AcharyaChanakya/simple-wikipedia huggingface dataset:
Milwaukee is crossed by Interstate 43 and Interstate 94, which come together downtown at the Marquette Interchange. The Interstate 894 bypass (which as of May 2015 also contains Interstate 41) runs through portions of the city's southwest side, and Interstate 794 comes out of the Marquette interchange eastbound, bends south along the lakefront and crosses the harbor over the Hoan Bridge, then ends near the Bay View neighborhood and becomes the "Lake Parkway" (WIS-794).
One of the distinctive traits of Milwaukee's residential areas are the neighborhoods full of so-called Polish flats. These are two-family homes with separate entrances, but with the units stacked one on top of another instead of side-by-side. This arrangement enables a family of limited means to purchase both a home and a modestly priced rental apartment unit. Since Polish-American immigrants to the area prized land ownership, this solution, which was prominent in their areas of settlement within the city, came to be associated with them.
The tallest building in the city is the U.S. Bank Center.
See also: Climate change in Wisconsin
Milwaukee's location in the Great Lakes Region often has rapidly changing weather, producing a humid continental climate (Köppen "Dfa"), with cold, windy, snowy winters, and warm, humid summers. The warmest month of the year is July, when the 24-hour average is 71.8 °F (22.1 °C), while January is the coldest month, with a 24-hour average of 22.3 °F (−5.4 °C).
Because of Milwaukee's proximity to Lake Michigan, a convection current forms around mid-afternoon in light wind, resulting in the so-called "lake breeze" – a smaller scale version of the more common sea breeze. The lake breeze is most common between the months of March and July. This onshore flow causes cooler temperatures to move inland usually 5 to 15 miles (8 to 24 km), with much warmer conditions persisting further inland. Because Milwaukee's official climate site, Milwaukee Mitchell International Airport, is only 3 miles (4.8 km) from the lake, seasonal temperature variations are less extreme than in many other locations of the Milwaukee metropolitan area.
As the sun sets, the convection current reverses and an offshore flow ensues causing a land breeze. After a land breeze develops, warmer temperatures flow east toward the lakeshore, sometimes causing high temperatures during the late evening. The lake breeze is not a daily occurrence and will not usually form if a southwest, west, or northwest wind generally exceeds 15 mph (24 km/h). The lake moderates cold air outbreaks along the lakeshore during winter months.
Aside from the lake's influence, overnight lows in downtown Milwaukee year-round are often much warmer than suburban locations because of the urban heat island effect. Onshore winds elevate daytime relative humidity levels in Milwaukee as compared to inland locations nearby.
Thunderstorms in the region can be dangerous and damaging, bringing hail and high winds. In rare instances, they can bring a tornado. However, almost all summer rainfall in the city is brought by these storms. In spring and fall, longer events of prolonged, lighter rain bring most of the precipitation. A moderate snow cover can be seen on or linger for many winter days, but even during meteorological winter, on average, over 40% of days see less than 1 inch (2.5 cm) on the ground.
Milwaukee tends to experience highs that are 90 °F (32 °C) on or above seven days per year, and lows at or below 0 °F (−18 °C) on six to seven nights. Extremes range from 105 °F (41 °C) set on July 24, 1934 down to −26 °F (−32 °C) on both January 17, 1982 and February 4, 1996. The 1982 event, also known as Cold Sunday, featured temperatures as low as −40 °F (−40 °C) in some of the suburbs as little as 10 miles (16 km) to the north of Milwaukee.
Diogenes syndrome
Diogenes syndrome is a behavioral disorder. It is not a disease, it is a psychological condition. People who have it do not look after themselves enough any more, this is called self-neglect. They do not wash, perhaps do not shave, etc. So they develop a strong odor (scent). Since they neglect their hygiene, other diseases related to poor hygiene usually accompany the disorder.
Very often this disorder affects old people who live alone.
Diogenes Syndrome is named after the behavior (but not necessarily the ideals) of Diogenes of Sinope. It is thought to be the end stage of personality disorder.
Marx and Engels Internet Archive
The Marxists Internet Archive (also known as MIA or Marxists.org) is a volunteer-based non-profit organization that has an Internet archive of Marxist writers and other similar authors, socialists, and others on their website.
The texts are available in many different languages.
