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RAID

March 28, 2023

This article is about the data storage technology. For the police unit, see RAID (French Police unit). For other uses, see Raid (disambiguation).

RAID (/reɪd/; "redundant array of inexpensive disks"[1] or "redundant array of independent disks"[2]) is a data storage virtualization technology that combines multiple physical disk drive components into one or more logical units for the purposes of data redundancy, performance improvement, or both. This is in contrast to the previous concept of highly reliable mainframe disk drives referred to as "single large expensive disk" (SLED).[3][1]

Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance. The different schemes, or data distribution layouts, are named by the word "RAID" followed by a number, for example RAID 0 or RAID 1. Each scheme, or RAID level, provides a different balance among the key goals: reliability, availability, performance, and capacity. RAID levels greater than RAID 0 provide protection against unrecoverable sector read errors, as well as against failures of whole physical drives.

History

The term "RAID" was invented by David Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987. In their June 1988 paper "A Case for Redundant Arrays of Inexpensive Disks (RAID)", presented at the SIGMOD Conference, they argued that the top-performing mainframe disk drives of the time could be beaten on performance by an array of the inexpensive drives that had been developed for the growing personal computer market. Although failures would rise in proportion to the number of drives, by configuring for redundancy, the reliability of an array could far exceed that of any large single drive.[4]

Although not yet using that terminology, the technologies of the five levels of RAID named in the June 1988 paper were used in various products prior to the paper's publication,[3] including the following:

  • Mirroring (RAID 1) was well established in the 1970s including, for example, Tandem NonStop Systems.
  • In 1977, Norman Ken Ouchi at IBM filed a patent disclosing what was subsequently named RAID 4.[5]
  • Around 1983, DEC began shipping subsystem mirrored RA8X disk drives (now known as RAID 1) as part of its HSC50 subsystem.[6]
  • In 1986, Clark et al. at IBM filed a patent disclosing what was subsequently named RAID 5.[7]
  • Around 1988, the Thinking Machines' DataVault used error correction codes (now known as RAID 2) in an array of disk drives.[8] A similar approach was used in the early 1960s on the IBM 353.[9][10]

Industry manufacturers later redefined the RAID acronym to stand for "redundant array of independent disks".[2][11][12][13]

Overview

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 simple XOR, but RAID 6 uses two separate parities based respectively on addition and multiplication in a particular Galois field or Reed–Solomon error correction.[14]

RAID can also 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".[15]

Standard levels

Main article: Standard RAID levels
 
Storage servers with 24 hard disk drives each and built-in hardware RAID controllers supporting various RAID levels

Originally, there were five standard levels of RAID, but many variations have evolved, including several nested levels and many non-standard levels (mostly 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:[16][17]

  • RAID 0 consists of striping, but no mirroring or parity. Compared to a spanned volume, the capacity of a RAID 0 volume is the same; it is the sum of the capacities of the drives in the set. But because striping distributes the contents of each file among all drives in the set, the failure of any drive causes the entire RAID 0 volume and all files to be lost. In comparison, a spanned volume preserves the files on the unfailing drives. The benefit of RAID 0 is that the throughput of read and write operations to any file is multiplied by the number of drives because, unlike spanned volumes, reads and writes are done concurrently.[11] The cost is increased vulnerability to drive failures—since any drive in a RAID 0 setup failing causes the entire volume to be lost, the average failure rate of the volume rises with the number of attached drives.
  • RAID 1 consists of data mirroring, without parity or striping. Data is written identically to two or more drives, thereby producing a "mirrored set" of drives. Thus, any read request can be serviced by any drive in the set. If a request is broadcast to every drive in the set, it can be serviced by the drive that accesses the data first (depending on its seek time and rotational latency), improving performance. Sustained read throughput, if the controller or software is optimized for it, approaches the sum of throughputs of every drive in the set, just as for RAID 0. Actual read throughput of most RAID 1 implementations is slower than the fastest drive. Write throughput is always slower because every drive must be updated, and the slowest drive limits the write performance. The array continues to operate as long as at least one drive is functioning.[11]
  • RAID 2 consists of bit-level striping with dedicated Hamming-code parity. All disk spindle rotation is synchronized and data is striped such that each sequential bit is on a different drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive.[11] This level is of historical significance only; although it was used on some early machines (for example, the Thinking Machines CM-2),[18] as of 2014 it is not used by any commercially available system.[19]
  • RAID 3 consists of 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.[11] Although implementations exist,[20] RAID 3 is not commonly used in practice.
  • RAID 4 consists of block-level striping with dedicated parity. This level was previously used by NetApp, but has now been largely replaced by a proprietary implementation of RAID 4 with two parity disks, called RAID-DP.[21] The main advantage of RAID 4 over RAID 2 and 3 is I/O parallelism: in RAID 2 and 3, a single read I/O operation requires reading the whole group of data drives, while in RAID 4 one I/O read operation does not have to spread across all data drives. As a result, more I/O operations can be executed in parallel, improving the performance of small transfers.[1]
  • RAID 5 consists of block-level striping with distributed parity. Unlike RAID 4, parity information is distributed among the drives, requiring all drives but one to 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.[11] Like all single-parity concepts, large RAID 5 implementations are susceptible to system failures because of trends regarding array rebuild time and the chance of drive failure during rebuild (see "Increasing rebuild time and failure probability" section, below).[22] Rebuilding an array requires reading all data from all disks, opening a chance for a second drive failure and the loss of the entire array.
  • RAID 6 consists of 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. RAID 6 requires a minimum of four disks. As with RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced.[11] With a RAID 6 array, using drives from multiple sources and manufacturers, it is possible to mitigate most of the problems associated with RAID 5. The larger the drive capacities and the larger the array size, the more important it becomes to choose RAID 6 instead of RAID 5.[23] RAID 10 also minimizes these problems.[24]

Nested (hybrid) RAID

Main article: Nested RAID levels

In what was originally termed hybrid RAID,[25] many storage controllers allow RAID levels to be nested. The elements of a RAID may be either individual drives or arrays themselves. Arrays are rarely nested more than one level deep.

The final array is known as the top array. When the top array is RAID 0 (such as in RAID 1+0 and RAID 5+0), most vendors omit the "+" (yielding RAID 10 and RAID 50, respectively).

