In
computer storage
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a compute ...
, the standard RAID levels comprise a basic set of
RAID
Raid, RAID or Raids may refer to:
Attack
* Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground
* Corporate raid, a type of hostile takeover in business
* Panty raid, a prankish raid by male college ...
("redundant array of independent disks" or "redundant array of inexpensive disks") configurations that employ the techniques of
striping,
mirroring
Mirroring is the behavior in which one person subconsciously imitates the gesture, speech pattern, or attitude of another. Mirroring often occurs in social situations, particularly in the company of close friends or family, often going unnotice ...
, or
parity
Parity may refer to:
* Parity (computing)
** Parity bit in computing, sets the parity of data for the purpose of error detection
** Parity flag in computing, indicates if the number of set bits is odd or even in the binary representation of the ...
to create large reliable data stores from multiple general-purpose computer
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with mag ...
s (HDDs). The most common types are RAID 0 (striping), RAID 1 (mirroring) and its variants, RAID 5 (distributed parity), and RAID 6 (dual parity). Multiple RAID levels can also be combined or ''
nested
''Nested'' is the seventh studio album by Bronx-born singer, songwriter and pianist Laura Nyro, released in 1978 on Columbia Records.
Following on from her extensive tour to promote 1976's ''Smile'', which resulted in the 1977 live album '' Sea ...
'', for instance RAID 10 (striping of mirrors) or RAID 01 (mirroring stripe sets). RAID levels and their associated data formats are standardized by the
Storage Networking Industry Association
The Storage Networking Industry Association (SNIA) is a registered 501(c)(6) non-profit trade association incorporated in December 1997. SNIA has more than 185 unique members, 2,000 active contributing members and over 50,000 IT end users and sto ...
(SNIA) in the Common RAID Disk Drive Format (DDF) standard. The numerical values only serve as identifiers and do not signify performance, reliability, generation, or any other metric.
While most RAID levels can provide good protection against and recovery from hardware defects or defective sectors/read errors (''hard errors''), they do not provide any protection against
data loss Data loss is an error condition in information systems in which information is destroyed by failures (like failed spindle motors or head crashes on hard drives) or neglect (like mishandling, careless handling or storage under unsuitable conditions) ...
due to catastrophic failures (fire, water) or ''soft errors'' such as user error, software malfunction, or malware infection. For valuable data, RAID is only one building block of a larger data loss prevention and recovery scheme – it cannot replace a
backup plan.
RAID 0
![RAID 0](https://upload.wikimedia.org/wikipedia/commons/9/9b/RAID_0.svg)
RAID 0 (also known as a ''stripe set'' or ''striped volume'') splits ("
stripes
Stripe, striped, or stripes may refer to:
Decorations
*Stripe (pattern), a line or band that differs in colour or tone from an adjacent surface
*Racing stripe, a vehicle decoration
*Service stripe, a decoration of the U.S. military
Entertainment
...
") data evenly across two or more disks, without
parity information, redundancy, or
fault tolerance
Fault tolerance is the property that enables a system to continue operating properly in the event of the failure of one or more faults within some of its components. If its operating quality decreases at all, the decrease is proportional to the ...
. Since RAID 0 provides no fault tolerance or redundancy, the failure of one drive will cause the entire array to fail; as a result of having data striped across all disks, the failure will result in total data loss. This configuration is typically implemented having speed as the intended goal. RAID 0 is normally used to increase performance, although it can also be used as a way to create a large logical
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...
out of two or more physical disks.
A RAID 0 setup can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk. For example, if a 120 GB disk is striped together with a 320 GB disk, the size of the array will be 120 GB × 2 = 240 GB. However, some RAID implementations would allow the remaining 200 GB to be used for other purposes.
The diagram in this section shows how the data is distributed into stripes on two disks, with A1:A2 as the first stripe, A3:A4 as the second one, etc. Once the stripe size is defined during the creation of a RAID 0 array, it needs to be maintained at all times. Since the stripes are accessed in parallel, an -drive RAID 0 array appears as a single large disk with a data rate times higher than the single-disk rate.
Performance
A RAID 0 array of drives provides data read and write transfer rates up to times as high as the individual drive rates, but with no data redundancy. As a result, RAID 0 is primarily used in applications that require high performance and are able to tolerate lower reliability, such as in
scientific computing
Computational science, also known as scientific computing or scientific computation (SC), is a field in mathematics that uses advanced computing capabilities to understand and solve complex problems. It is an area of science that spans many disc ...
or
computer gaming
A personal computer game, also known as a PC game or computer game, is a type of video game played on a personal computer (PC) rather than a video game console or arcade machine. Its defining characteristics include: more diverse and user-deter ...