The North Atlantic Treaty Organization (NATO), or North Atlantic Alliance, the Atlantic Alliance, the Western Alliance, is a military alliance. It was established by the North Atlantic Treaty in 1949 and it was signed in Washington, DC, United States, on April 4, 1949. Its headquarters are in Brussels, Belgium. Its other official name means the same in French: "Organisation du traité de l'Atlantique nord" ("OTAN").
NATO has two official languages, English and French, as defined in Article 14 of the North Atlantic Treaty.
Its members in 1949 were the United States, Belgium, the Netherlands, Luxembourg, France, the United Kingdom, Canada, Portugal, Italy, Norway, Denmark and Iceland. On 18 February 1952, Greece and Turkey also joined.
When West Germany joined the organization on 9 May 1955 it was described as "a decisive turning point in the history of our continent" by Halvard Lange, Foreign Minister of Norway at the time., the result was the Warsaw Pact, signed on 14 May 1955 by the Soviet Union and its satellite states as response to NATO.
After the Cold War in 1999 three former communist countries, Hungary, the Czech Republic, and Poland joined NATO. On 29 March 2004 seven more Northern European and Eastern European countries joined NATO: Estonia, Latvia and Lithuania and also Slovenia, Slovakia, Bulgaria, and Romania.
Croatia and Albania received NATO membership invitation on 3 April 2008. The Republic of Macedonia received only conditional invitation because it was vetoed by Greece because of its name dispute with Greece.
Montenegro joined on 5 June 2017. Having changed its name to end the dispute,
North Macedonia joined NATO on 27 March 2020 and became its 30th member.
Ireland officially joined NATO on 8 September 2020 as an observer.
Felidae is a family of mammals, in the order Carnivora. Animals that belong to Felidae are called "felids".
The family is of quite recent origin: the first fossils are from the Oligocene, 25 million years ago (mya). There are 41 living species, including the domestic cat, and they are monophyletic: all descended from the same ancestor.
Felids tend to have lithe and flexible bodies with muscular limbs. In most species, the tail is a third or a half the length of the body, with some exceptions (the bobcat and margay). The limbs are digitigrade, meaning they walk on their toes. The paws have three soft toe pads and retractible claws (usually). The tongue of felids is covered with horny papillae, which rasp meat from prey and aid in grooming.
The Felidae has three subfamilies:
More examples of felines are the big cats – the lion, tiger, leopard, jaguar, snow leopard, clouded leopard, puma, and cheetah - and other wild cats, for example the lynx, caracal, and bobcat. All felines, also the small pet cat, are predators that can hunt many animals.
The felids evolved in Asia and spread across continents by crossing land bridges. DNA sequence analysis shows that ancient cats evolved into eight main lines. There were at least 10 migrations (in both directions) from continent to continent via the Bering land bridge and Isthmus of Panama. The "Panthera" genus is the oldest and the "Felis" genus is the youngest. 60 percent of the modern species of cats developed within the last million years.
The felids' closest relatives are the linsangs, and at one remove the group of civets, hyenas, mongooses, and Madagascar carnivores, with whom they share the Suborder Feliformia. All felid species share a genetic anomaly that prevents them from tasting sweetness.
Genetic research has provided a basis for a more concise classification for the living members of the cat family based on genotypical groupings. Specifically, eight genetic lineages have been identified:
The last four lineages (5, 6, 7, 8) are more related to each other than to any of the first four (1, 2, 3, 4), and so form a clade within the Felinae subfamily of family Felidae.
1668
Andrei Dmitrievich Sakharov (Russian: Андре́й Дми́триевич Са́харов, May 21 1921 – December 14 1989), was a Soviet nuclear physicist. He was also a well-known dissident and human rights activist. Sakharov was an advocate of nuclear disarmament and civil liberties and reforms in the Soviet Union.
Sakharov was a very clever scientist. He graduated from college during World War II but was kept out of the Red Army to do scientific research for the government. Sakharov helped the Soviets develop the hydrogen bomb in the 1950s. But he also made many peaceful uses of nuclear power.
The bomb he had helped make started Sakharov to be a dissident. He argued with Nikita Khrushchev against unneeded nuclear tests. He was afraid that these tests put people's lives and healths in danger without need. He began asking, too, for more freedom of speech. The government removed him from his secret work and his special job as a scientist. Sakharov began to see what unhappy lives most Soviet citizens led.
