A Redundant Array of Independent Disks (RAID) controller is a piece of hardware or system software that control and manage the RAID level of the RAID storage, it is largely used in servers because they have a large space of storage that importantly be recoverable and secure.
The RAID controller is also largely used in clustered environments like Network-attached storage (NAS), which are standalone devices, usually SCSI, that have a network interface but no PC. NAS is possibly include a built-in RAID controller to allow all the hard disks to be fault tolerant and function as a single volume.
RAID combines two or more physical hard disks into a single logical unit by using either special hardware or software. Hardware solutions often are designed to present themselves to the attached system as a single hard drive, so that the operating system would be unaware of the technical workings. For example, you might configure a 1TB RAID 5 array using three 500GB hard drives in hardware RAID, the operating system would simply be presented with a "single" 1TB volume. Software solutions are typically implemented in the operating system and would present the RAID drive as a single volume to applications running upon the operating system.
There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Different RAID levels use one or more of these techniques, depending on the system requirements. RAID's main aim can be either to improve reliability and availability of data, ensuring that important data is available more often than not (e.g. a database of customer orders), or merely to improve the access speed to files (e.g. for a system that delivers video on demand TV programs to many viewers).
RAID is a wide used fault-tolerance technology that provide a duplication of data stored across drives so that if a drive fails, the other drives in the solution can provide the data. The benefit is that if a drive fails, you can wait to fix the problem at the end of the business day, knowing that the redundancy of your solution will take care of the missing data.
There are different types of RAID, known as levels, and each level provides a different benefit with a different type of redundancy. The next parts of this group of posts will give you a full description of popular RAID levels supported by different network operating systems.
Normal storage concept.