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{{About|the data storage technology|other uses|Raid (disambiguation)}}
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{{Merge from|Standard RAID levels|discuss=Talk:RAID#Merger proposal|date=January 2014}}


'''RAID''' is a storage technology that combines multiple [[disk drive]] components into a logical unit for the purposes of data redundancy and performance improvement. Data is distributed across the drives in one of several ways, referred to as [[Standard RAID levels|RAID levels]], depending on the specific level of [[redundancy (engineering)|redundancy]] and performance required.
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The term "RAID" was first used by [[David Patterson (scientist)|David Patterson]], [[Garth A. Gibson]], and [[Randy Katz]] at the [[University of California, Berkeley]] in 1987, standing for '''redundant array of inexpensive disks'''.<ref name="patterson">David A. Patterson, Garth Gibson, and Randy H. Katz: [http://www-2.cs.cmu.edu/~garth/RAIDpaper/Patterson88.pdf A Case for Redundant Arrays of Inexpensive Disks (RAID)]. University of California Berkeley. 1988.</ref> Industry RAID manufacturers later tended to interpret the acronym as standing for '''redundant array of independent disks'''.<ref>"Originally referred to as Redundant Array of Inexpensive Disks, the concept of RAID was first developed in the late 1980s by Patterson, Gibson, and Katz of the University of California at Berkeley. (The RAID Advisory Board has since substituted the term Inexpensive with Independent.)" Storagecc Area Network Fundamentals; Meeta Gupta; Cisco Press; ISBN 978-1-58705-065-7; Appendix A.</ref><ref name="Patterson_1994">{{Cite journal |url=http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.41.3889 |first1=Peter |last1=Chen |first2=Edward |last2=Lee |first3=Garth |last3=Gibson |first4=Randy |last4=Katz |first5=David |last5=Patterson |title=RAID: High-Performance, Reliable Secondary Storage |work=ACM Computing Surveys |volume=26 |pages=145–185 |year=1994}}</ref><ref>{{Cite journal| last1=Donald| first1=L.| lastauthoramp=yes| title=MCSA/MCSE 2006 JumpStart Computer and Network Basics| publisher=SYBEX| place=Glasgow| edition=2nd| year=2003}}</ref><ref>{{Cite book|url=http://foldoc.org/RAID|title=Redundant Arrays of Independent Disks from FOLDOC|work=Free On-line Dictionary of Computing|publisher=[http://www.doc.ic.ac.uk/ Imperial College Department of Computing]|editor-last=Howe|editor-first=Denis|accessdate=2011-11-10}}</ref>
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RAID is now used as an [[umbrella term]] for [[computer data storage]] schemes that can divide and replicate [[Data (computing)|data]] among multiple physical drives: RAID is an example of [[storage virtualization]] and the array can be accessed by the [[operating system]] as one single drive.<ref group="note">The physical drives are said to be "''in'' a RAID", however the more common, somewhat repetitive parlance is to say that they are "in a RAID array". See [[RAS syndrome]].</ref> The different schemes or architectures are named by the word RAID followed by a number (e.g. RAID&nbsp;0, RAID&nbsp;1). Each scheme provides a different balance between the key goals: [[Reliability engineering|reliability]] and [[availability]], [[Computer performance|performance]] and [[capacity]]. RAID levels greater than RAID&nbsp;0 provide protection against unrecoverable (sector) read errors, as well as whole disk failure.
 
== History ==
The term RAID was first defined by [[David A. Patterson (scientist)|David A. Patterson]], [[Garth A. Gibson]] and [[Randy Katz]] at the University of California, Berkeley, in 1987. They studied the possibility of using two or more drives to appear as a single device to the host system and published a paper: ''"A Case for Redundant Arrays of Inexpensive Disks (RAID)"'' in June 1988 at the [[SIGMOD]] conference.<ref name="patterson">{{Cite conference |last=Patterson |first=David |authorlink=David A. Patterson (scientist)|coauthors=[[Garth A. Gibson|Gibson, Garth A.]]; [[Randy Katz|Katz, Randy]] |year=1988 |title=A Case for Redundant Arrays of Inexpensive Disks (RAID) |booktitle=[[SIGMOD]] Conferences |pages=109–116 |url=http://www.eecs.berkeley.edu/Pubs/TechRpts/1987/CSD-87-391.pdf | accessdate=2006-12-31 | format=PDF}}</ref>
 
Each of the five levels of RAID named in the paper were well established in the art prior to the paper's publications, for example:
* Around 1983, [[DEC]] began shipping subsystem mirrored RA8X disk drives (now known as ''RAID&nbsp;1'') as part of its HSC50 subsystem.<ref>{{Cite web
| url = http://www.textfiles.com/bitsavers/pdf/dec/ci/EK-HS571-TM-001_HSC_hwTech.pdf
| title = HSC50/70 Hardware Technical Manual
| format = PDF | page = 29, 32
| date = July 1986 | accessdate = 2014-01-03
| publisher = [[DEC]]
}}</ref>
* Around 1988, the [[Thinking Machines Corporation|Thinking Machines]] DataVault used error correction codes (now known as ''RAID&nbsp;2'') in an array of disk drives.<ref>{{US patent reference |number=4899342 |y=1988 |m=02 |d=01 |inventor=Norman Ken Ouchi |title=Method and Apparatus for Operating Multi-Unit Array of Memories}}</ref> A similar approach was used in 1970s on the [[History of IBM magnetic disk drives#IBM 3330|IBM 3330]].<ref>{{Cite web
| url = http://www-03.ibm.com/ibm/history/exhibits/storage/storage_3330.html
| title = IBM 3330 data storage
| accessdate = 2014-01-02
| publisher = [[IBM]]
| work = IBM.com
}}</ref>
* In 1977, Norman Ken Ouchi at [[IBM]] filed a patent disclosing what was subsequently named ''RAID&nbsp;4''.<ref>{{US patent reference |number=4092732 |y=1978 |m=05 |d=30 |inventor=Norman Ken Ouchi |title=System for Recovering Data Stored in Failed Memory Unit}}</ref>
* In 1986, Clark et al. at IBM filed a patent disclosing what was subsequently named ''RAID&nbsp;5''.<ref>{{US patent reference |number=4761785 |y=1988 |m=08 |d=02 |inventor=Brian E. Clark, et al. |title=Parity Spreading to Enhance Storage Access}}</ref>
 