  • RAID 0+1: creates two stripes and mirrors them. If a single drive failure occurs then one of the mirrors has failed, at this point it is running effectively as RAID 0 with no redundancy. Significantly higher risk is introduced during a rebuild than RAID 1+0 as all the data from all the drives in the remaining stripe has to be read rather than just from one drive, increasing the chance of an unrecoverable read error (URE) and significantly extending the rebuild window.[26][27][28]
  • RAID 1+0: (see: RAID 10) 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.[29]
  • JBOD RAID N+N: With JBOD (just a bunch of disks), it is possible to concatenate disks, but also volumes such as RAID sets. With larger drive capacities, write delay and rebuilding time increase dramatically (especially, as described above, with RAID 5 and RAID 6). By splitting a larger RAID N set into smaller subsets and concatenating them with linear JBOD,[clarification needed] write and rebuilding time will be reduced. If a hardware RAID controller is not capable of nesting linear JBOD with RAID N, then linear JBOD can be achieved with OS-level software RAID in combination with separate RAID N subset volumes created within one, or more, hardware RAID controller(s). Besides a drastic speed increase, this also provides a substantial advantage: the possibility to start a linear JBOD with a small set of disks and to be able to expand the total set with disks of different size, later on (in time, disks of bigger size become available on the market). There is another advantage in the form of disaster recovery (if a RAID N subset happens to fail, then the data on the other RAID N subsets is not lost, reducing restore time).[citation needed]

Non-standard levels

Main article: 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 1 with two drives, and a standard RAID 1+0 with four drives; however, it can include any number of drives, including odd numbers. With its "far" layout, MD RAID 10 can run both striped and mirrored, even with only two drives in f2 layout; this runs mirroring with striped reads, giving the read performance of RAID 0. Regular RAID 1, as provided by Linux software RAID, does not stripe reads, but can perform reads in parallel.[29][30][31]
  • Hadoop has a RAID system that generates a parity file by xor-ing a stripe of blocks in a single HDFS file.[32]
  • BeeGFS, the parallel file system, has internal striping (comparable to file-based RAID0) and replication (comparable to file-based RAID10) options to aggregate throughput and capacity of multiple servers and is typically based on top of an underlying RAID to make disk failures transparent.
  • Declustered RAID scatters dual (or more) copies of the data across all disks (possibly hundreds) in a storage subsystem, while holding back enough spare capacity to allow for a few disks to fail. The scattering is based on algorithms which give the appearance of arbitrariness. When one or more disks fail the missing copies are rebuilt into that spare capacity, again arbitrarily. Because the rebuild is done from and to all the remaining disks, it operates much faster than with traditional RAID, reducing the overall impact on clients of the storage system.

Implementations

The distribution of data across multiple drives can be managed either by dedicated computer hardware or by software. A software solution may be part of the operating system, part of the firmware and drivers supplied with a standard drive controller (so-called "hardware-assisted software RAID"), or it may reside entirely within the hardware RAID controller.

Hardware-based

Main article: RAID controller

Hardware RAID controllers can be configured through card BIOS or Option ROM before an operating system is booted, and after the operating system is booted, proprietary configuration utilities are available from the manufacturer of each controller. Unlike the network interface controllers for Ethernet, which can usually be configured and serviced entirely through the common operating system paradigms like ifconfig in Unix, without a need for any third-party tools, each manufacturer of each RAID controller usually provides their own proprietary software tooling for each operating system that they deem to support, ensuring a vendor lock-in, and contributing to reliability issues.[33]

For example, in FreeBSD, in order to access the configuration of Adaptec RAID controllers, users are required to enable Linux compatibility layer, and use the Linux tooling from Adaptec,[34] potentially compromising the stability, reliability and security of their setup, especially when taking the long-term view.[33]

Some other operating systems have implemented their own generic frameworks for interfacing with any RAID controller, and provide tools for monitoring RAID volume status, as well as facilitation of drive identification through LED blinking, alarm management and hot spare disk designations from within the operating system without having to reboot into card BIOS. For example, this was the approach taken by OpenBSD in 2005 with its bio(4) pseudo-device and the bioctl utility, which provide volume status, and allow LED/alarm/hotspare control, as well as the sensors (including the drive sensor) for health monitoring;[35] this approach has subsequently been adopted and extended by NetBSD in 2007 as well.[36]

Software-based

Software RAID implementations are provided by many modern operating systems. Software RAID can be implemented as:

  • A layer that abstracts multiple devices, thereby providing a single virtual device (such as Linux kernel's md and OpenBSD's softraid)
  • A more generic logical volume manager (provided with most server-class operating systems such as Veritas or LVM)
  • A component of the file system (such as ZFS, Spectrum Scale or Btrfs)
  • A layer that sits above any file system and provides parity protection to user data (such as RAID-F)[37]

Some advanced file systems are designed to organize data across multiple storage devices directly, without needing the help of a third-party logical volume manager:

  • ZFS supports the equivalents of RAID 0, RAID 1, RAID 5 (RAID-Z1) single-parity, RAID 6 (RAID-Z2) double-parity, and a triple-parity version (RAID-Z3) also referred to as RAID 7.[38] 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 and illumos, and is also available on FreeBSD and Linux. Open-source ZFS implementations are actively developed under the OpenZFS umbrella project.[39][40][41][42][43]
  • Spectrum Scale, initially developed by IBM for media streaming and scalable analytics, supports declustered RAID protection schemes up to n+3. A particularity is the dynamic rebuilding priority which runs with low impact in the background until a data chunk hits n+0 redundancy, in which case this chunk is quickly rebuilt to at least n+1. On top, Spectrum Scale supports metro-distance RAID 1.[44]
  • Btrfs supports RAID 0, RAID 1 and RAID 10 (RAID 5 and 6 are under development).[45][46]
  • XFS was originally designed to provide an integrated volume manager that supports concatenating, mirroring and striping of multiple physical storage devices.[47] However, the implementation of XFS in Linux kernel lacks the integrated volume manager.[48]

Many operating systems provide RAID implementations, including the following:

  • Hewlett-Packard's OpenVMS operating system supports RAID 1. The mirrored disks, called a "shadow set", can be in different locations to assist in disaster recovery.[49]
  • Apple's macOS and macOS Server support RAID 0, RAID 1, and RAID 1+0.[50][51]
  • FreeBSD supports RAID 0, RAID 1, RAID 3, and RAID 5, and all nestings via GEOM modules and ccd.[52][53][54]
  • Linux's md supports RAID 0, RAID 1, RAID 4, RAID 5, RAID 6, and all nestings.[55] Certain reshaping/resizing/expanding operations are also supported.[56]
  • Microsoft Windows supports RAID 0, RAID 1, and RAID 5 using various software implementations. Logical Disk Manager, introduced with Windows 2000, allows for the creation of RAID 0, RAID 1, and RAID 5 volumes by using dynamic disks, but this was limited only to professional and server editions of Windows until the release of Windows 8.[57][58] Windows XP can be modified to unlock support for RAID 0, 1, and 5.[59] Windows 8 and Windows Server 2012 introduced 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. These options are similar to RAID 1 and RAID 5, but are implemented at a higher abstraction level.[60]
  • NetBSD supports RAID 0, 1, 4, and 5 via its software implementation, named RAIDframe.[61]
  • OpenBSD supports RAID 0, 1 and 5 via its software implementation, named softraid.[62]