.
Some benchmarks of desktop applications show RAID 0 performance to be marginally better than a single drive. Another article examined these claims and concluded that "striping does not always increase performance (in certain situations it will actually be slower than a non-RAID setup), but in most situations it will yield a significant improvement in performance". Synthetic benchmarks show different levels of performance improvements when multiple HDDs or SSDs are used in a RAID 0 setup, compared with single-drive performance. However, some synthetic benchmarks also show a drop in performance for the same comparison.
RAID 1
![RAID 1](https://upload.wikimedia.org/wikipedia/commons/b/b7/RAID_1.svg)
RAID 1 consists of an exact copy (or ''
mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
'') of a set of data on two or more disks; a classic RAID 1 mirrored pair contains two disks. This configuration offers no parity, striping, or spanning of disk space across multiple disks, since the data is mirrored on all disks belonging to the array, and the array can only be as big as the smallest member disk. This layout is useful when read performance or reliability is more important than write performance or the resulting data storage capacity.
The array will continue to operate so long as at least one member drive is operational.
Performance
Any read request can be serviced and handled by any drive in the array; thus, depending on the nature of I/O load, random read performance of a RAID 1 array may equal up to the sum of each member's performance, while the write performance remains at the level of a single disk. However, if disks with different speeds are used in a RAID 1 array, overall write performance is equal to the speed of the slowest disk.
Synthetic benchmarks show varying levels of performance improvements when multiple HDDs or SSDs are used in a RAID 1 setup, compared with single-drive performance. However, some synthetic benchmarks also show a drop in performance for the same comparison.
RAID 2
![RAID2 arch](https://upload.wikimedia.org/wikipedia/commons/b/b5/RAID2_arch.svg)
RAID 2, which is rarely used in practice, stripes data at the
bit
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
(rather than block) level, and uses a
Hamming code for
error correction. The disks are synchronized by the controller to spin at the same angular orientation (they reach index at the same time), so it generally cannot service multiple requests simultaneously.
However, depending with a high rate
Hamming code, many spindles would operate in parallel to simultaneously transfer data so that "very high data transfer rates" are possible as for example in the
DataVault where 32 data bits were transmitted simultaneously.
With all hard disk drives implementing internal error correction, the complexity of an external Hamming code offered little advantage over parity so RAID 2 has been rarely implemented; it is the only original level of RAID that is not currently used.
RAID 3
![RAID 3](https://upload.wikimedia.org/wikipedia/commons/f/f9/RAID_3.svg)
RAID 3, which is rarely used in practice, consists of
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
-level striping with a dedicated
parity disk. One of the characteristics of RAID 3 is that it generally cannot service multiple requests simultaneously, which happens because any single block of data will, by definition, be spread across all members of the set and will reside in the same physical location on each disk. Therefore, any
I/O operation requires activity on every disk and usually requires synchronized spindles.
This makes it suitable for applications that demand the highest transfer rates in long sequential reads and writes, for example
uncompressed video
Uncompressed video is digital video that either has never been compressed or was generated by decompressing previously compressed digital video. It is commonly used by video cameras, video monitors, video recording devices (including general-purp ...
editing. Applications that make small reads and writes from random disk locations will get the worst performance out of this level.
The requirement that all disks spin synchronously (in a
lockstep
In the United States, lockstep marching or simply lockstep is marching in a very close single file in such a way that the leg of each person in the file moves in the same way and at the same time as the corresponding leg of the person immediately ...
) added design considerations that provided no significant advantages over other RAID levels. Both RAID 3 and RAID 4 were quickly replaced by RAID 5.
RAID 3 was usually implemented in hardware, and the performance issues were addressed by using large disk caches.
RAID 4
![RAID 4](https://upload.wikimedia.org/wikipedia/commons/a/ad/RAID_4.svg)
RAID 4 consists of
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
-level striping with a dedicated
parity disk. As a result of its layout, RAID 4 provides good performance of random reads, while the performance of random writes is low due to the need to write all parity data to a single disk, unless the filesystem is RAID-4-aware and compensates for that.
An advantage of RAID 4 is that it can be quickly extended online, without parity recomputation, as long as the newly added disks are completely filled with 0-bytes.
In diagram 1, a read request for block A1 would be serviced by disk 0. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently by disk 1.
