What is Redundant Array of Independent Disks?

Researchers are constantly trying to improve secondary storage media by raising their, performance, capacity as well as reliability. But any physical media has a limit to its performance and capacity. When it gets harder to make further improvements in physical storage media and its drive then researchers generally look for other methods for improvement. Mass storage devices are a result of such improvements that researchers have resorted to for larger capacity secondary devices. The concept is to use multiple units of storage media being used as a single secondary storage device. One attempt of this kind in direction of improving disk performance is to have multiple components in parallel. A number of fundamental questions for these kinds of systems are:

How are the disks arranged? 

May be as an array of disks.

Can separate I/O requests be handled by this kind of system in parallel? 

Yes however only if the disk accesses are from separate disks.

Can a single I/O request be handled in parallel?

Yes although the data block requested should be available on separate disks.

Can this array of disks be used for increasing reliability?

Yes however for that redundancy of data is necessary.