If a boot drive fails, the system has to be sophisticated enough to be able to boot from the remaining drive or drives. For instance, consider a computer whose disk is configured as RAID 1 (mirrored drives); if the first drive in the array fails, then a first-stage boot loader might not be sophisticated enough to attempt loading the second-stage boot loader from the second drive as a fallback. The second-stage boot loader for FreeBSD is capable of loading a kernel from such an array.[63]

Firmware- and driver-based

 
A SATA 3.0 controller that provides RAID functionality through proprietary firmware and drivers
See also: MD RAID external metadata

Software-implemented RAID is not always compatible with the system's boot process, and it is generally impractical for desktop versions of Windows. However, hardware RAID controllers are expensive and proprietary. To fill this gap, inexpensive "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 bootup, the RAID is implemented by the firmware and, once the operating system has been more completely loaded, the drivers take over control. Consequently, such controllers may not work when driver support is not available for the host operating system.[64] An example is Intel Rapid Storage Technology, implemented on many consumer-level motherboards.[65][66]

Because some minimal hardware support is involved, this implementation is also called "hardware-assisted software RAID",[67][68][69] "hybrid model" RAID,[69] or even "fake RAID".[70] If RAID 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 system's drivers take over.[69]

Integrity

Data scrubbing (referred to in some environments as patrol read) involves periodic reading and checking by the RAID controller of all the blocks in an array, including those not otherwise accessed. This detects bad blocks before use.[71] Data scrubbing 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.[72]

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, using consumer-marketed drives with RAID can be risky, and so-called "enterprise class" drives limit this error recovery time to reduce risk.[citation needed] 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 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 (such as the Caviar Black line), making such drives unsuitable for use in RAID configurations.[73] However, Western Digital enterprise class drives are shipped from the factory with TLER enabled. Similar technologies are used by Seagate, Samsung, and Hitachi. 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.[73] 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 RAID setups.[73]

While RAID may protect against physical drive failure, the data is still exposed to operator, software, hardware, and virus destruction. Many studies cite operator fault as a common source of malfunction,[74][75] such as a server operator replacing the incorrect drive in a faulty RAID, and disabling the system (even temporarily) in the process.[76]

An array can be overwhelmed by catastrophic failure that exceeds its recovery capacity and the entire array is at risk of physical damage by fire, natural disaster, and human forces, however backups can be stored off site. An array is also vulnerable to controller failure because it is not always possible to migrate it to a new, different controller without data loss.[77]

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.[11] In practice, the chances for a second failure before the first has been recovered (causing data loss) are higher than the chances for 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 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.[78]

Unrecoverable read errors during rebuild

Unrecoverable read errors (URE) present as sector read failures, also known as latent sector errors (LSE). The associated media assessment measure, unrecoverable bit error (UBE) rate, is typically guaranteed to be less than one bit in 1015[disputed – discuss] for enterprise-class drives (SCSI, FC, SAS or SATA), and less than one bit in 1014[disputed – discuss] for desktop-class drives (IDE/ATA/PATA or SATA). Increasing drive capacities and large RAID 5 instances have led to the maximum error rates being insufficient to guarantee a successful recovery, due to the high likelihood of such an error occurring on one or more remaining drives during a RAID set rebuild.[11][obsolete source][79][deprecated source?] When rebuilding, parity-based schemes such as RAID 5 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.[80]

Double-protection parity-based schemes, such as RAID 6, attempt to address this issue by providing redundancy that allows double-drive failures; as a downside, such schemes suffer from elevated write penalty—the number of times the storage medium must be accessed during a single write operation.[81] Schemes that duplicate (mirror) data in a drive-to-drive manner, such as RAID 1 and RAID 10, have a lower risk from UREs than those using parity computation or mirroring between striped sets.[24][82] Data scrubbing, as a background process, can be used to detect and recover from UREs, effectively reducing the risk of them happening during RAID rebuilds and causing double-drive failures. The recovery of UREs involves remapping of affected underlying disk sectors, utilizing the drive's sector remapping pool; in case of UREs detected during background scrubbing, data redundancy provided by a fully operational RAID set allows the missing data to be reconstructed and rewritten to a remapped sector.[83][84]

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, during which time other drives may fail or yet undetected read errors may surface. The rebuild time is also limited if the entire array is still in operation at reduced capacity.[85] Given an array with only one redundant drive (which applies to RAID levels 3, 4 and 5, and to "classic" two-drive RAID 1), a second drive 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.[22]

Some commentators have declared that RAID 6 is only a "band aid" in this respect, because it only kicks the problem a little further down the road.[22] However, according to the 2006 NetApp study of Berriman et al., the chance of failure decreases by a factor of about 3,800 (relative to RAID 5) for a proper implementation of RAID 6, even when using commodity drives.[86][citation not found] Nevertheless, if the currently observed technology trends remain unchanged, in 2019 a RAID 6 array will have the same chance of failure as its RAID 5 counterpart had in 2010.[86][unreliable source?]

Mirroring schemes such as RAID 10 have a bounded recovery time as they require the copy of a single failed drive, compared with parity schemes such as RAID 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.[86][unreliable source?]

Atomicity

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. This is commonly termed the write hole which is a known data corruption issue in older and low-end RAIDs, caused by interrupted destaging of writes to disk.[87] The write hole can be addressed with write-ahead logging. This was fixed in mdadm by introducing a dedicated journaling device (to avoid performance penalty, typically, SSDs and NVMs are preferred) for that purpose.[88][89]

This is a little understood and rarely mentioned failure mode for redundant storage systems that do not utilize transactional features. Database researcher Jim Gray wrote "Update in Place is a Poison Apple" during the early days of relational database commercialization.[90]

Write-cache reliability

There are concerns about write-cache reliability, specifically regarding devices equipped with a write-back cache, which is a caching system that reports the data as written as soon as it is written to cache, as opposed to when it is written to the non-volatile medium. If the system experiences a power loss or other major failure, the data may be irrevocably lost from the cache before reaching the non-volatile storage. For this reason good write-back cache implementations include mechanisms, such as redundant battery power, to preserve cache contents across system failures (including power failures) and to flush the cache at system restart time.[91]