== Standard levels ==
{{Main|Standard RAID levels}}
 
A number of standard schemes have evolved. These are called ''levels''. Originally, there were five RAID levels, but many variations have evolved—notably several [[Nested RAID levels|nested levels]] and many [[Non-standard RAID levels|non-standard levels]] (mostly [[proprietary software|proprietary]]). RAID levels and their associated data formats are standardized by the [[Storage Networking Industry Association]] (SNIA) in the Common RAID Disk Drive Format (DDF) standard:<ref>{{Cite web|url=http://www.snia.org/tech_activities/standards/curr_standards/ddf/ |title=Common RAID Disk Drive Format (DDF) standard |publisher=SNIA |work=SNIA.org |date= |accessdate=2012-08-26}}</ref><ref>{{Cite web |url=http://www.snia.org/education/dictionary |title=SNIA Dictionary |publisher=SNIA |work=SNIA.org |accessdate=2010-08-24}}</ref>
 
; [[RAID 0]]
: RAID&nbsp;0 comprises [[Data striping|striping]] (but no [[parity bit|parity]] or [[Disk mirroring|mirroring]]). This level provides no data redundancy nor fault tolerance, but improves performance through parallelism of read and write operations across multiple drives. RAID&nbsp;0 has no error detection mechanism, so the failure of one disk causes the loss of all data on the array.<ref name="Patterson_1994" />
 
; [[RAID 1]]
: RAID&nbsp;1 comprises mirroring (without parity or striping). Data are written identically to two (or more) drives, thereby producing a "mirrored set". The read request is serviced by any of the drives containing the requested data. This can improve performance if data is read from the disk with the least [[seek time|seek latency]] and [[rotational latency]]. Conversely, write performance can be degraded because all drives must be updated; thus the write performance is determined by the slowest drive. The array continues to operate as long as at least one drive is functioning.<ref name="Patterson_1994" />
 
; [[RAID 2]]
: RAID&nbsp;2 comprises bit-level striping with dedicated Hamming-code parity. All disk spindle rotation is synchronized and data is [[Data striping|striped]] such that each sequential [[bit]] is on a different drive. [[Hamming code|Hamming-code]] parity is calculated across corresponding bits and stored on at least one parity drive.<ref name="Patterson_1994" /> This level is of historical significance only. Although it was used on some early machines (e.g. the [[Thinking Machines Corporation|Thinking Machines]] CM-2),<ref>{{Cite book |title=Structured Computer Organization 6th ed. |author=Andrew S. Tanenbaum |page=95}}</ref> it is not used by any current commercially available systems.<ref>{{Cite book |title=Computer Architecture: A Quantitative Approach, 4th ed |first1=John |last1=Hennessy |first2=David |last2=Patterson |year=2006 |page=362 |isbn=978-0123704900}}</ref>
 
; [[RAID 3]]
: RAID&nbsp;3 comprises byte-level striping with dedicated parity. All disk spindle rotation is synchronized and data is striped such that each sequential [[byte]] is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive.<ref name="Patterson_1994" /> Although implementations exist,<ref>{{Cite web|url=http://www.freebsd.org/doc/handbook/geom-raid3.html|title=FreeBSD Handbook, Chapter 20.5 GEOM: Modular Disk Transformation Framework|accessdate=2012-12-20}}</ref> RAID&nbsp;3 is not commonly used in practice.
 
; [[RAID 4]]
: RAID&nbsp;4 comprises block-level striping with dedicated parity.{{Citation needed|date=February 2013}}
: RAID&nbsp;4 was previously used primarily by [[NetApp]], but has now been largely replaced by an implementation of RAID&nbsp;6 ([[Non-standard RAID levels#Double parity|RAID-DP]]).<ref>{{Cite web|url=http://www.netapp.com/us/library/technical-reports/tr-3298.html|title=RAID-DP:NetApp Implementation of Double Parity RAID for Data Protection. NetApp Technical Report TR-3298|first1=Jay|last1=White|first2=Chris|last2=Lueth|date=May 2010|accessdate=2013-03-02}}</ref>
 
; [[RAID 5]]
: RAID&nbsp;5 comprises 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&nbsp;5 requires at least three disks.<ref name="Patterson_1994" />
 
; [[RAID 6]]
: RAID&nbsp;6 comprises block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, as large-capacity drives take longer to restore. As with RAID&nbsp;5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced.<ref name="Patterson_1994" />
 
=== 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, <math>n</math>; 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&nbsp;3, this gives an array space efficiency of <math>1 - (1/n) = 1 - (1/3) = 2/3 \approx 67%</math> thus, if each drive in this example has a capacity of 250&nbsp;GB, then the array has a total capacity of 750&nbsp;GB but the capacity that is usable for data storage is only 500&nbsp;GB.
* Array failure rate is given as an expression in terms of the number of drives, <math>n</math>, and the drive failure rate, <math>r</math> (which is assumed identical and independent for each drive) and can be seen to be a [[Bernoulli trial]].{{Citation needed|date=September 2013}} For example, if each of three drives has a failure rate of 5% over the next three years, and these drives are arranged in RAID&nbsp;3, then this gives an array failure rate over the next three years of:
:<math>
\begin{align} 1 - (1 - r)^{n} - nr(1 - r)^{n - 1} & = 1 - (1 - 5\%)^{3} - 3 \times 5\% \times (1 - 5\%)^{3 - 1} \\
& = 1 - 0.95^{3} - 0.15 \times 0.95^{2} \\
& = 1 - 0.857375 - 0.135375 \\
& = 0.00725 \\
& \approx 0.7\% \end{align}
</math>
 
