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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 In computer data storage, data striping is the technique of segmenting logically sequential data, such as a file, so that consecutive segments are stored on different physical storage devices. Striping is useful when a processing device request ...
, mirroring, or parity 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 magnet ...
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 '' Seas ...
'', 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 In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "back up", w ...
plan.


RAID 0

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). The de ...
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 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

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 In computer science and telecommunication, Hamming codes are a family of linear error-correcting codes. Hamming codes can detect one-bit and two-bit errors, or correct one-bit errors without detection of uncorrected errors. By contrast, the sim ...
for
error correction In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communica ...
. 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 In computer science and telecommunication, Hamming codes are a family of linear error-correcting codes. Hamming codes can detect one-bit and two-bit errors, or correct one-bit errors without detection of uncorrected errors. By contrast, the sim ...
, 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 The DataVault was Thinking Machines' mass storage system, storing five gigabytes of data, expandable to ten gigabytes with transfer rates of 40 megabytes per second. Eight DataVaults could be operated in parallel for a combined data transfer rate o ...
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, 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-pur ...
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 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 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 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
ASIC An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-efficien ...
s 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 \mathbb_2 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, subtra ...
GF(m) with m=2^k. This field is isomorphic to a polynomial field F_2 (p(x)) 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 ...
p(x) of degree k over \mathbb_2. We will represent the data elements D as polynomials \mathbf=d_x^ + d_x^ + ... + d_1x + d_0 in the Galois field. Let \mathbf_0,...,\mathbf_ \in GF(m) correspond to the stripes of data across hard drives encoded as field elements in this manner. We will use \oplus to denote addition in the field, and concatenation to denote multiplication. The reuse of \oplus is intentional: this is because addition in the finite field \mathbb_2 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 g^i is different for each non-negative i. This means each element of the field, except the value 0, can be written as a power of g. A finite field is guaranteed to have at least one generator. Pick one such generator g, and define \mathbf and \mathbf as follows: As before, the first checksum \mathbf is just the XOR of each stripe, though interpreted now as a polynomial. The effect of g^i can be thought of as the action of a carefully chosen
linear feedback shift register In computing, a linear-feedback shift register (LFSR) is a shift register whose input bit is a linear function of its previous state. The most commonly used linear function of single bits is exclusive-or (XOR). Thus, an LFSR is most often a sh ...
on the data chunk. Unlike the bit shift in the simplified example, which could only be applied k times before the encoding began to repeat, applying the operator g multiple times is guaranteed to produce m=2^k-1 unique invertible functions, which will allow a chunk length of k to support up to 2^k-1 data pieces. If one data chunk is lost, the situation is similar to the one before. In the case of two lost data chunks, we can compute the recovery formulas algebraically. Suppose that \mathbf_i and \mathbf_j are the lost values with i \neq j, then, using the other values of D, we find constants A and B: We can solve for D_i in the second equation and plug it into the first to find D_j = (g^\oplus1)^ (g^B\oplus A), and then D_i=A\oplus D_j. Unlike P, The computation of Q is relatively CPU intensive, as it involves polynomial multiplication in F_2 (p(x)). This can be mitigated with a hardware implementation or by using an
FPGA A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturinghence the term '' field-programmable''. The FPGA configuration is generally specified using a hardware de ...
.


Comparison

The following table provides an overview of some considerations for standard RAID levels. In each case, array space efficiency is given as an expression in terms of the number of drives, ; this expression designates a fractional value between zero and one, representing the fraction of the sum of the drives' capacities that is available for use. For example, if three drives are arranged in RAID 3, this gives an array space efficiency of ; thus, if each drive in this example has a capacity of 250 GB, then the array has a total capacity of 750 GB but the capacity that is usable for data storage is only 500 GB. Different RAID configurations can also detect failure during so called
data scrubbing Data scrubbing is an error correction technique that uses a background task to periodically inspect main memory or storage for errors, then corrects detected errors using redundant data in the form of different checksums or copies of data. Data ...
. Historically disks were subject to lower reliability and RAID levels were also used to detect which disk in the array had failed in addition to that a disk had failed. Though as noted by Patterson et. al. even at the inception of RAID many (though not all) disks were already capable of finding internal errors using error correcting codes. In particular it is/was sufficient to have a mirrored set of disks to detect a failure, but two disks were not sufficient to detect ''which'' had failed in a disk array without error correcting features. Modern RAID arrays depend for the most part on a disk's ability to identify itself as faulty which can be detected as part of a scrub. The redundant information is used to reconstruct the missing data, rather than to identify the faulted drive. Drives are considered to have faulted if they experience an
unrecoverable read error 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 s ...
, which occurs after a drive has retried many times to read data and failed. Enterprise drives may also report failure in far fewer tries than consumer drives as part of
TLER In computing, error recovery control (ERC) (Western Digital: time-limited error recovery (TLER), Samsung/Hitachi: command completion time limit (CCTL)) is a feature of hard disks which allow a system administrator to configure the amount of time a ...
to ensure a read request is fulfilled in a timely manner.


System implications

In measurement of the I/O performance of five filesystems with five storage configurations—single SSD, RAID 0, RAID 1, RAID 10, and RAID 5 it was shown that
F2FS F2FS (Flash-Friendly File System) is a flash file system initially developed by Samsung Electronics for the Linux kernel. The motive for F2FS was to build a file system that, from the start, takes into account the characteristics of NAND flash ...
on RAID 0 and RAID 5 with eight SSDs outperforms
EXT4 ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3. ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for ...
by 5 times and 50 times, respectively. The measurements also suggest that the RAID controller can be a significant bottleneck in building a RAID system with high speed SSDs.


Nested RAID

Combinations of two or more standard RAID levels. They are also known as RAID 0+1 or RAID 01, RAID 0+3 or RAID 03, RAID 1+0 or RAID 10, RAID 5+0 or RAID 50, RAID 6+0 or RAID 60, and RAID 10+0 or RAID 100.


Non-standard variants

In addition to standard and nested RAID levels, alternatives include
non-standard RAID levels Although all RAID implementations differ from the specification to some extent, some companies and open-source projects have developed non-standard RAID implementations that differ substantially from the standard. Additionally, there are non-RAI ...
, and
non-RAID drive architectures The most widespread standard for configuring multiple hard disk drives is RAID (Redundant Array of Inexpensive/Independent Disks), which comes in a number of standard configurations and non-standard configurations. Non-RAID drive architectures a ...
. Non-RAID drive architectures are referred to by similar terms and acronyms, notably
JBOD The most widespread standard for configuring multiple hard disk drives is RAID (Redundant Array of Inexpensive/Independent Disks), which comes in a number of standard configurations and non-standard configurations. Non-RAID drive architectures a ...
("just a bunch of disks"), SPAN/BIG, and
MAID A maid, or housemaid or maidservant, is a female domestic worker. In the Victorian era domestic service was the second largest category of employment in England and Wales, after agricultural work. In developed Western nations, full-time maids ...
("massive array of idle disks").


Notes


References


Further reading

*
Redundant Arrays of Inexpensive Disks (RAIDs)
chapter 38 from the ''Operating Systems: Three Easy Pieces'' book by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau


External links




RAID 5 parity explanation and checking tool

RAID Calculator for Standard RAID Levels and Other RAID Tools


{{RAID RAID Computer data storage