See also

References

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  2. ^ a b "Originally referred to as Redundant Array of Inexpensive Disks, the term RAID was first published 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.)" Storage Area Network Fundamentals; Meeta Gupta; Cisco Press; ISBN 978-1-58705-065-7; Appendix A.
  3. ^ a b Katz, Randy H. (October 2010). "RAID: A Personal Recollection of How Storage Became a System" (PDF). eecs.umich.edu. IEEE Computer Society. Retrieved 2015-01-18. We were not the first to think of the idea of replacing what Patterson described as a slow large expensive disk (SLED) with an array of inexpensive disks. For example, the concept of disk mirroring, pioneered by Tandem, was well known, and some storage products had already been constructed around arrays of small disks.
  4. ^ Hayes, Frank (November 17, 2003). "The Story So Far". Computerworld. Retrieved November 18, 2016. Patterson recalled the beginnings of his RAID project in 1987. [...] 1988: David A. Patterson leads a team that defines RAID standards for improved performance, reliability and scalability.
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External links

 
Wikimedia Commons has media related to Redundant array of independent disks.
  • "Empirical Measurements of Disk Failure Rates and Error Rates", by Jim Gray and Catharine van Ingen, December 2005
  • The Mathematics of RAID-6, by H. Peter Anvin
  • Does Fake RAID Offer Any Advantage Over Software RAID? – Discussion on superuser.com
  • (RAID 3, 4 and 5 versus RAID 10)
  • A Clean-Slate Look at Disk Scrubbing