{| class="wikitable" style="text-align: center;"
|-
! Level !! Description !! Minimum # of drives<ref group="limit" name="NonDegenerate">Assumes a non-degenerate minimum number of drives</ref> !! Space efficiency !! Fault tolerance !! Array failure rate<ref group="limit" name="IdenticalRate">Assumes independent, identical rate of failure amongst drives</ref> !! Read performance !! Write performance !! Figure
|-
| [[RAID 0|RAID&nbsp;0]]
| Block-level [[Data striping|striping]] without [[Parity bit|parity]] or [[Disk mirroring|mirroring]]
| 2
| 1
| 0 (none)
| <math>1-(1-r)^{n}</math>
| nX
| nX
| [[File:RAID 0.svg|100px|RAID Level 0]]
|-
| [[RAID 1|RAID&nbsp;1]]
| Mirroring without parity or striping
| 2
| 1/n
| n−1 drives
| <math>r^{n}</math>
| nX<ref group="limit">Theoretical maximum, as low as 1X in practice</ref>
| 1X
| [[File:RAID 1.svg|100px|RAID Level 1]]
|-
| [[RAID 2|RAID&nbsp;2]]
| Bit-level striping with dedicated [[Hamming code|Hamming-code]] parity
|3
|1 − 1/n ⋅ log<sub>2</sub>(n-1)
| RAID&nbsp;2 can recover from one drive failure or repair corrupt data or parity when a corrupted bit's corresponding data and parity are good.
| ''(Varies)''
| ''(Varies)''
| ''(Varies)''
| [[File:RAID2 arch.svg|150px|RAID Level 2]]
|-
| [[RAID 3|RAID&nbsp;3]]
| Byte-level striping with dedicated parity
| 3
| 1 − 1/n
| 1 drive
| <math>1-(1-r)^{n}-nr(1-r)^{n-1}</math>
| (n−1)X
| (n−1)X<ref group="limit" name="FastHardware">Assumes hardware is fast enough to support</ref>
| [[File:RAID 3.svg|150px|RAID Level 3]]
|-
| [[RAID 4|RAID&nbsp;4]]
| Block-level striping with dedicated parity
| 3
| 1 − 1/n
| 1 drive
| <math>1-(1-r)^{n}-nr(1-r)^{n-1}</math>
| (n−1)X
| (n−1)X<ref group="limit" name="FastHardware" />
| [[File:RAID 4.svg|150px|RAID Level 4]]
|-
| [[RAID 5|RAID&nbsp;5]]
| Block-level striping with distributed parity
| 3
| 1 − 1/n
| 1 drive
| <math>1-(1-r)^{n}-nr(1-r)^{n-1}</math>
| (n−1)X<ref group="limit" name="FastHardware" />
| (n−1)X<ref group="limit" name="FastHardware" />
| [[File:RAID 5.svg|150px|RAID Level 5]]
|-
| [[RAID 6|RAID&nbsp;6]]
| Block-level striping with double distributed parity
| 4
| 1 − 2/n
| 2 drives
| <math>1-(1-r)^{n}-nr(1-r)^{n-1}-{n\choose 2}r^{2}(1-r)^{n-2}</math>
| (n−2)X<ref group="limit" name="FastHardware" />
| (n−2)X<ref group="limit" name="FastHardware" />
| [[File:RAID 6.svg|175px|RAID Level 6]]
|-
| [[RAID 10|RAID&nbsp;10]]
| Mirroring without parity, and block-level striping
| 4
| 2/n
| 1 drive / span<ref group="limit">Raid&nbsp;10 can only lose one drive per span up to the max of 2/n drives.</ref>
|
| nX
| (n/2)X
| [[File:RAID 10.png|150px|RAID Level 10]]
|-
! Level !! Description !! Minimum # of drives<ref group="limit" name="NonDegenerate" /> !! Space efficiency !! Fault tolerance !! Array failure rate<ref group="limit" name="IdenticalRate" /> !! Read performance !! Write performance !! Figure
|}
 
{{Reflist|group="limit"}}
 
== Nested (hybrid) RAID ==
{{Main|Nested RAID levels}}
In what was originally termed '''hybrid RAID''',<ref name="Vijayan">{{cite book
| last = Vijayan
| first = S.
| coauthors = Selvamani, S. ; Vijayan, S
| title = Proceedings of the 1995 International Conference on Parallel Processing: Volume 1
| url = http://books.google.com/?id=QliANH5G3_gC&dq=%22hybrid+raid%22
| year = 1995
| publisher = [[CRC Press]]
| isbn = 0-8493-2615-X
| pages = I–146''ff''
| chapter = Dual-Crosshatch Disk Array: A Highly Reliable Hybrid-RAID Architecture
}}</ref>
many storage controllers allow RAID levels to be nested. The elements of a ''RAID'' may be either individual drives or RAIDs themselves. However, if a RAID is itself an element of a larger RAID, it is unusual for its elements to be themselves RAIDs.
 
The final RAID is known as the top array. When the top array is a RAID&nbsp;0 (such as in RAID&nbsp;1+0 and RAID&nbsp;5+0), most vendors omit the "+" (yielding RAID&nbsp;10 and RAID&nbsp;50, respectively).
 
* RAID&nbsp;0+1: striped sets in a mirrored set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.
: The key difference from RAID&nbsp;1+0 is that RAID&nbsp;0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.
* RAID&nbsp;1+0: (a.k.a. RAID&nbsp;10) mirrored sets in a striped set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.
: The key difference from RAID&nbsp;0+1 is that RAID&nbsp;1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives.<ref name="layton-lm">Jeffrey B. Layton: [http://www.linux-mag.com/id/7928?hq_e=el&hq_m=1151565&hq_l=36&hq_v=3fa9646c7f "Intro to Nested-RAID: RAID-01 and RAID-10"], Linux Magazine, January 6, 2011</ref>
 
== RAID parity ==
{{Further2|[[Parity bit]]}}
Many RAID levels employ an error protection scheme called "parity", a widely used method in information technology to provide fault tolerance in a given set of data. Most use the simple [[exclusive or|XOR]] parity described in this section, but RAID&nbsp;6 uses two separate parities based respectively on addition and multiplication in a particular Galois Field or [[Reed–Solomon error correction]].<ref>Dawkins, Bill and Jones, Arnold. [http://www.snia.org/tech_activities/standards/curr_standards/ddf/SNIA-DDFv1.2.pdf "Common RAID Disk Data Format Specification"] ''[Storage Networking Industry Association]'' Colorado Springs, 28 July 2006. Retrieved on 22 February 2011.</ref>
 
== Non-standard levels ==
{{Main|Non-standard RAID levels}}
 
Many configurations other than the basic numbered RAID levels are possible, and many companies, organizations, and groups have created their own non-standard configurations, in many cases designed to meet the specialized needs of a small niche group.  Such configurations include the following:
 
* [[Linux MD RAID 10]] provides a general RAID driver that in its "near" layout defaults to a standard RAID&nbsp;1 with two drives, and a standard RAID&nbsp;1+0 with four drives; though, it can include any number of drives, including odd numbers.  With its "far" layout, MD RAID&nbsp;10 can run both striped and mirrored, even with only two drives in <code>f2</code> layout; this runs mirroring with striped reads, giving the read performance of RAID&nbsp;0.  Regular RAID&nbsp;1, as provided by [[Linux software RAID]], does not stripe reads, but can perform reads in parallel.<ref name="layton-lm" /><ref>{{Cite web |title=Performance, Tools & General Bone-Headed Questions |url=http://www.tldp.org/HOWTO/Software-RAID-0.4x-HOWTO-8.html |publisher=tldp.org |accessdate=2013-12-25}}</ref><ref>{{Cite web |title=Main Page - Linux-raid |url=http://linux-raid.osdl.org/ |publisher=osdl.org |date=2010-08-20 |accessdate=2010-08-24}}</ref>
* [[Hadoop]] has a RAID system that generates a parity file by xor-ing a stripe of blocks in a single HDFS file.<ref>{{Cite web|url=http://hadoopblog.blogspot.com/2009/08/hdfs-and-erasure-codes-hdfs-raid.html |title=Hdfs Raid |publisher=Hadoopblog.blogspot.com |date=2009-08-28 |accessdate=2010-08-24}}</ref>
 
== Data backup ==
A RAID system used as [[secondary storage]] is not an alternative to [[Backup|backing up]] data. In RAID levels greater than zero, a RAID protects from catastrophic data loss caused by physical damage or errors on a single drive within the array (or two drives in, say, RAID&nbsp;6). However, a true backup system has other important features such as the ability to restore an earlier version of data, which is needed both to protect against [[Computer software|software]] errors that write unwanted data to secondary storage, and also to recover from [[user error]] and malicious data deletion.
 
A RAID can be overwhelmed by catastrophic failure that exceeds its recovery capacity and, of course, the entire array is at risk of physical damage by fire, natural disaster, and human forces, while backups can be stored off site. A RAID is also vulnerable to controller failure because it is not always possible to migrate a RAID to a new, different controller without data loss.<ref>{{Cite web |url=http://www.tomshardware.com/reviews/RAID-MIGRATION-ADVENTURE,1640.html |title=The RAID Migration Adventure |accessdate=2010-03-10}}</ref>
 
== Implementations ==
[[File:Storage-Product-with-RAID-Function.jpg|thumb|right|Storage products with RAID functionality]]
 
The distribution of data across multiple drives can be managed either by dedicated [[computer hardware]] or by [[Computer software|software]]. A software solution may be part of the operating system, or it may be part of the firmware and drivers supplied with a hardware RAID controller.
 
=== Software-based RAID ===
{{Refimprove section|date=December 2013}}
 
Software RAID implementations are now provided by many [[operating system]]s. Software RAID can be implemented as:
* A layer that abstracts multiple devices, thereby providing a single [[Virtualization|virtual device]] (e.g. Linux's [[mdadm|md]])
* A more generic logical volume manager (provided with most server-class operating systems, e.g. [[Veritas file system|Veritas]] or [[Logical Volume Manager (Linux)|LVM]])
* A component of the file system (e.g. [[ZFS]] or [[Btrfs]])
 
==== Volume manager support ====
{{expand section|date=December 2013}}
 
==== File-system support ====
Some advanced [[file system]]s are designed to organize data across multiple storage devices directly (without needing the help of a third-party logical volume manager):
* [[ZFS]] supports equivalents of RAID&nbsp;0, RAID&nbsp;1, RAID&nbsp;5 (RAID&nbsp;Z), RAID&nbsp;6 (RAID&nbsp;Z2) and a triple-parity version RAID&nbsp;Z3. As it always stripes over top-level vdevs, it supports equivalents of the 1+0, 5+0, and 6+0 nested RAID levels (as well as striped triple-parity sets) but not other nested combinations. ZFS is the native file system on [[Solaris (operating system)|Solaris]] and also available on FreeBSD and Linux.<ref>{{Cite web|title=ZFS on Linux|url=http://zfsonlinux.org/|accessdate=2013-07-15}}</ref>
* [[Btrfs]] supports RAID&nbsp;0, RAID&nbsp;1 and RAID&nbsp;10 (RAID&nbsp;5 and 6 are under development).<ref>{{cite web|title = Btrfs Wiki: Feature List|date = 2012-11-07|accessdate = 2012-11-16|url = https://btrfs.wiki.kernel.org/index.php/Main_Page#Features}}</ref><ref>{{cite web|title = Btrfs Wiki: Changelog|date = 2012-10-01|accessdate = 2012-11-14|url = https://btrfs.wiki.kernel.org/index.php/Changelog}}</ref><!--Although in Wiki format, this is documentation and changelog used by btrfs, a GPL project-->
 