Persecution only made him speak out more. He asked for more freedom of religion and defended others who had spoken out and been put into jail. The secret police began to watch him and to make files of information against him. After Sakharov criticized activities of the USSR's Arab allies, Arab terrorists came to his apartment and held him, his wife, and his stepson hostage for an hour. "Do you want to kill us?" cried Sakharov's wife. "We can do worse things than kill you," they replied. After the terrorists left without doing anything, Sakharov tried to tell the police about it. The police did not really care.
The only thing that protected Sakharov was many people knew about him outside the Soviet Union. The Communists were afraid that if something happened to him, it would give the world a bad image of the USSR. But when Sakharov criticized the Soviet invasion of Afghanistan in 1979, they became very angry and took him to the city of Gorky. There he was watched by the police outside his apartment. He tried to go on a hunger strike, not eating anything. The Soviet Union was afraid he might die, so they force-fed him (forcing him to eat).
Sakharov kept on asking for "glasnost" (openness). In 1986, Mikhail Gorbachev, who agreed with Sakharov and thought there should be "glasnost", let the dissident scientist return home. Sakharov continued to call for reform, and in December 1989, he died of heart failure, at the age of 68. . But two years later his wish became true with the dissolution of the Soviet Union. .
1686
Year 1686 was a common year that started on a Tuesday when using the Gregorian calendar.
1st century
The 1st century was the century that lasted from year 1 to 100.
During this period Europe, North Africa and the Near East fell under increasing domination by the Roman Empire. It continued expanding under the emperor Claudius (43). The reforms introduced by Augustus during his long reign stabilized the empire. Later in the century the Julio-Claudian Dynasty, which had been founded by Augustus came to an end with the death of Nero in 68. There followed the famous Year of Four Emperors, a brief period of civil war and instability, which was finally brought to an end by Vespasian, 9th Roman emperor, and founder of the Flavian Dynasty.
China continued to be dominated by the Han Dynasty, despite a 14-year interruption by the Xin dynasty under Wang Mang. Han rule was restored in 23. Wang Mang's rule represents the watershed between the Western/Former Han and the Eastern/Later Han. The capital was also moved from Chang'an to Luoyang.
According to the New Testament, during the reign of Tiberius, Jesus, a Jewish religious leader from Galilee, was crucified in Jerusalem on the charge of blasphemy for claiming to be the Son of God. But "God raised him from the dead" three days later, see Resurrection of Jesus. Over the next few decades his followers, following the Great Commission, including the apostle Paul, carried his message throughout the Greek-speaking regions of Asia Minor, eventually introducing it to Rome itself. Roman rulers began to persecute the new sect almost immediately (the emperor Nero accused the Christians of starting the fires that destroyed much of Rome in 64 AD), and would continue to do so for centuries, sometimes vigorously, and other times passively. Christian tradition records that all of Christ's apostles except John the Evangelist suffered martyrdom.
In the 4th century, Christianity was eventually taken up by the emperor Constantine. One of his successors Julian the Apostate renounced it for paganism and again persecuted the Church. However, by the end of the 4th century, Emperor Theodosius I proclaimed Christianity as the state religion of the Roman Empire.
Note: years before or after the 1st century are in "italics".
The Equidae is a family of odd-toed ungulate mammals. The family has only one surviving genus: "Equus". Horses, donkeys and zebras belong in this genus.
Animals of the "Equus" genus can have hybrid offspring, but they are usually sterile.
They can have many different colours.
The Lynx is a small cat. It is a genus with four species. The four species are in Eurasia, Spain, Canada and America, and do not overlap much.
"Lynx" have short tails, and usually some hair on the ears. They have large paws (feet) padded for walking on snow, and long whiskers on the face. The color of the body is from light brown to grey and is sometimes marked with dark brown spots, especially on the legs. They weigh about 5 kg or about 11 pounds (approximately the size of a large domestic cat) and can weigh up to about 30 kg (66 pounds). They are between 70 and 150 cm long, with a relative short tail - 5-25 cm. They live for about 20 years.
