RAID

RAID, an acronym for redundant array of inexpensive disks or redundant array of independent disks, 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. This concept was first defined by David A. Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 as redundant array of inexpensive disks.[1] Marketers representing industry RAID manufacturers later reinvented the term to describe a redundant array of independent disks as a means of dissociating a low-cost expectation from RAID technology.[2]

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 by the word RAID followed by 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[3]. This array distributes data across multiple disks, but the array is seen by 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 by using error checking and/or mirroring, the array as a whole can be made more reliable by 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.

RAID 0

RAID 0 (striped disks) distributes data across multiple disks in ways that gives improved speed at any given instant. If one disk fails, however, all of the data on the array will be lost, as there is neither parity nor mirroring. In this regard, RAID 0 is somewhat of a misnomer, in that RAID 0 is non-redundant. A RAID 0 array requires a minimum of two drives. A RAID 0 configuration can be applied to a single drive provided that the RAID controller is hardware and not software (i.e. OS-based arrays) and allows for such configuration. This allows a single drive to be added to a controller already containing another RAID configuration when the user does not wish to add the additional drive to the existing array. In this case, the controller would be set up as RAID only (as opposed to SCSI in non-RAID configuration), which requires that each individual drive be a part of some sort of RAID array.

RAID 1

RAID 1 mirrors the contents of the disks, making a form of 1:1 ratio realtime mirroring. The contents of each disk in the array are identical to that of every other disk in the array. A RAID 1 array requires a minimum of two drives.

RAID 3, RAID 4

RAID 3 or 4 (striped disks with dedicated parity) combines three or more disks in a way that protects data against loss of any one disk. Fault tolerance is achieved by adding an extra disk to the array, which is dedicated to storing parity information; the overall capacity of the array is reduced by one disk. A RAID 3 or 4 array requires a minimum of three drives: two to hold striped data, and a third for parity. With the minimum three drives needed for RAID 3, the storage efficiency is 66 percent. With six drives, the storage efficiency is 87 percent.

RAID 5

Striped set with distributed parity or interleave parity requiring 3 or more disks. Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive. A single drive failure in the set will result in reduced performance of the entire set until the failed drive has been replaced and rebuilt.

RAID 6

RAID 6 (striped disks with dual parity) combines four or more disks in a way that protects data against loss of any two disks. For example, if the goal is to create 10x1TB of usable space in a RAID 6 configuration, we need two additional disks for the parity data.

RAID 10

RAID 1+0 (or 10) is a mirrored data set (RAID 1) which is then striped (RAID 0), hence the "1+0" name. A RAID 1+0 array requires a minimum of four drives – two mirrored drives to hold half of the striped data, plus another two mirrored for the other half of the data. In Linux, MD RAID 10 is a non-nested RAID type like RAID 1 that only requires a minimum of two drives and may give read performance on the level of RAID 0.

RAID 01

RAID 0+1 (or 01) is a striped data set (RAID 0) which is then mirrored (RAID 1). A RAID 0+1 array requires a minimum of four drives: two to hold the striped data, plus another two to mirror the first pair.

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