==== Operating-system support ====
Many operating systems provide basic RAID functionality independently{{clarify|does this mean physical disk-level or what?|date=December 2013}} of volume management:
* Apple's [[OS X]] and [[OS X Server]] support RAID&nbsp;0, RAID&nbsp;1, and RAID&nbsp;1+0.<ref>{{Cite web|url= http://support.apple.com/kb/TA24359|title= Mac OS X: How to combine RAID sets in Disk Utility|accessdate= 2010-01-04}}</ref><ref>{{Cite web|url= http://www.apple.com/server/macosx/technology/file-system.html|title= Apple Mac OS X Server File Systems|accessdate= 2008-04-23}}</ref>
* [[FreeBSD]] supports RAID&nbsp;0, RAID&nbsp;1, RAID&nbsp;3, and RAID&nbsp;5, and all nestings via [[GEOM]] modules and ccd.<ref>{{Cite web|url=http://www.freebsd.org/cgi/man.cgi?query=geom|title=FreeBSD System Manager's Manual page for GEOM(8)|accessdate=2009-03-19}}</ref><ref>{{Cite web|url=http://lists.freebsd.org/pipermail/freebsd-geom/2006-July/001356.html|title=freebsd-geom mailing list - new class / geom_raid5|accessdate=2009-03-19}}</ref><ref>{{Cite web|url=http://www.freebsd.org/cgi/man.cgi?query=ccd|title=FreeBSD Kernel Interfaces Manual for CCD(4)|accessdate=2009-03-19}}</ref>
* [[Linux]]'s [[mdadm|md]] supports RAID&nbsp;0, RAID&nbsp;1, RAID&nbsp;4, RAID&nbsp;5, RAID&nbsp;6, and all nestings.<ref>{{Cite web|url=http://tldp.org/HOWTO/Software-RAID-HOWTO.html|title=The Software-RAID HOWTO|accessdate=2008-11-10}}</ref><ref>{{Cite web|url=http://linux-raid.osdl.org/index.php/RAID_setup|title=RAID setup|accessdate=2008-11-10}} {{Dead link|date=September 2010|bot=H3llBot}}</ref> Certain reshaping/resizing/expanding operations are also supported.<ref>{{cite web|url=https://raid.wiki.kernel.org/index.php/RAID_setup|title=RAID setup|accessdate=2010-09-30}}</ref>
* [[Microsoft]]'s server operating systems support RAID&nbsp;0, RAID&nbsp;1, and RAID&nbsp;5. Some of the Microsoft desktop operating systems support RAID. For example, Windows XP Professional supports RAID level 0, in addition to spanning multiple drives, but only if using dynamic disks and volumes. Windows XP can be modified to support RAID&nbsp;0, 1, and 5.<ref>{{cite web|url=http://www.tomshardware.com/reviews/windowsxp-make-raid-5-happen,925.html |title=Using Windows XP to Make RAID&nbsp;5 Happen |publisher=Tomshardware.com |date= |accessdate=2010-08-24}}</ref> [[Windows 8]] and [[Windows Server 2012]] introduces a RAID-like feature known as Storage Spaces, which also allows users to specify mirroring, parity, or no redundancy on a folder-by-folder basis.<ref>{{Cite web|last=Sinofsky|first=Steven|title=Virtualizing storage for scale, resiliency, and efficiency|url=http://blogs.msdn.com/b/b8/archive/2012/01/05/virtualizing-storage-for-scale-resiliency-and-efficiency.aspx|publisher=Microsoft}}</ref>
* [[NetBSD]] supports RAID&nbsp;0, 1, 4, and 5 via its software implementation, named RAIDframe.<ref>{{Cite web|title=NetBSD 1.4 Release Announcement|url=http://www.netbsd.org/releases/formal-1.4/NetBSD-1.4.html|first=Perry|last=Metzger|publisher=The NetBSD Foundation|work=NetBSD.org|date=1999-05-12|accessdate=2013-01-30}}</ref>
 
=== {{Anchor|FAKE}}Firmware/driver-based RAID ===
{{See also|MD RAID external metadata}}
 
A RAID implemented at the level of an operating system is not always compatible with the system's boot process, and it is generally impractical for desktop versions of Windows (as described above). However, hardware RAID controllers are expensive and proprietary. To fill this gap, cheap "RAID controllers" were introduced that do not contain a dedicated RAID controller chip, but simply a standard drive controller chip with proprietary firmware and drivers; during early stage bootup, the RAID is implemented by the firmware, and once the operating system has been more completely loaded, then the drivers take over control. Consequently, such controllers may not work when driver support is not available for the host operating system.<ref>{{Cite web|title=SATA RAID FAQ|url=https://ata.wiki.kernel.org/index.php/SATA_RAID_FAQ|publisher=Ata.wiki.kernel.org |date=2011-04-08 |accessdate=2012-08-26}}</ref> An example is [[Intel Matrix RAID]], implemented on many consumer-level motherboards.<ref>[https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/s1-raid-approaches.html Red Hat Enterprise Linux - Storage Administrator Guide - RAID Types]</ref><ref name="RusselCrawford2011">{{cite book|author1=Charlie Russel|author2=Sharon Crawford|author3=Andrew Edney|title=Working with Windows Small Business Server 2011 Essentials|url=http://books.google.com/books?id=R2gJ9kcX2ywC&pg=PA90|year=2011|publisher=O'Reilly Media, Inc.|isbn=978-0-7356-5670-3|page=90}}</ref>
 