"Lynx" are similar to a regular house cat, but larger. They have a powerful body on short, furry legs attached to hefty feet, and a bobbed, black-tipped tail. The fur is spotted and yellowish-brown to grey. It has a collar of fur around the face, giving a triangular shape. They also have long black ear tufts.
"Lynx" live in the high altitude forests with many shrubs, reeds and grass. They hunt only on the ground, but can climb trees and swim.
They can be found in the some places in northern Scandinavia, in North America and also in the Himalayas. Since the 1990s people try to move part of the population of the Eurasian lynx to Germany. They can also be found in Białowieża Forest (northeastern Poland), and southern Spain.
The Colorado Division of Wildlife tried to move other wild "Lynx" populations from Canada back to the United States.
"Lynx" are usually solitary, although a small group may travel and hunt together occasionally. Mating takes place in the late winter and once a year the female gives birth to between two and four kittens. The young stay with the mother for one more winter, a total of around nine months, before moving out to live on their own as young adults.
"Lynx" make their dens in crevices or under ledges. They feed on a wide range of animals from white-tailed deer, reindeer, roe deer, small red deer, and chamois, to smaller, more usual prey: snowshoe hares, fish, foxes, sheep, squirrels, mice, turkeys and other birds, and goats. They also eat ptarmigans,voles, and grouse.
The Iberian lynx is the most endangered feline in the world. There are only two populations (together, 300 animals), in the southern Spain.
Hunting lynxes is illegal in many countries.
Shania Twain (born Eilleen Regina Edwards; August 28, 1965, in Timmins, Ontario, Canada) is a Canadian singer-songwriter and record producer. She was married a long time to music producer Mutt Lange. She is very successful in the country and pop music genres. She had a residency show at Caesars Palace in Las Vegas, Nevada, in late 2012.
Her 1995 album "The Woman in Me" brought her fame. Her 1997 album "Come On Over" became the bestselling album of all time by a female musician in any genre and the bestselling country album of all time. It has sold more than 40 million copies worldwide.
In 2017, she released the album "Now", her first in 15 years. She wrote and co-produced all of the songs. It entered at the top of the charts in Australia, Canada, the UK, and the US, where it opened with 137,000 combined copies and streams.
Twain married music producer Robert John "Mutt" Lange on December 28, 1993. They have a son, Eja (pronounced "Asia") D'Angelo, who was born on August 12, 2001. On May 15, 2008, a spokesperson for Mercury Nashville said that Twain and Lange were separating. This was after Lange allegedly had an affair with Twain's best friend, Marie-Anne Thiébaud. Their divorce was finalized on June 9, 2010. On December 20, 2010, Twain's manager said that Twain was engaged to Swiss Frédéric Thiébaud (the ex-husband of her former best friend), an executive at Nestlé. They married on January 1, 2011, in Rincón, Puerto Rico.
Twain is a longtime vegetarian. In 2001 she was voted PETA's very first Sexiest Vegetarian Alive.
In September 2010, it was confirmed that Twain was to release her first biography, "From This Moment On", on May 3, 2011. The cover work for the book was released on March 2, 2011.
In addition to the awards for her singles and albums, Twain has a number of personal honours:
Twain has released thirty-five singles from four studio albums, one greatest hits album, and one soundtrack album.
The Grass Roots
Dionne Warwick
Dionne Warwick (born December 12, 1940) is an American R&B singer. She was born in East Orange, New Jersey.
Warwick sang many songs written by Burt Bacharach and Hal David. Many of these songs were hits.
In 1990, Warwick hosted and executive produced a talk show, "Dionne and Friends". She was a contestant in the 2011 season of "The Apprentice".
Warwick's sister Dee Dee Warwick (1942-2008), their aunt Cissy Houston (born 1933) and cousin Whitney Houston (1963-2012) were singers.
Barenaked Ladies are a Canadian band. They formed in Toronto, Canada in 1988. The band is made up of four men. In 1991, the band was taken off the program for a New Year's Eve concert in Toronto because someone thought the name was bad. The band started as a duo with Ed Roberstson and Steven Page. Lead singer Page left the band in 2009 to begin a solo career. He was also arrested in 2008 for having the illegal drug cocaine.
The current band members are:
Ed Robertson is also the host of a television programme, called "Ed's Up". On the programme, he travels around Canada by airplane. A plane he was flying crashed into trees in 2008. Roberston and the other passengers were not hurt.