March 28, 2023
raid, this, article, about, data, storage, technology, police, unit, french, police, unit, other, uses, raid, disambiguation, redundant, array, inexpensive, disks, redundant, array, independent, disks, data, storage, virtualization, technology, that, combines,. This article is about the data storage technology For the police unit see RAID French Police unit For other uses see Raid disambiguation RAID r eɪ d redundant array of inexpensive disks 1 or redundant array of independent disks 2 is a data storage virtualization technology that combines multiple physical disk drive components into one or more logical units for the purposes of data redundancy performance improvement or both This is in contrast to the previous concept of highly reliable mainframe disk drives referred to as single large expensive disk SLED 3 1 Data is distributed across the drives in one of several ways referred to as RAID levels depending on the required level of redundancy and performance The different schemes or data distribution layouts are named by the word RAID followed by a number for example RAID 0 or RAID 1 Each scheme or RAID level provides a different balance among the key goals reliability availability performance and capacity RAID levels greater than RAID 0 provide protection against unrecoverable sector read errors as well as against failures of whole physical drives Contents 1 History 2 Overview 3 Standard levels 4 Nested hybrid RAID 5 Non standard levels 6 Implementations 6 1 Hardware based 6 2 Software based 6 3 Firmware and driver based 7 Integrity 8 Weaknesses 8 1 Correlated failures 8 2 Unrecoverable read errors during rebuild 8 3 Increasing rebuild time and failure probability 8 4 Atomicity 8 5 Write cache reliability 9 See also 10 References 11 External linksHistory EditThe term RAID was invented by David Patterson Garth A Gibson and Randy Katz at the University of California Berkeley in 1987 In their June 1988 paper A Case for Redundant Arrays of Inexpensive Disks RAID presented at the SIGMOD Conference they argued that the top performing mainframe disk drives of the time could be beaten on performance by an array of the inexpensive drives that had been developed for the growing personal computer market Although failures would rise in proportion to the number of drives by configuring for redundancy the reliability of an array could far exceed that of any large single drive 4 Although not yet using that terminology the technologies of the five levels of RAID named in the June 1988 paper were used in various products prior to the paper s publication 3 including the following Mirroring RAID 1 was well established in the 1970s including for example Tandem NonStop Systems In 1977 Norman Ken Ouchi at IBM filed a patent disclosing what was subsequently named RAID 4 5 Around 1983 DEC began shipping subsystem mirrored RA8X disk drives now known as RAID 1 as part of its HSC50 subsystem 6 In 1986 Clark et al at IBM filed a patent disclosing what was subsequently named RAID 5 7 Around 1988 the Thinking Machines DataVault used error correction codes now known as RAID 2 in an array of disk drives 8 A similar approach was used in the early 1960s on the IBM 353 9 10 Industry manufacturers later redefined the RAID acronym to stand for redundant array of independent disks 2 11 12 13 Overview EditMany 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 simple XOR but RAID 6 uses two separate parities based respectively on addition and multiplication in a particular Galois field or Reed Solomon error correction 14 RAID can also 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 15 Standard levels EditMain article Standard RAID levels Storage servers with 24 hard disk drives each and built in hardware RAID controllers supporting various RAID levels Originally there were five standard levels of RAID but many variations have evolved including several nested levels and many non standard levels mostly 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 16 17 RAID 0 consists of striping but no mirroring or parity Compared to a spanned volume the capacity of a RAID 0 volume is the same it is the sum of the capacities of the drives in the set But because striping distributes the contents of each file among all drives in the set the failure of any drive causes the entire RAID 0 volume and all files to be lost In comparison a spanned volume preserves the files on the unfailing drives The benefit of RAID 0 is that the throughput of read and write operations to any file is multiplied by the number of drives because unlike spanned volumes reads and writes are done concurrently 11 The cost is increased vulnerability to drive failures since any drive in a RAID 0 setup failing causes the entire volume to be lost the average failure rate of the volume rises with the number of attached drives RAID 1 consists of data mirroring without parity or striping Data is written identically to two or more drives thereby producing a mirrored set of drives Thus any read request can be serviced by any drive in the set If a request is broadcast to every drive in the set it can be serviced by the drive that accesses the data first depending on its seek time and rotational latency improving performance Sustained read throughput if the controller or software is optimized for it approaches the sum of throughputs of every drive in the set just as for RAID 0 Actual read throughput of most RAID 1 implementations is slower than the fastest drive Write throughput is always slower because every drive must be updated and the slowest drive limits the write performance The array continues to operate as long as at least one drive is functioning 11 RAID 2 consists of bit level striping with dedicated Hamming code parity All disk spindle rotation is synchronized and data is striped such that each sequential bit is on a different drive Hamming code parity is calculated across corresponding bits and stored on at least one parity drive 11 This level is of historical significance only although it was used on some early machines for example the Thinking Machines CM 2 18 as of 2014 update it is not used by any commercially available system 19 RAID 3 consists of 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 11 Although implementations exist 20 RAID 3 is not commonly used in practice RAID 4 consists of block level striping with dedicated parity This level was previously used by NetApp but has now been largely replaced by a proprietary implementation of RAID 4 with two parity disks called RAID DP 21 The main advantage of RAID 4 over RAID 2 and 3 is I O parallelism in RAID 2 and 3 a single read I O operation requires reading the whole group of data drives while in RAID 4 one I O read operation does not have to spread across all data drives As a result more I O operations can be executed in parallel improving the performance of small transfers 1 RAID 5 consists of block level striping with distributed parity Unlike RAID 4 parity information is distributed among the drives requiring all drives but one to 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 11 Like all single parity concepts large RAID 5 implementations are susceptible to system failures because of trends regarding array rebuild time and the chance of drive failure during rebuild see Increasing rebuild time and failure probability section below 22 Rebuilding an array requires reading all data from all disks opening a chance for a second drive failure and the loss of the entire array RAID 6 consists of 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 RAID 6 requires a minimum of four disks As with RAID 5 a single drive failure results in reduced performance of the entire array until the failed drive has been replaced 11 With a RAID 6 array using drives from multiple sources and manufacturers it is possible to mitigate most of the problems associated with RAID 5 The larger the drive capacities and the larger the array size the more important it becomes to choose RAID 6 instead of RAID 5 23 RAID 10 also minimizes these problems 24 Nested hybrid RAID EditMain article Nested RAID levels In what was originally termed hybrid RAID 25 many storage controllers allow RAID levels to be nested The elements of a RAID may be either individual drives or arrays themselves Arrays are rarely nested more than one level deep The final array is known as the top array When the top array is RAID 0 such as in RAID 1 0 and RAID 5 0 most vendors omit the yielding RAID 10 and RAID 50 respectively RAID 0 1 creates two stripes and mirrors them If a single drive failure occurs then one of the mirrors has failed at this point it is running effectively as RAID 0 with no redundancy Significantly higher risk is introduced during a rebuild than RAID 1 0 as all the data from all the drives in the remaining stripe has to be read rather than just from one drive increasing the chance of an unrecoverable read error URE and significantly extending the rebuild window 26 27 28 RAID 1 0 see RAID 10 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 29 JBOD RAID N N With JBOD just a bunch of disks it is possible to concatenate disks but also volumes such as RAID sets With larger drive capacities write