RAID (redundant array of independent disks; originally redundant array of inexpensive disks) is a technology that increases disk reliability in a multi disk configuration by allowing one disk to fail while still maintaining all the data. It does this by placing all data across multiple disks instead of independently on individual disks. In this way, the lost data from one disk can be recreated from the data on the remaining disks.
In addition, by placing data on multiple disks, I/O operations can overlap in a balanced way, improving performance. Since multiple disks increases the mean time between failure (MTBF), storing data redundantly also increases fault-tolerance.
A RAID appears to the operating system to be a single logical hard disk. RAID employs the technique of striping, which involves partitioning each drive's storage space into units ranging from a sector (512 bytes) up to several megabytes. The stripes of all the disks are interleaved and addressed in order.
In a single-user system where large records, such as medical or other scientific images, are stored, the stripes are typically set up to be small (perhaps 512 bytes) so that a single record spans all disks and can be accessed quickly by reading all disks at the same time.
In a multi-user system, better performance requires establishing a stripe wide enough to hold the typical or maximum size record. This allows overlapped disk I/O across drives.
Redundant Arrays of Independent (or Inexpensive) Disks, or RAID, is an evolving technology that offers significant advantages in storage capacity, performance, and reliability to firms that have requirements for more information than can be readily stored and accessed on a single personal computer. A RAID system comprises two main components: an array of four or more disks and a RAID controller. The RAID controller is an electronic device that provides the interface between the host computer and the array of disks. It makes the array of disks look like one very large, very fast, very reliable disk to the host computer. From the viewpoint of the host computer, this large virtual disk operates seamlessly and transparently just like any other disk; it does not require changes to the computer’s operating system or application software.
There are number of different RAID levels, with RAID 5 being the most common:
