failure through the use of multiple disks and by the use of data distribution and correction techniques.

RAID can be software, hardware or combination of both.

SOFTWARE RAID uses more system resources as more disk ports and channels are required. It may have lower

cost than hardware RAID because it has no dedicated RAID controller but has lower performance.

HARDWARE RAID offloads parity generation and checking. It also allows for greater disk capacity per disk port.

It requires expensive RAID controller.

RAID has got many levels.

Level 0: Also known as disk stripping, because it uses a disk file system called a strip set. This level does not

provide fault tolerance. Data is divided into blocks and is spread in a fixed order amond all the disks in the array.

This level improves read and write performance by spreading operations across multiple disks, so that operations

can be performed independently.

Level1: This level is also known as disk mirrroring because it uses a disk file system called a mirror set. This level

provides fault tolerance.

It provides a  redundant, identical copy of a selected disk. All data written to the primary disk is written to the

mirror disk. It also generally improves read performance but may degrade write performance.

Level 2: RAID level 2 uses error correction algorithm that employs disk stripping strategy that breaks a file into

bytes and spreads it across multiple disks. The error correction method requires several disks. RAID level 2 is

more advanced than level 0, because it provides fault tolerance, but is not as efficient as other RAID levels and

is hence not generally used.

Level 3: It is similar to level 2, because it uses the same stripping method as level 2, but it requires only one disk

for parity data. This level suffers from a write bottleneck, because all parity data is written to a single drive, but

provides some read and write performance improvement.

Level 4: It is similar to level 3, because the stripping method stands the same and requires only one disk for

parity data, but it employs striped data in much larger blocks or segments. This level is not as efficient as level 5

because(as in level 3) all parity data is written to a single drive, so it also suffers from a write bottleneck.

Level 5: Known as Stripping with parity. It is most popular and is similar to level 4 in that it stripes the data in

large blocks across all the disks in the array. It differs in that it writes tha parity across all the disks. The data 

redundancy is provided by the parity information. Data and parity information are arranged on the disk array so

that the two are always on different disks. It also has better performance than level 1 and provides fault tolerance.

Virtualizing storage networks enables two key additional capabilities:

• The ability to mask or hide volumes from servers that are not authorized to access those volumes, providing an additional level of security.
• The ability to change and grow volumes on the fly to meet the needs of individual servers.

Essentially, anything other than a locally attached disk drive might be viewed in this light. Typically, storage virtualization applies to larger SAN (storage area network) arrays, but it is just as accurately applied to the logical partitioning of a local desktop hard drive, redundant array of independent disks (RAID), volume management, virtual memory, file systems and virtual tape. A very simple example is folder redirection in Windows, which lets the information in a folder be stored on any network-accessible drive. Much more powerful (and more complex) approaches include SANs. Large enterprises have long benefited from SAN technologies, in which storage is uncoupled from servers and attached directly to the network. By sharing storage on the network, SANs enable highly scalable and flexible storage resource allocation, high efficiency backup solutions, and better storage utilization.