RAID is an acronym for redundant array of independent disks, also known as redundant array of inexpensive disks. This is a technology that allows high levels of storage reliability from low-cost and less reliable PC-class disk-drive components, via the technique of arranging the devices into arrays for redundancy.
RAID is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. The different schemes/architectures are named the word RAID followed a number, as in RAID 0, RAID 1, etc. RAID’s various designs involve two key design goals: increase data reliability and/or increase input/output performance. When multiple physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across multiple disks, but the array is seen the computer user and operating system as one single disk. RAID can be set up to serve several different purposes.
RAID combines two or more physical hard disks into a single logical unit using special hardware or software. Hardware solutions are often 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, if one were to configure a hardware-based RAID-5 volume using three 250 GB hard drives (two drives for data, and one for parity) the operating system would be presented with a single 500 GB volume. Software solutions are typically implemented in the operating system and would present the RAID volume as a single drive to applications running within the operating system.
There are three key concepts in RAID: mirroring, the writing of identical data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant parity data is stored to allow problems to be detected and possibly repaired (known as fault tolerance). Different RAID schemes use one or more of these techniques, depending on the system requirements. The purpose of using RAID is to improve reliability and availability of data, ensuring that important data is not harmed in case of hardware failure, and/or to increase the speed of file input/output.
Each RAID scheme affects reliability and performance in different ways. Every additional disk included in an array increases the likelihood that one will fail, but using error checking and/or mirroring, the array as a whole can be made more reliable the ability to survive and recover from a failure. Basic mirroring can speed up the reading of data, as a system can read different data from multiple disks at the same time, but it may be slow for writing if the configuration requires that all disks must confirm that the data is correctly written. Striping, often used for increasing performance, writes each bit to a different disk, allowing the data to be reconstructed from multiple disks faster than a single disk could send the same data. Error checking typically will slow down performance as data needs to be read from multiple places and then compared. The design of any RAID scheme is often a compromise in one or more respects, and understanding the requirements of a system is important. Modern disk arrays typically provide the facility to select an appropriate RAID configuration.There are differ levels of RAID which are termed as RAID 0, RAID 1, RAD 2, RAID 10, etc.