RAID (redundant array of independent disks; originally redundant array of inexpensive disks) is a way of storing the same data in different places (thus, redundantly) on multiple hard disks. 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.

RAID systems provide large amounts of storage by making the data available on several disks readily available to the host computer. RAID systems may contain as many as 75 disks similar to disks used in personal computers. With current technology each disk may provide 10 gigabytes (billion bytes) or more of information. Thus, RAID systems may contain several hundred gigabytes for computer databases, computer networks, video production and editing, prepress, medical imaging and other applications.

Owing to their electronic design, the performance of electronic devices such as CPUs and networks has continued to grow at a rapid pace. Unfortunately, the electro-mechanical design of computer disks has limited their performance growth. Indeed, microprocessor performance has been doubling about every two years, while disk performance has taken 10 years to double.

The electronic controllers in RAID systems overcome this limitation by striping data across the array of disks and by using parallel data paths. Striping data simply means that when the host computer sends information to the RAID system, the controller writes a portion of that information (a stripe) on each of several disks. Thus, the data is distributed across the disks rather than being written only on one disk. The RAID controller also uses parallel data paths, so that it can perform the operations of reading and writing information to several disks simultaneously. With these capabilities, a RAID system can write information to the disk array or read information from the disk array at speeds as high as 35 megabytes (million bytes) per second. In contrast, with a single disk the transfer rate is only about 10 megabytes per second.

RAID systems also provide high reliability and data availability through a technique called parity checking. In this scheme, when the RAID controller writes information on the disks, it also writes redundant information called parity bits. These parity bits can be computed in parallel to other operations, so that RAID systems suffer no performance penalty when computing parity. This parity information has the fascinating property that the RAID controller can re-compute the information that was on a disk should the disk or its connections fail. Advanced RAID systems will reconstruct the data from a failed disk onto a spare disk, so that the RAID system continues to operate at high performance without loss of data even if one of the component disks fails or is removed from the system!

With increasing demands for mass storage capacity, performance, and reliability in their computer systems, many firms are adopting RAID technology to complement their computer systems. RAID systems keep large transaction data bases online, they provide real-time video and information for broadcast, and they provide rapid access to large electronic files. With the advantages RAID systems offer, they are becoming used in an increasing number of business and scientific applications.

There are at least nine types of RAID plus a non-redundant array (RAID-0):

RAID-0. This technique has striping but no redundancy of data. It offers the best performance but no fault-tolerance.

RAID-1. This type is also known as disk mirroring and consists of at least two drives that duplicate the storage of data. There is no striping. Read performance is improved since either disk can be read at the same time. Write performance is the same as for single disk storage. RAID-1 provides the best performance and the best fault-tolerance in a multi-user system.

RAID-2. This type uses striping across disks with some disks storing error checking and correcting (ECC) information. It has no advantage over RAID-3.

RAID-3. This type uses striping and dedicates one drive to storing parity information. The embedded error checking (ECC) information is used to detect errors. Data recovery is accomplished by calculating the exclusive OR (XOR) of the information recorded on the other drives. Since an I/O operation addresses all drives at the same time, RAID-3 cannot overlap I/O. For this reason, RAID-3 is best for single-user systems with long record applications.

RAID-4. This type uses large stripes, which means you can read records from any single drive. This allows you to take advantage of overlapped I/O for read operations. Since all write operations have to update the parity drive, no I/O overlapping is possible. RAID-4 offers no advantage over RAID-5.

RAID-5. This type includes a rotating parity array, thus addressing the write limitation in RAID-4. Thus, all read and write operations can be overlapped. RAID-5 stores parity information but not redundant data (but parity information can be used to reconstruct data). RAID-5 requires at least three and usually five disks for the array. It's best for multi-user systems in which performance is not critical or which do few write operations.

RAID-6. This type is similar to RAID-5 but includes a second parity scheme that is distributed across different drives and thus offers extremely high fault- and drive-failure tolerance. There are few or no commercial examples currently.

RAID-7. This type includes a real-time embedded operating system as a controller, caching via a high-speed bus, and other characteristics of a stand-alone computer. One vendor offers this system.

RAID-10. This type offers an array of stripes in which each stripe is a RAID-1 array of drives. This offers higher performance than RAID-1 but at much higher cost.

RAID-53. This type offers an array of stripes in which each stripe is a RAID-3 array of disks. This offers higher performance than RAID-3 but at much higher cost.

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