RAID 5
![RAID 5](https://upload.wikimedia.org/wikipedia/commons/6/64/RAID_5.svg)
RAID 5 consists of block-level striping with distributed parity. Unlike in RAID 4, parity information is distributed among the drives. It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost.
RAID 5 requires at least three disks.
There are many layouts of data and parity in a RAID 5 disk drive array depending upon the sequence of writing across the disks,
that is:
#the sequence of data blocks written, left to right or right to left on the disk array, of disks 0 to N.
#the location of the parity block at the beginning or end of the stripe.
#the location of the first block of a stripe with respect to parity of the previous stripe.
The figure to the right shows 1) data blocks written left to right, 2) the parity block at the end of the stripe and 3) the first block of the next stripe not on the same disk as the parity block of the previous stripe. It can be designated as a ''Left Asynchronous'' RAID 5 layout
and this is the only layout identified in the last edition of ''The Raid Book'' published by the defunct ''Raid Advisory Board.'' In a ''Synchronous'' layout the data first block of the next stripe is written on the same drive as the parity block of the previous stripe.
In comparison to RAID 4, RAID 5's distributed parity evens out the stress of a dedicated parity disk among all RAID members. Additionally, write performance is increased since all RAID members participate in the serving of write requests. Although it will not be as efficient as a striping (RAID 0) setup, because parity must still be written, this is no longer a bottleneck.
Since parity calculation is performed on the full stripe, small changes to the array experience ''write amplification'': in the worst case when a single, logical sector is to be written, the original sector and the according parity sector need to be read, the original data is removed from the parity, the new data calculated into the parity and both the new data sector and the new parity sector are written.
RAID 6
![RAID 6](https://upload.wikimedia.org/wikipedia/commons/7/70/RAID_6.svg)
RAID 6 extends RAID 5 by adding another
parity block; thus, it uses
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
-level striping with two parity blocks distributed across all member disks.
As in RAID 5, there are many layouts of RAID 6 disk arrays depending upon the direction the data blocks are written, the location of the parity blocks with respect to the data blocks and whether or not the first data block of a subsequent stripe is written to the same drive as the last parity block of the prior stripe. The figure to the right is just one of many such layouts.
According to the Storage Networking Industry Association (SNIA), the definition of RAID 6 is: "Any form of RAID that can continue to execute read and write requests to all of a RAID array's virtual disks in the presence of any two concurrent disk failures. Several methods, including dual check data computations (parity and
Reed–Solomon), orthogonal dual parity check data and diagonal parity, have been used to implement RAID Level 6."
Performance
RAID 6 does not have a performance penalty for read operations, but it does have a performance penalty on write operations because of the overhead associated with parity calculations. Performance varies greatly depending on how RAID 6 is implemented in the manufacturer's storage architecture—in software, firmware, or by using firmware and specialized
ASICs for intensive parity calculations. RAID 6 can read up to the same speed as RAID 5 with the same number of physical drives.
When either diagonal or orthogonal dual parity is used, a second parity calculation is necessary for write operations. This doubles CPU overhead for RAID-6 writes, versus single-parity RAID levels. When a Reed Solomon code is used, the second parity calculation is unnecessary. Reed Solomon has the advantage of allowing all redundancy information to be contained within a given stripe.
General parity system
It is possible to support a far greater number of drives by choosing the parity function more carefully. The issue we face is to ensure that a system of equations over the finite field
has a unique solution, so we will turn to the theory of polynomial equations. Consider the
Galois field
In mathematics, a finite field or Galois field (so-named in honor of Évariste Galois) is a field that contains a finite number of elements. As with any field, a finite field is a set on which the operations of multiplication, addition, subtr ...
with
. This field is isomorphic to a polynomial field
for a suitable
irreducible polynomial
In mathematics, an irreducible polynomial is, roughly speaking, a polynomial that cannot be factored into the product of two non-constant polynomials. The property of irreducibility depends on the nature of the coefficients that are accepted f ...
of degree
over
. We will represent the data elements
as polynomials
in the Galois field. Let
correspond to the stripes of data across hard drives encoded as field elements in this manner. We will use
to denote addition in the field, and concatenation to denote multiplication. The reuse of
is intentional: this is because addition in the finite field
represents to the XOR operator, so computing the sum of two elements is equivalent to computing XOR on the polynomial coefficients.
A
generator
Generator may refer to:
* Signal generator, electronic devices that generate repeating or non-repeating electronic signals
* Electric generator, a device that converts mechanical energy to electrical energy.
* Generator (circuit theory), an eleme ...
of a field is an element of the field such that
is different for each non-negative