delay and rebuilding time increase dramatically especially as described above with RAID 5 and RAID 6 By splitting a larger RAID N set into smaller subsets and concatenating them with linear JBOD clarification needed write and rebuilding time will be reduced If a hardware RAID controller is not capable of nesting linear JBOD with RAID N then linear JBOD can be achieved with OS level software RAID in combination with separate RAID N subset volumes created within one or more hardware RAID controller s Besides a drastic speed increase this also provides a substantial advantage the possibility to start a linear JBOD with a small set of disks and to be able to expand the total set with disks of different size later on in time disks of bigger size become available on the market There is another advantage in the form of disaster recovery if a RAID N subset happens to fail then the data on the other RAID N subsets is not lost reducing restore time citation needed Non standard levels EditMain article 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 1 with two drives and a standard RAID 1 0 with four drives however it can include any number of drives including odd numbers With its far layout MD RAID 10 can run both striped and mirrored even with only two drives in f2 layout this runs mirroring with striped reads giving the read performance of RAID 0 Regular RAID 1 as provided by Linux software RAID does not stripe reads but can perform reads in parallel 29 30 31 Hadoop has a RAID system that generates a parity file by xor ing a stripe of blocks in a single HDFS file 32 BeeGFS the parallel file system has internal striping comparable to file based RAID0 and replication comparable to file based RAID10 options to aggregate throughput and capacity of multiple servers and is typically based on top of an underlying RAID to make disk failures transparent Declustered RAID scatters dual or more copies of the data across all disks possibly hundreds in a storage subsystem while holding back enough spare capacity to allow for a few disks to fail The scattering is based on algorithms which give the appearance of arbitrariness When one or more disks fail the missing copies are rebuilt into that spare capacity again arbitrarily Because the rebuild is done from and to all the remaining disks it operates much faster than with traditional RAID reducing the overall impact on clients of the storage system Implementations EditThe distribution of data across multiple drives can be managed either by dedicated computer hardware or by software A software solution may be part of the operating system part of the firmware and drivers supplied with a standard drive controller so called hardware assisted software RAID or it may reside entirely within the hardware RAID controller Hardware based Edit Main article RAID controller Hardware RAID controllers can be configured through card BIOS or Option ROM before an operating system is booted and after the operating system is booted proprietary configuration utilities are available from the manufacturer of each controller Unlike the network interface controllers for Ethernet which can usually be configured and serviced entirely through the common operating system paradigms like ifconfig in Unix without a need for any third party tools each manufacturer of each RAID controller usually provides their own proprietary software tooling for each operating system that they deem to support ensuring a vendor lock in and contributing to reliability issues 33 For example in FreeBSD in order to access the configuration of Adaptec RAID controllers users are required to enable Linux compatibility layer and use the Linux tooling from Adaptec 34 potentially compromising the stability reliability and security of their setup especially when taking the long term view 33 Some other operating systems have implemented their own generic frameworks for interfacing with any RAID controller and provide tools for monitoring RAID volume status as well as facilitation of drive identification through LED blinking alarm management and hot spare disk designations from within the operating system without having to reboot into card BIOS For example this was the approach taken by OpenBSD in 2005 with its bio 4 pseudo device and the bioctl utility which provide volume status and allow LED alarm hotspare control as well as the sensors including the drive sensor for health monitoring 35 this approach has subsequently been adopted and extended by NetBSD in 2007 as well 36 Software based Edit Software RAID implementations are provided by many modern operating systems Software RAID can be implemented as A layer that abstracts multiple devices thereby providing a single virtual device such as Linux kernel s md and OpenBSD s softraid A more generic logical volume manager provided with most server class operating systems such as Veritas or LVM A component of the file system such as ZFS Spectrum Scale or Btrfs A layer that sits above any file system and provides parity protection to user data such as RAID F 37 Some advanced file systems are designed to organize data across multiple storage devices directly without needing the help of a third party logical volume manager ZFS supports the equivalents of RAID 0 RAID 1 RAID 5 RAID Z1 single parity RAID 6 RAID Z2 double parity and a triple parity version RAID Z3 also referred to as RAID 7 38 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 and illumos and is also available on FreeBSD and Linux Open source ZFS implementations are actively developed under the OpenZFS umbrella project 39 40 41 42 43 Spectrum Scale initially developed by IBM for media streaming and scalable analytics supports declustered RAID protection schemes up to n 3 A particularity is the dynamic rebuilding priority which runs with low impact in the background until a data chunk hits n 0 redundancy in which case this chunk is quickly rebuilt to at least n 1 On top Spectrum Scale supports metro distance RAID 1 44 Btrfs supports RAID 0 RAID 1 and RAID 10 RAID 5 and 6 are under development 45 46 XFS was originally designed to provide an integrated volume manager that supports concatenating mirroring and striping of multiple physical storage devices 47 However the implementation of XFS in Linux kernel lacks the integrated volume manager 48 Many operating systems provide RAID implementations including the following Hewlett Packard s OpenVMS operating system supports RAID 1 The mirrored disks called a shadow set can be in different locations to assist in disaster recovery 49 Apple s macOS and macOS Server support RAID 0 RAID 1 and RAID 1 0 50 51 FreeBSD supports RAID 0 RAID 1 RAID 3 and RAID 5 and all nestings via GEOM modules and ccd 52 53 54 Linux s md supports RAID 0 RAID 1 RAID 4 RAID 5 RAID 6 and all nestings 55 Certain reshaping resizing expanding operations are also supported 56 Microsoft Windows supports RAID 0 RAID 1 and RAID 5 using various software implementations Logical Disk Manager introduced with Windows 2000 allows for the creation of RAID 0 RAID 1 and RAID 5 volumes by using dynamic disks but this was limited only to professional and server editions of Windows until the release of Windows 8 57 58 Windows XP can be modified to unlock support for RAID 0 1 and 5 59 Windows 8 and Windows Server 2012 introduced 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 These options are similar to RAID 1 and RAID 5 but are implemented at a higher abstraction level 60 NetBSD supports RAID 0 1 4 and 5 via its software implementation named RAIDframe 61 OpenBSD supports RAID 0 1 and 5 via its software implementation named softraid 62 If a boot drive fails the system has to be sophisticated enough to be able to boot from the remaining drive or drives For instance consider a computer whose disk is configured as RAID 1 mirrored drives if the first drive in the array fails then a first stage boot loader might not be sophisticated enough to attempt loading the second stage boot loader from the second drive as a fallback The second stage boot loader for FreeBSD is capable of loading a kernel from such an array 63 Firmware and driver based Edit A SATA 3 0 controller that provides RAID functionality through proprietary firmware and drivers See also MD RAID external metadata Software implemented RAID is not always compatible with the system s boot process and it is generally impractical for desktop versions of Windows However hardware RAID controllers are expensive and proprietary To fill this gap inexpensive 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 bootup the RAID is implemented by the firmware and once the operating system has been more completely loaded the drivers take over control Consequently such controllers may not work when driver support is not available for the host operating system 64 An example is Intel Rapid Storage Technology implemented on many consumer level motherboards 65 66 Because some minimal hardware support is involved this implementation is also called hardware assisted software RAID 67 68 69 hybrid model RAID 69 or even fake RAID 70 If RAID 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 system s drivers take over 69 Integrity EditData scrubbing referred to in