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.

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.
 

To create an optimal cost-effective RAID configuration, we need to simultaneously achieve the following goals:
 

• Maximize the number of disks being accessed in parallel.
• Minimize the amount of disk space being used for redundant data.
• Minimize the overhead required to achieve the above goals.