Originally, the term RAID was defined as redundant array of inexpensive disks, but now it usually refers to a redundant array of independent disks. RAID is a data storage technology that combines multiple physical disk drive components into a single logical unit for the purposes of data redundancy & performance improvement.
Before RAID devices became popular, most systems used a single drive to store data. This arrangement is sometimes referred to as a SLED (single large expensive disk). The term “RAID” was invented by David Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 in a paper titled “A Case for Redundant Arrays of Inexpensive Disks (RAID). “
RAID is the way of combining several independent and relatively small disks into a single storage of a large size. RAID devices use many different architectures, called levels, depending on the desired balance between performance and fault tolerance. It is used to improve disk I/O (performance) and reliability of your server or workstation. A RAID can be deployed using both software and hardware. RAID levels describe how data is distributed across the drives.
Standard RAID Levels
RAID 0 consists of striping, without mirroring or parity. This RAID level doesn’t provide fault tolerance but increases the system performance. If one drive fails then all data in the array is lost.
RAID 1 consists of data mirroring, without parity or striping. Data is written identically to two drives, thereby producing a “mirrored set” of drives. Using this technique provides fault tolerance in the loss of no more than one member disk. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.
RAID 2 consists of bit-level striping with dedicated Hamming-code parity. All disk spindle rotation is synchronized and data is striped such that each sequential bit is on a different drive.
RAID 3 consists of byte-level striping with dedicated parity. All disk spindle rotation is synchronized and data is striped such that each sequential byte is on a different drive.
RAID 4 consists of block-level striping with dedicated parity. This level was previously used by NetApp, but has now been largely replaced by a proprietary implementation of RAID 4 with two parity disks, called RAID-DP.
RAID 5 consists of block-level striping with distributed parity. RAID 5 requires at least three disks, two disk for data uses and one disk for parity. The parity information is striped across each drive, allowing the array to function even if one drive were to fail.
Deployment of RAID
There are two type of RAID Deployment available. One is Hardware RAID, and another is Software RAID.
In a hardware RAID setup, the drives connect to a special RAID controller inserted in a fast PCI-Express (PCI-e) slot in a motherboard. When storage drives are connected directly to the motherboard without a RAID controller, RAID configuration is managed by utility software in the operating system, and thus referred to as a software RAID setup. Both hardware and software redundant RAID arrays may support the use of hot spare disks. Such disks are physically installed in the array and are inactive until an active disk fails. The RAID controller automatically replaces the failed drive with the spare and starts the rebuilding process for the affected array.