some environments as patrol read involves periodic reading and checking by the RAID controller of all the blocks in an array including those not otherwise accessed This detects bad blocks before use 71 Data scrubbing 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 72 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 using consumer marketed drives with RAID can be risky and so called enterprise class drives limit this error recovery time to reduce risk citation needed 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 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 such as the Caviar Black line making such drives unsuitable for use in RAID configurations 73 However Western Digital enterprise class drives are shipped from the factory with TLER enabled Similar technologies are used by Seagate Samsung and Hitachi 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 73 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 RAID setups 73 While RAID may protect against physical drive failure the data is still exposed to operator software hardware and virus destruction Many studies cite operator fault as a common source of malfunction 74 75 such as a server operator replacing the incorrect drive in a faulty RAID and disabling the system even temporarily in the process 76 An array can be overwhelmed by catastrophic failure that exceeds its recovery capacity and the entire array is at risk of physical damage by fire natural disaster and human forces however backups can be stored off site An array is also vulnerable to controller failure because it is not always possible to migrate it to a new different controller without data loss 77 Weaknesses EditCorrelated failures Edit 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 11 In practice the chances for a second failure before the first has been recovered causing data loss are higher than the chances for 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 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 78 Unrecoverable read errors during rebuild Edit Unrecoverable read errors URE present as sector read failures also known as latent sector errors LSE The associated media assessment measure unrecoverable bit error UBE rate is typically guaranteed to be less than one bit in 1015 disputed discuss for enterprise class drives SCSI FC SAS or SATA and less than one bit in 1014 disputed discuss for desktop class drives IDE ATA PATA or SATA Increasing drive capacities and large RAID 5 instances have led to the maximum error rates being insufficient to guarantee a successful recovery due to the high likelihood of such an error occurring on one or more remaining drives during a RAID set rebuild 11 obsolete source 79 deprecated source When rebuilding parity based schemes such as RAID 5 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 80 Double protection parity based schemes such as RAID 6 attempt to address this issue by providing redundancy that allows double drive failures as a downside such schemes suffer from elevated write penalty the number of times the storage medium must be accessed during a single write operation 81 Schemes that duplicate mirror data in a drive to drive manner such as RAID 1 and RAID 10 have a lower risk from UREs than those using parity computation or mirroring between striped sets 24 82 Data scrubbing as a background process can be used to detect and recover from UREs effectively reducing the risk of them happening during RAID rebuilds and causing double drive failures The recovery of UREs involves remapping of affected underlying disk sectors utilizing the drive s sector remapping pool in case of UREs detected during background scrubbing data redundancy provided by a fully operational RAID set allows the missing data to be reconstructed and rewritten to a remapped sector 83 84 Increasing rebuild time and failure probability Edit 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 during which time other drives may fail or yet undetected read errors may surface The rebuild time is also limited if the entire array is still in operation at reduced capacity 85 Given an array with only one redundant drive which applies to RAID levels 3 4 and 5 and to classic two drive RAID 1 a second drive 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 22 Some commentators have declared that RAID 6 is only a band aid in this respect because it only kicks the problem a little further down the road 22 However according to the 2006 NetApp study of Berriman et al the chance of failure decreases by a factor of about 3 800 relative to RAID 5 for a proper implementation of RAID 6 even when using commodity drives 86 citation not found Nevertheless if the currently observed technology trends remain unchanged in 2019 a RAID 6 array will have the same chance of failure as its RAID 5 counterpart had in 2010 86 unreliable source Mirroring schemes such as RAID 10 have a bounded recovery time as they require the copy of a single failed drive compared with parity schemes such as RAID 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 86 unreliable source Atomicity Edit 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 This is commonly termed the write hole which is a known data corruption issue in older and low end RAIDs caused by interrupted destaging of writes to disk 87 The write hole can be addressed with write ahead logging This was fixed in mdadm by introducing a dedicated journaling device to avoid performance penalty typically SSDs and NVMs are preferred for that purpose 88 89 This is a little understood and rarely mentioned failure mode for redundant storage systems that do not utilize transactional features Database researcher Jim Gray wrote Update in Place is a Poison Apple during the early days of relational database commercialization 90 Write cache reliability Edit There are concerns about write cache reliability specifically regarding devices equipped with a write back cache which is a caching system that reports the data as written as soon as it is written to cache as opposed to when it is written to the non volatile medium If the system experiences a power loss or other major failure the data may be irrevocably lost from the cache before reaching the non volatile storage For this reason good write back cache implementations include mechanisms such as redundant battery power to preserve cache contents across system failures including power failures and to flush the cache at system restart time 91 See also EditDisk Data Format Network attached storage NAS Non RAID drive architectures Redundant array of independent memory Self Monitoring Analysis and Reporting Technology S M A R T References Edit a b c Patterson David Gibson Garth A Katz Randy 1988 A Case for Redundant Arrays of Inexpensive Disks RAID PDF SIGMOD Conferences Retrieved 2006 12 31 a b Originally referred to as Redundant Array of Inexpensive Disks the term RAID was first published 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 Storage Area Network Fundamentals Meeta Gupta Cisco Press ISBN 978 1 58705 065 7 Appendix A a b Katz Randy H October 2010 RAID A Personal Recollection of How Storage Became a System PDF eecs umich edu IEEE Computer Society Retrieved 2015 01 18 We were not the first to think of the idea of replacing what Patterson described as a slow large expensive disk SLED with an array of inexpensive disks For example the concept of disk mirroring pioneered by Tandem was well known and some storage products had already been constructed around arrays of small disks Hayes Frank November 17 2003 The Story So Far Computerworld Retrieved November 18 2016 Patterson recalled the beginnings of his RAID project in 1987 1988 David A Patterson leads a team that defines RAID standards for improved performance reliability and scalability US patent 4092732 Norman Ken Ouchi System for Recovering Data Stored in Failed Memory Unit issued 1978 05 30 HSC50 70 Hardware Technical Manual PDF DEC July 1986 pp 29 32 Retrieved 2014 01 03 US patent 4761785 Brian E Clark et al Parity Spreading to Enhance Storage Access issued 1988 08 02 US patent 4899342 David Potter et al Method and Apparatus for Operating Multi Unit Array of Memories issued 1990 02 06 See also The Connection Machine 1988 IBM 7030 Data Processing System Reference Manual PDF bitsavers trailing edge com IBM 1960 p 157 Retrieved 2015 01 17 Since a large number of bits are handled in parallel it is practical to use error checking and correction ECC bits and each 39 bit byte is composed of 32 data bits and seven ECC bits The ECC bits accompany all data transferred to or from the high speed disks and on reading are used to correct a single bit error in a byte and detect double and most multiple errors in a byte IBM Stretch aka IBM 7030 Data Processing System brouhaha com 2009 06 18 Retrieved 2015 01 17 A typical IBM 7030 Data Processing System might have been comprised of the following units IBM 353 Disk Storage Unit similar to IBM 1301 Disk File but much faster 2 097 152 2 21 72 bit words 64 data bits and 8 ECC bits 125 000 words per second a b c d e f g h i Chen Peter Lee Edward Gibson Garth Katz Randy Patterson David 1994 RAID High Performance Reliable Secondary Storage ACM