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39-847: [REDACTED] Look up ADG in Wiktionary, the free dictionary. ADG or Adg may refer to: Adg , a village in Iran Symbols and initialisms [ edit ] ADG, Asociación Deportiva Guanacasteca , Costa Rican soccer team A US Navy hull classification symbol: Degaussing ship (ADG) Active Data Guard Acyclic Directed Graph Additional Director General of Police , senior police rank in India Advanced Data Guarding Aerospace , Defense & Government Air-driven generator Airfield Defence Guards Alpha Delta Gamma ,

78-792: A vendor lock-in , and contributing to reliability issues. 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, potentially compromising the stability, reliability and security of their setup, especially when taking the long-term view. 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

117-550: A RAID 6 array will have the same chance of failure as its RAID 5 counterpart had in 2010. 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. A system crash or other interruption of

156-600: 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. 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 10 for enterprise-class drives ( SCSI , FC , SAS or SATA), and less than one bit in 10 for desktop-class drives (IDE/ATA/PATA or SATA). Increasing drive capacities and large RAID 5 instances have led to

195-487: A dedicated RAID controller chip, but simply a standard drive controller chip, or the chipset built-in RAID function, 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. An example

234-462: A downside, such schemes suffer from elevated write penalty—the number of times the storage medium must be accessed during a single write operation. 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. Data scrubbing , as a background process, can be used to detect and recover from UREs, effectively reducing

273-470: A few ways: Write hole 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. There are concerns about write-cache reliability, specifically regarding devices equipped with a write-back cache , which

312-477: 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. Given an array with only one redundant drive (which applies to RAID levels 3, 4 and 5, and to "classic" two-drive RAID 1),

351-438: A national fraternity Amlan Das Gupta , Indian scholar Andegerebinha dialect , an Aboriginal language of Australia (ISO 639-3 code) Art Directors Guild Aubrey de Grey Australian Directors' Guild Autonomous Design Group , an anti-capitalist artist collective Lenawee County Airport (IATA: ADG) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with

390-438: A national fraternity Amlan Das Gupta , Indian scholar Andegerebinha dialect , an Aboriginal language of Australia (ISO 639-3 code) Art Directors Guild Aubrey de Grey Australian Directors' Guild Autonomous Design Group , an anti-capitalist artist collective Lenawee County Airport (IATA: ADG) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with

429-672: A particular Galois field or Reed–Solomon error correction . 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". 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

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468-487: 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. 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. Data scrubbing checks for bad blocks on each storage device in an array, but also uses

507-438: 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. Some commentators have declared that RAID 6

546-503: 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. However, Western Digital enterprise class drives are shipped from

585-403: 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. The write hole can be addressed in

624-411: Is Intel Rapid Storage Technology , implemented on many consumer-level motherboards. Because some minimal hardware support is involved, this implementation is also called "hardware-assisted software RAID", "hybrid model" RAID, or even "fake RAID". 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

663-482: Is RAID 0 (such as in RAID ;1+0 and RAID 5+0), most vendors omit the "+" (yielding RAID 10 and RAID 50, respectively). 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

702-498: 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

741-490: Is also vulnerable to controller failure because it is not always possible to migrate it to a new, different controller without data loss. 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. In practice,

780-584: 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

819-605: 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. The term "RAID"

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858-419: Is only a "band aid" in this respect, because it only kicks the problem a little further down the road. 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. Nevertheless, if the currently observed technology trends remain unchanged, in 2019

897-586: The Storage Networking Industry Association (SNIA) in the Common RAID Disk Drive Format (DDF) standard: In what was originally termed hybrid RAID , 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

936-463: 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. 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

975-416: 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

1014-520: The data is still exposed to operator, software, hardware, and virus destruction. Many studies cite operator fault as a common source of malfunction, such as a server operator replacing the incorrect drive in a faulty RAID, and disabling the system (even temporarily) in the process. 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

1053-563: 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. 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. While RAID may protect against physical drive failure,

1092-422: The following: Industry manufacturers later redefined the RAID acronym to stand for "redundant array of independent disks". 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

1131-489: The following: 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 RAID controllers can be configured through card BIOS or Option ROM before an operating system

1170-615: The 💕 [REDACTED] Look up ADG in Wiktionary, the free dictionary. ADG or Adg may refer to: Adg , a village in Iran Symbols and initialisms [ edit ] ADG, Asociación Deportiva Guanacasteca , Costa Rican soccer team A US Navy hull classification symbol: Degaussing ship (ADG) Active Data Guard Acyclic Directed Graph Additional Director General of Police , senior police rank in India Advanced Data Guarding Aerospace , Defense & Government Air-driven generator Airfield Defence Guards Alpha Delta Gamma ,

1209-458: 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. 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, including

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1248-469: The help of a third-party logical volume manager: Many operating systems provide RAID implementations, including the following: 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

1287-579: 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. 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. Double-protection parity-based schemes, such as RAID 6, attempt to address this issue by providing redundancy that allows double-drive failures; as

1326-648: 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; this approach has subsequently been adopted and extended by NetBSD in 2007 as well. Software RAID implementations are provided by many modern operating systems . Software RAID can be implemented as: Some advanced file systems are designed to organize data across multiple storage devices directly, without needing

1365-690: 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. 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. Western Digital's desktop drives used to have

1404-427: 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. Drive capacity has grown at

1443-443: The title ADG . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=ADG&oldid=1240643582 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages ADG From Misplaced Pages,

1482-450: The title ADG . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=ADG&oldid=1240643582 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Advanced Data Guarding Data

1521-536: Was invented by David Patterson , Garth 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

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