Why we need Redundant Array of Independent Disks (RAID)

The need for RAID can be summarized in two points given below. The two keywords are Redundant and Array.
 

• An array of multiple disks accessed in parallel will give greater throughput than a single disk.
• Redundant data on multiple disks provides fault tolerance.

Provided that the RAID hardware and software perform true parallel accesses on multiple drives, there will be a performance improvement over a single disk. With a single hard disk, you cannot protect yourself against the costs of a disk failure, the time required to obtain and install a replacement disk, reinstall the operating system, restore files from backup tapes, and repeat all the data entry performed since the last backup was made.
With multiple disks and a suitable redundancy scheme, your system can stay up and running when a disk fails, and even while the replacement disk is being installed and its data restored.