Because there is some minimal hardware support involved, this implementation approach is also called "hardware-assisted software RAID",<ref>http://www.freebsd.org/doc/handbook/geom-graid.html</ref><ref name="KrutzConley2007">{{cite book|author1=Ronald L. Krutz|author2=James Conley|title=Wiley Pathways Network Security Fundamentals|url=http://books.google.com/books?id=Gdux_6ckDYwC&pg=PA422|year=2007|publisher=John Wiley & Sons|isbn=978-0-470-10192-6|page=422}}</ref><ref name="AdaptecWP" /> "hybrid model" RAID,<ref name="AdaptecWP" /> or even "fake RAID".<ref name="Smith2010">{{cite book|author=Gregory Smith|title=PostgreSQL 9.0: High Performance|url=http://books.google.com/books?id=OWOAu0GcsqoC&pg=PT72|year=2010|publisher=Packt Publishing Ltd|isbn=978-1-84951-031-8|page=31}}</ref> If RAID&nbsp;5 is supported, the hardware may provide a hardware XOR accelerator. An advantage of this model over the pure software RAID is that—if using a redundancy mode—the boot drive is protected from failure (due to the firmware) during the boot process even before the operating systems drivers take over.<ref name="AdaptecWP">[http://www.adaptec.com/nr/rdonlyres/14b2fd84-f7a0-4ac5-a07a-214123ea3dd6/0/4423_sw_hwraid_10.pdf Hardware RAID vs. Software RAID: Which Implementation is Best for my Application? Adaptec Whitepaper]</ref>
 
=== Hardware RAID ===
{{Expand section|date=December 2013}}
 
=== Data scrubbing / Patrol read ===
{{Main|Data scrubbing}}
 
Data scrubbing involves periodic reading and checking by the RAID controller of all the blocks in a RAID, including those not otherwise accessed. This detects bad blocks before use.<ref>
Ulf Troppens, Wolfgang Mueller-Friedt, Rainer Erkens, Rainer Wolafka, Nils Haustein. Storage Networks Explained: Basics and Application of Fibre Channel SAN, NAS, ISCSI, InfiniBand and FCoE. John Wiley and Sons, 2009. p.39</ref>
 
In some environments, documentation refers to data scrubbing as ''patrol read''. Patrol reading checks for bad blocks on each storage device in an array, but also uses the redundancy of the array to recover bad blocks on a single drive and to reassign the recovered data to spare blocks elsewhere on the drive.<ref>
Dell Computers, Background Patrol Read for Dell PowerEdge RAID Controllers, By Drew Habas and John Sieber, Reprinted from Dell Power Solutions, February 2006 http://www.dell.com/downloads/global/power/ps1q06-20050212-Habas.pdf</ref>
 
=== RAID with solid-state drives ===
RAID can provide data security with [[solid-state drives]] (SSDs) without the expense of an all-SSD system. For example, a fast SSD can be mirrored with a mechanical drive. For this configuration to provide a significant speed advantage an appropriate controller is needed that uses the fast SSD for all read operations. [[Adaptec]] calls this "hybrid RAID",<ref>{{Cite web | title=Adaptec Hybrid RAID Solutions | url=http://www.adaptec.com/nr/pdfs/hybrid-raid_wp.pdf | publisher=Adaptec | work=Adaptec.com | year=2012 | accessdate=2013-09-07}}</ref> the same term as is sometimes used for [[#Nested (hybrid) RAID|nested RAID]].
 
== Weaknesses ==
 
=== Correlated failures ===
In practice, the drives are often the same age (with similar wear) and subject to the same environment. Since many drive failures are due to mechanical issues (which are more likely on older drives), this violates the assumptions of independent, identical rate of failure amongst drives; failures are in fact statistically correlated.<ref name="Patterson_1994" /> In practice, the chances of a second failure before the first has been recovered (causing data loss) is not as unlikely as four random failures. In a study of about 100,000 drives, the probability of two drives in the same cluster failing within one hour was four times larger than predicted by the [[Exponential distribution|exponential statistical distribution]]—which characterizes processes in which events occur continuously and independently at a constant average rate. The probability of two failures in the same 10-hour period was twice as large as predicted by an exponential distribution.<ref name="schroeder">[http://www.usenix.org/events/fast07/tech/schroeder.html Disk Failures in the Real World: What Does an MTTF of 1,000,000 Hours Mean to You?] Bianca Schroeder and [[Garth A. Gibson]]</ref>
 
=== Unrecoverable read errors during rebuild ===
Unrecoverable read errors (URE) present as sector read failures. The [[unrecoverable bit error|unrecoverable bit-error]] (UBE) rate is typically specified at one bit in 10<sup>15</sup> for enterprise class drives ([[SCSI]], [[Fibre Channel|FC]], [[Serial Attached SCSI|SAS]]), and one bit in 10<sup>14</sup> for desktop class drives (IDE/ATA/PATA, SATA). Increasing drive capacities and large RAID&nbsp;5 redundancy groups have led to an increasing inability to successfully rebuild a RAID group after a drive failure because an unrecoverable sector is found on the remaining drives.<ref name="Patterson_1994" /><ref name="mojo2010">{{Cite web|title=Does RAID 6 stop working in 2019?|url=http://storagemojo.com/2010/02/27/does-raid-6-stops-working-in-2019/|first=Robin|last=Harris|publisher=TechnoQWAN|work=StorageMojo.com|date=2010-02-27|accessdate=2013-12-17}}</ref> Parity schemes such as RAID&nbsp;5 when rebuilding are particularly prone to the effects of UREs as they affect not only the sector where they occur but also reconstructed blocks using that sector for parity computation; typically an URE during a RAID&nbsp;5 rebuild leads to a complete rebuild failure.<ref>J.L. Hafner, V. Deenadhaylan, K. Rao, and J.A. Tomlin. "Matrix methods for lost data reconstruction in erasure codes. USENIX Conference on File and Storage Technologies, p15-30, Dec. 13-16, 2005.</ref>
 
