedded in almost all modern disk drives.

RAID 3

Data striping with bit interleave and parity checking. RAID 3 is similar to lever 2, but more reliable. Data striping is done across the drives, one byte at a time. Usually, 4 or 5 provide very high data transfer rates. One drive is dedicated to storing parity information. The failure of a single drive can be compensated using the parity drive to reconstruct the failed drive contents. Since the parity drive is accessed on every write operation, data writing tends to be slower. The failure of two drives or more is a problem. RAID 3 is used in data-intensive environments with long sequential records to speed up data transfer. However, it does not allow multiple I/O operations to be overlapped and requires synchronized spindle drives to avoid performance degradation with short records.

RAID 4

Block interleave data striping with parity checking. As in level 3, RAID 4 uses a single parity drive and block data striping like in RAID 0. The drives in this RAID level function individually, with an individual drive reading a block of data. A failure of the controller is catastrophic. Offers no advantages over RAID 5 and does not support multiple simultaneous write operations.

RAID 5

Block interleave, data striping with distributed check data on all drives. The one to use for NetWare. Parity information is distributed across all drives. RAID 5 efficiency goes up as the number of disks increases. You can use hot spares to rebuild a failed drive on "the fly." The best choice in multi-user environments, which are not write performance sensitive. However, at least three, and more typically five drives, are required for RAID 5 arrays.

RAID 6

Extension to RAID 5, which adds a log-structured file system providing a mapping between a disk drive's physical sectors and their logical representation. As information is written, it is placed in sequential physical disk sectors.

RAID 10

Stripped array whose segments are RAID 1 arrays and contain the same fault tolerance as RAID 1. High I/O rates are achieved by stripping RAID 1 segments. Excellent solution for those considering RAID 1 since it provides good write performance but is an expensive solution.

RAID 53

Implemented as striped RAID 0 array whose segments are RAID 3 arrays. RAID 53 also contains the same fault tolerance and overhead as RAID 3. Excellent solution for those considering RAID 3 since it provides additional write performance but is an expensive solution and requires all drives to have the same synchronization.

What is RAID 0/1?

Some computer and RAID manufacturers have created a scheme for labeling their devices. For example, RAID 0/1 would mean that it combines the striping of RAID 0 with the mirroring of RAID 1.

2. A raid is a large group of people, larger than a single group, that team up together to take on difficult quests and bosses in an MMORPG (massively-multiplayer online role-playing game).

3. In general, a raid may describe a sudden attack on someone or something.

Array, Computer acronyms, Game terms, Hard drive terms, Redundant