Computing Surveys 26 2 145 185 CiteSeerX 10 1 1 41 3889 doi 10 1145 176979 176981 S2CID 207178693 Donald L 2003 MCSA MCSE 2006 JumpStart Computer and Network Basics 2nd ed Glasgow SYBEX Howe Denis ed Redundant Arrays of Independent Disks from FOLDOC Free On line Dictionary of Computing Imperial College Department of Computing Retrieved 2011 11 10 Dawkins Bill and Jones Arnold Common RAID Disk Data Format Specification Archived 2009 08 24 at the Wayback Machine Storage Networking Industry Association Colorado Springs 28 July 2006 Retrieved on 22 February 2011 Adaptec Hybrid RAID Solutions PDF Adaptec com Adaptec 2012 Retrieved 2013 09 07 Common RAID Disk Drive Format DDF standard SNIA org SNIA Retrieved 2012 08 26 SNIA Dictionary SNIA org SNIA Retrieved 2010 08 24 Tanenbaum Andrew S Structured Computer Organization 6th ed p 95 Hennessy John Patterson David 2006 Computer Architecture A Quantitative Approach 4th ed p 362 ISBN 978 0123704900 FreeBSD Handbook Chapter 20 5 GEOM Modular Disk Transformation Framework Retrieved 2012 12 20 White Jay Lueth Chris May 2010 RAID DP NetApp Implementation of Double Parity RAID for Data Protection NetApp Technical Report TR 3298 Retrieved 2013 03 02 a b c Newman Henry 2009 09 17 RAID s Days May Be Numbered EnterpriseStorageForum Retrieved 2010 09 07 Why RAID 6 stops working in 2019 ZDNet 22 February 2010 a b Lowe Scott 2009 11 16 How to protect yourself from RAID related Unrecoverable Read Errors UREs Techrepublic Retrieved 2012 12 01 Vijayan S Selvamani S Vijayan S 1995 Dual Crosshatch Disk Array A Highly Reliable Hybrid RAID Architecture Proceedings of the 1995 International Conference on Parallel Processing Volume 1 CRC Press pp I 146ff ISBN 978 0 8493 2615 8 via Google Books Why is RAID 1 0 better than RAID 0 1 aput net Retrieved 2016 05 23 RAID 10 Vs RAID 01 RAID 1 0 Vs RAID 0 1 Explained with Diagram www thegeekstuff com Retrieved 2016 05 23 Comparing RAID 10 and RAID 01 SMB IT Journal www smbitjournal com Retrieved 2016 05 23 a b Jeffrey B Layton Intro to Nested RAID RAID 01 and RAID 10 Usurped Linux Magazine January 6 2011 Performance Tools amp General Bone Headed Questions tldp org Retrieved 2013 12 25 Main Page Linux raid osdl org 2010 08 20 Archived from the original on 2008 07 05 Retrieved 2010 08 24 Hdfs Raid Hadoopblog blogspot com 2009 08 28 Retrieved 2010 08 24 a b 3 8 Hackers of the Lost RAID OpenBSD Release Songs OpenBSD 2005 11 01 Retrieved 2019 03 23 Long Scott Adaptec Inc 2000 aac 4 Adaptec AdvancedRAID Controller driver BSD Cross Reference FreeBSD aac Adaptec AdvancedRAID Controller driver FreeBSD Manual Pages FreeBSD Raadt Theo de 2005 09 09 RAID management support coming in OpenBSD 3 8 misc Mailing list OpenBSD Murenin Constantine A 2010 05 21 1 1 Motivation 4 Sensor Drivers 7 1 NetBSD envsys sysmon OpenBSD Hardware Sensors Environmental Monitoring and Fan Control MMath thesis University of Waterloo UWSpace hdl 10012 5234 Document ID ab71498b6b1a60ff817b29d56997a418 RAID over File System Retrieved 2014 07 22 ZFS Raidz Performance Capacity and Integrity calomel org Retrieved 26 June 2017 ZFS illumos illumos org 2014 09 15 Archived from the original on 2019 03 15 Retrieved 2016 05 23 Creating and Destroying ZFS Storage Pools Oracle Solaris ZFS Administration Guide Oracle Corporation 2012 04 01 Retrieved 2014 07 27 20 2 The Z File System ZFS freebsd org Archived from the original on 2014 07 03 Retrieved 2014 07 27 Double Parity RAID Z raidz2 Solaris ZFS Administration Guide Oracle Corporation Retrieved 2014 07 27 Triple Parity RAIDZ raidz3 Solaris ZFS Administration Guide Oracle Corporation Retrieved 2014 07 27 Deenadhayalan Veera 2011 General Parallel File System GPFS Native RAID PDF UseNix org IBM Retrieved 2014 09 28 Btrfs Wiki Feature List 2012 11 07 Retrieved 2012 11 16 Btrfs Wiki Changelog 2012 10 01 Retrieved 2012 11 14 Trautman Philip Mostek Jim Scalability and Performance in Modern File Systems linux xfs sgi com Retrieved 2015 08 17 Linux RAID Setup XFS kernel org 2013 10 05 Retrieved 2015 08 17 Hewlett Packard Enterprise HPE Support document HPE Support Center support hpe com Mac OS X How to combine RAID sets in Disk Utility Retrieved 2010 01 04 Apple Mac OS X Server File Systems Retrieved 2008 04 23 FreeBSD System Manager s Manual page for GEOM 8 Retrieved 2009 03 19 freebsd geom mailing list new class geom raid5 6 July 2006 Retrieved 2009 03 19 FreeBSD Kernel Interfaces Manual for CCD 4 Retrieved 2009 03 19 The Software RAID HowTo Retrieved 2008 11 10 mdadm 8 Linux man page Linux Die net Retrieved 2014 11 20 Windows Vista support for large sector hard disk drives Microsoft 2007 05 29 Archived from the original on 2007 07 03 Retrieved 2007 10 08 You cannot select or format a hard disk partition when you try to install Windows Vista Windows 7 or Windows Server 2008 R2 Microsoft 14 September 2011 Archived from the original on 3 March 2011 Retrieved 17 December 2009 Using Windows XP to Make RAID 5 Happen Tom s Hardware 19 November 2004 Retrieved 24 August 2010 Sinofsky Steven Virtualizing storage for scale resiliency and efficiency Microsoft Metzger Perry 1999 05 12 NetBSD 1 4 Release Announcement NetBSD org The NetBSD Foundation Retrieved 2013 01 30 OpenBSD softraid man page OpenBSD org Retrieved 2018 02 03 FreeBSD Handbook Chapter 19 GEOM Modular Disk Transformation Framework Retrieved 2009 03 19 SATA RAID FAQ Ata wiki kernel org 2011 04 08 Retrieved 2012 08 26 Red Hat Enterprise Linux Storage Administrator Guide RAID Types redhat com Russel Charlie Crawford Sharon Edney Andrew 2011 Working with Windows Small Business Server 2011 Essentials O Reilly Media Inc p 90 ISBN 978 0 7356 5670 3 via Google Books Block Warren 19 5 Software RAID Devices freebsd org Retrieved 2014 07 27 Krutz Ronald L Conley James 2007 Wiley Pathways Network Security Fundamentals John Wiley amp Sons p 422 ISBN 978 0 470 10192 6 via Google Books a b c Hardware RAID vs Software RAID Which Implementation is Best for my Application Adaptec Whitepaper PDF adaptec com Smith Gregory 2010 PostgreSQL 9 0 High Performance Packt Publishing Ltd p 31 ISBN 978 1 84951 031 8 via Google Books 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 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 a b c Error Recovery Control with Smartmontools 2009 Archived from the original on September 28 2011 Retrieved September 29 2017 Gray Jim Oct 1990 A census of Tandem system availability between 1985 and 1990 PDF IEEE Transactions on Reliability IEEE 39 4 409 418 doi 10 1109 24 58719 S2CID 2955525 Archived from the original PDF on 2019 02 20 Murphy Brendan Gent Ted 1995 Measuring system and software reliability using an automated data collection process Quality and Reliability Engineering International 11 5 341 353 doi 10 1002 qre 4680110505 Patterson D Hennessy J 2009 574 The RAID Migration Adventure 10 July 2007 Retrieved 2010 03 10 Disk Failures in the Real World What Does an MTTF of 1 000 000 Hours Mean to You Bianca Schroeder and Garth A Gibson Harris Robin 2010 02 27 Does RAID 6 stop working in 2019 StorageMojo com TechnoQWAN Retrieved 2013 12 17 J L Hafner V Dheenadhayalan K Rao and J A Tomlin Matrix methods for lost data reconstruction in erasure codes USENIX Conference on File and Storage Technologies Dec 13 16 2005 Miller Scott Alan 2016 01 05 Understanding RAID Performance at Various Levels Recovery Zone StorageCraft Retrieved 2016 07 22 Kagel Art S March 2 2011 RAID 5 versus RAID 10 or even RAID 3 or RAID 4 miracleas com Archived from the original on November 3 2014 Retrieved October 30 2014 Baker M Shah M Rosenthal D S H Roussopoulos M Maniatis P Giuli T Bungale P April 2006 A fresh look at the reliability of long term digital storage EuroSys2006 221 234 doi 10 1145 1217935 1217957 ISBN 1595933220 S2CID 7655425 Bairavasundaram L N Goodson G R Pasupathy S Schindler J June 12 16 2007 An analysis of latent sector errors in disk drives PDF Proceedings of SIGMETRICS 07 289 300 doi 10 1145 1254882 1254917 ISBN 9781595936394 S2CID 14164251 Patterson D Hennessy J 2009 Computer Organization and Design New York Morgan Kaufmann Publishers pp 604 605 a b c Leventhal Adam 2009 12 01 Triple Parity RAID and Beyond ACM Queue Association of Computing Machinery Retrieved 2012 11 30 Write Hole in RAID5 RAID6 RAID1 and Other Arrays ZAR team Retrieved 15 February 2012 ANNOUNCE mdadm 3 4 A tool for managing md Soft RAID under Linux LWN net lwn net A journal for MD RAID5 LWN net lwn net Jim Gray The Transaction Concept Virtues and Limitations Archived 2008 06 11 at the Wayback Machine Invited Paper VLDB 1981 144 154 Definition of write back cache at SNIA dictionary www snia org External links Edit Wikimedia Commons has media related to Redundant array of independent disks Empirical Measurements of Disk Failure Rates and Error Rates by Jim Gray and Catharine van Ingen December 2005 The Mathematics of RAID 6 by H Peter Anvin Does Fake RAID Offer Any Advantage Over Software RAID Discussion on superuser com BAARF Battle Against Any Raid Five RAID 3 4 and 5 versus RAID 10 A Clean Slate Look at Disk Scrubbing Retrieved from https en wikipedia org w index php title RAID amp oldid 1146991004, wikipedia, wiki, book, books, library,

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