Double protection schemes such as RAID&nbsp;6 are attempting to address this issue, but suffer from a very high write penalty. Schemes that duplicate (mirror) data such as RAIDs&nbsp;1 and 10 have a lower risk from UREs than those using parity computation.<ref name="UREs" /> [[#Data scrubbing / Patrol read|Background scrubbing]] can be used to detect and recover from UREs (which are latent and invisibly compensated for dynamically by the RAID controller) as a background process, by reconstruction from the redundant RAID data and then re-writing and re-mapping to a new sector; and so reduce the risk of double-failures to the RAID system.<ref>M.Baker, M.Shah, D.S.H. Rosenthal, M.Roussopoulos, P.Maniatis, T.Giuli, and P.Bungale. 'A fresh look at the reliability of long-term digital storage." EuroSys2006, Apr. 2006.</ref><ref>{{Cite web |title=L.N. Bairavasundaram, GR Goodson, S. Pasupathy, J.Schindler. "An analysis of latent sector errors in disk drives". Proceedings of SIGMETRICS'07, June 12-16,2007. |url=http://research.cs.wisc.edu/adsl/Publications/latent-sigmetrics07.pdf}}</ref>
 
=== Increasing rebuild time and failure probability ===
Drive capacity has grown at a much faster rate than transfer speed, and error rates have only fallen a little in comparison. Therefore, larger capacity drives may take hours, if not days, to rebuild. The re-build time is also limited if the entire array is still in operation at reduced capacity.<ref>Patterson, D., Hennessy, J. (2009). ''Computer Organization and Design''. New York: Morgan Kaufmann Publishers. pp 604-605.</ref> Given a RAID with only one drive of redundancy (RAIDs 3, 4, and 5), a second failure would cause complete failure of the array. Even though individual drives' [[mean time between failure]] (MTBF) have increased over time, this increase has not kept pace with the increased storage capacity of the drives. The time to rebuild the array after a single drive failure, as well as the chance of a second failure during a rebuild, have increased over time.<ref name="StorageForum">{{Cite web |url=http://www.enterprisestorageforum.com/technology/features/article.php/3839636 |title=RAID's Days May Be Numbered |last=Newman |first=Henry |date=2009-09-17 |accessdate=2010-09-07 |work=EnterpriseStorageForum}}</ref>
 
Some commentators have declared that RAID&nbsp;6 is only a "band aid" in this respect, because it only pushes the problem a little further down the road.<ref name="StorageForum" /> However, according to a 2006 [[NetApp]] study of Berriman et al., the chance of failure decreases by a factor of about 3,800 (relative to RAID&nbsp;5) for a proper implementation of RAID&nbsp;6, even when using commodity drives.<ref name="ACMQ" /> Nevertheless, if the currently observed technology trends remain unchanged, in 2019 a RAID&nbsp;6 array will have the same chance of failure as its RAID&nbsp;5 counterpart had in 2010.<ref name="ACMQ" /><ref name="mojo2010" />
 
Mirroring schemes such as RAID&nbsp;10 have a bounded recovery time as they require the copy of a single failed drive, compared with parity schemes such as RAID&nbsp;6, which require the copy of all blocks of the drives in an array set. Triple parity schemes, or triple mirroring, have been suggested as one approach to improve resilience to an additional drive failure during this large rebuild time.<ref name="ACMQ">{{Cite web|title=Triple-Parity RAID and Beyond. ACM Queue, Association of Computing Machinery|url=https://queue.acm.org/detail.cfm?id=1670144|first=Adam|last=Leventhal|date=2009-12-01|accessdate=2012-11-30}}</ref>
 
=== Atomicity: including parity inconsistency due to system crashes ===
A system crash or other interruption of a write operation can result in states where the parity is inconsistent with the data due to non-atomicity of the write process, such that the parity cannot be used for recovery in the case of a disk failure (the so-called RAID 5 write hole - see below).<ref name="Patterson_1994" />
 
This is a little understood and rarely mentioned failure mode for redundant storage systems that do not utilize transactional features. Database researcher [[Jim Gray (computer scientist)|Jim Gray]] wrote "Update in Place is a Poison Apple" during the early days of relational database commercialization.<ref>[http://www.informatik.uni-trier.de/~ley/db/conf/vldb/Gray81.html Jim Gray: The Transaction Concept: Virtues and Limitations] (Invited Paper) [http://www.informatik.uni-trier.de/~ley/db/conf/vldb/vldb81.html#Gray81 VLDB 1981]: 144-154</ref>
 
=== {{Anchor|WRITE-HOLE}}RAID write hole ===
The RAID write hole is a known data corruption issue in older and low-end RAIDs, caused by interrupted destaging of writes to disk.<ref name="RRG">{{Cite web|title="Write hole" in RAID5, RAID6, RAID1, and other arrays|url=http://www.raid-recovery-guide.com/raid5-write-hole.aspx|publisher=ZAR team|accessdate=15 February 2012}}</ref>
 
=== Write cache reliability ===
A concern about write cache reliability exists, specifically regarding devices equipped with a write-back cache—a caching system that reports the data as written as soon as it is written to cache, as opposed to the non-volatile medium.<ref>{{Cite web|url= http://www.snia.org/education/dictionary/w/|title=Definition of write-back cache at SNIA dictionary}}</ref>
 
=== Drive error recovery algorithms ===
Frequently, a RAID controller is configured to "drop" a component drive (that is, to assume a component drive has failed) if the drive has been unresponsive for eight seconds or so; this might cause the array controller to drop a good drive because that drive has not been given enough time to complete its internal error recovery procedure. Consequently, desktop drives can be risky in a RAID, and so-called "enterprise class" drives limit this error recovery time to reduce risk.{{Citation needed|date=October 2013}}
 
Western Digital's desktop drives used to have a specific fix. A utility called WDTLER.exe limited a drive's error recovery time. The utility enabled [[Time-Limited Error Recovery|TLER (time limited error recovery)]], which limits the error recovery time to seven seconds. Around September 2009, Western Digital disabled this feature in their desktop drives (e.g. the Caviar Black line), making such drives unsuitable for use in a RAID.<ref name="csc.liv.ac.uk">{{Cite web|title=Error Recovery Control with Smartmontools|url=http://www.csc.liv.ac.uk/~greg/projects/erc/|accessdate=2011}}</ref>
 
However, Western Digital enterprise class drives are shipped from the factory with TLER enabled. Similar technologies are used by Seagate, Samsung, and Hitachi. Of course, for non-RAID usage, an enterprise class drive with a short error recovery timeout that cannot be changed is therefore less suitable than a desktop drive.<ref name="csc.liv.ac.uk" />
 
In late 2010, the [[Smartmontools]] program began supporting the configuration of ATA Error Recovery Control, allowing the tool to configure many desktop class hard drives for use in a RAID.<ref name="csc.liv.ac.uk" />
 
=== Scenarios other than disk failure ===
While RAID may protect against physical drive failure, the data are still exposed to operator, software, hardware, and virus destruction. Many studies cite operator fault as the most common source of malfunction,<ref>These studies are: Gray, J (1990), Murphy and Gent (1995), Kuhn (1997), and Enriquez P. (2003).</ref> such as a server operator replacing the incorrect drive in a faulty RAID, and disabling the system (even temporarily) in the process.<ref>Patterson, D., Hennessy, J. (2009), 574.</ref>
 
=== RAID 5 in enterprise environments ===
RAID 5 is seriously affected by the general trends regarding array rebuild time and chance of failure during rebuild.<ref name="StorageForum" />
 
In August 2012, Dell posted an advisory against the use of RAID&nbsp;5 in any configuration and of RAID&nbsp;50 with "Class 2 7200&nbsp;RPM drives of 1&nbsp;TB and higher capacity".<ref>{{Cite web|url=http://community.spiceworks.com/topic/251735-new-raid-level-recommendations-from-dell|title=New RAID level recommendations from Dell|first=Mikko|last=Peltoniemi|date=2012-08-07|accessdate=2012-12-01}}</ref>
 
With a RAID&nbsp;6 array, using drives from multiple sources and manufacturers, it is possible to mitigate most of the problems associated with RAID&nbsp;5. The larger the drive capacities and the larger the array size, the more important it becomes to choose RAID&nbsp;6 instead of RAID&nbsp;5.<ref name="zdnet">{{Cite web|url=http://www.zdnet.com/blog/storage/why-raid-6-stops-working-in-2019/805|title=Why RAID&nbsp;6 stops working in 2019|publisher=[[ZDNet]]|date=22 February 2010}}</ref> RAID&nbsp;10 also minimizes these problems.<ref name="UREs">{{Cite web|url=http://www.techrepublic.com/blog/datacenter/how-to-protect-yourself-from-raid-related-unrecoverable-read-errors-ures/1752|author=Scott Lowe|title=How to protect yourself from RAID-related Unrecoverable Read Errors (UREs). Techrepublic.|date=2009-11-16|accessdate=2012-12-01}}</ref>
 
=== Software RAID issues ===
If a boot drive fails, the system has to be sophisticated enough to be able to boot off the remaining drive or drives. For instance, consider a computer being booted from a RAID&nbsp;1 (mirrored drives); if the first drive in the RAID&nbsp;1 fails, then a [[Bootloader|first-stage boot loader]] might not be sophisticated enough to attempt loading the [[Booting#Second-stage boot loader|second-stage boot loader]] from the second drive as a fallback. The second-stage boot loader for FreeBSD is capable of loading a [[kernel (computing)|kernel]] from a RAID&nbsp;1.<ref>{{Cite web|url=http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/geom-mirror.html|title=FreeBSD Handbook|work=Chapter 19 GEOM: Modular Disk Transformation Framework|accessdate= 2009-03-19}}</ref>
 
== See also ==
* [[Non-RAID drive architectures]]
* [[Redundant array of independent memory]] (RAIM)
 
== Notes ==
{{Reflist|group="note"}}
 
== References ==
{{Reflist|30em}}
 
== External links ==
{{Commons|Redundant array of independent disks}}
* {{Dmoz|Computers/Hardware/Storage/Subsystems/RAID}}
* [http://research.microsoft.com/research/pubs/view.aspx?msr_tr_id=MSR-TR-2005-166 "Empirical Measurements of Disk Failure Rates and Error Rates"], by Jim Gray and Catharine van Ingen, December 2005
* [https://www.kernel.org/pub/linux/kernel/people/hpa/raid6.pdf The mathematics of RAID-6], by [[H. Peter Anvin]]
* [http://superuser.com/questions/245928/does-fake-raid-offer-any-advantage-over-software-raid Does fake RAID offer any advantage over software RAID?] – Discussion on [[superuser.com]]
* [http://www.servethehome.com/difference-hardware-raid-hbas-software-raid/ Differences between Hardware RAID, HBAs, and Software RAID] – ServeTheHome.com (details more relative advantages and disadvantages)
* [http://www.servethehome.com/anatomy-hardware-raid-controller/ Anatomy of a Hardware RAID Controller] – ServeTheHome.com (focused on contemporary SAS products)
* [ftp://ftp.dell.com/app/3q03-Dum.pdf Comparing RAID Implementation Methods] – Dell.com
* [http://www.miracleas.com/BAARF/BAARF2.html BAARF: Battle Against Any Raid Five] (RAID&nbsp;3, 4 and 5 versus RAID&nbsp;10)
 
[[Category:RAID| ]]
[[Category:Fault-tolerant computer systems]]
 
{{Link GA|de}}

Revision as of 09:35, 3 March 2014

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