The theme song to the television series "The Big Bang Theory" is performed by Barenaked Ladies.
The band have made 14 albums:
Paul Revere &amp; the Raiders
Paul Revere & the Raiders were an American pop group who were popular in the 1960s. They started in Portland, Oregon in 1960.
There are four versions of the game each with different breeds.
They are: "Dalmatian and Friends", "Chihuahua and Friends", "Labrador and Friends" and "Dachshund and Friends". All breeds are unlockable in every version and a player can unlock dogs by increasing his trainer points. However, the player can only have three dogs at home. To get more the player must drop off one at the Dog Hotel. Every player can only drop off five dogs at a time.
If the player does not play the game for a long period of time, the dog may run away. So players need to play with their dogs daily.
A sequel, "Nintendogs + Cats", was released for the Nintendo 3DS in 2011.
Luigi is a video game character made by Nintendo. The famous game designer, Shigeru Miyamoto, made him. Luigi is the little bit younger but taller fraternal twin brother of the Nintendo mascot, Mario. He is in many games all over the "Mario" series, almost always as an assistant to his brother Mario.
The first game Luigi was in was "Mario Bros. (Game and Watch)" (1983) as a character seen working in a factory. He later appeared playable in the 1983 arcade game "Mario Bros" and was played by the second character. It was also done in "Super Mario Bros.", "Super Mario Bros. 3", "Super Mario World", and other games. The first game where he was present as the first character was "Super Mario Bros. 2". In the Mario series, he is usually not the main character. However, he has his own games where is the main character, like Luigi's Mansion and Mario is Missing!. He also appears in many spin-offs, such as Mario Kart, Mario Party and the Super Smash Bros. series. Luigi has also been in every show of the three DiC TV series based on the Nintendo Entertainment System and Super Nintendo Entertainment System games.
At first he was made as a palette swap of Mario with a green color instead of red. Later in the series, he became his own character with his own personality and abilities. As his part in the "Mario" series progressed, Luigi became taller and thinner than his brother. Nintendo called 2013 "The Year of Luigi" to mark the thirty years of the existence of the character. Similarly, games being released in 2013 bring attention to Luigi, such as , "", "Dr. Luigi", and the level pack "New Super Luigi U" for "New Super Mario Bros. U". In "Super Mario 3D World" a Luigi version of "Mario Bros." called "Luigi Bros." can be unlocked.
The events leading to the creation of Luigi began in 1982, during the making of "Donkey Kong". The Nintendo designer Shigeru Miyamoto had created Mario (who used to be "Jumpman") hoping that he would be able to present the character in many different pars in future games. Miyamoto had been given ideas by the game "Joust" to create a game where two players can play at the same time. This was the cause of his development of the game "Mario Bros." in 1983, with Luigi as the brother of Mario and the second playable character. The name Luigi is said to have been inspired by a pizza business near the head office of Nintendo of America in Redmond, Washington, called "Mario & Luigi's". Miyamoto noticed that the word "ruiji" means "similar" in the Japanese language, and that Luigi was designed to have the same size, shape and gameplay of Mario.
At first in "Donkey Kong", Mario was a carpenter. In "Mario Bros.", Mario and Luigi were designed as Italian plumbers by Miyamoto, on the suggestion of a partner. Software limits at the time—almost the same as those that gave Mario his special look in "Donkey Kong"—meant the first appearance of Luigi was limited to a simple palette swap of Mario designed to be the second player. The characters were in every way the same, other than their colors. The green color for Luigi would last as one of his physical characteristics in future releases.
After the success of "Mario Bros.", Luigi was presented to a wider market in 1985 with the release of the console game "Super Mario Bros. "Once again his part was limited to a palette swap of Mario, acting as the second-player in almost the same way as "Mario Bros. "The Japan-only form of "Super Mario Bros. 2" in 1986 (released in the west as " "in the future) marked the start of the development of Luigi becoming a separate character. As with his last appearances, Luigi was still a palette swap of Mario. However, his moves were not the same. Luigi could now jump high up and a great distance more than his brother, at the cost of motion reaction and exactness.
While this form of "Super Mario Bros. 2" was released in Japan, it was thought to be too hard for the American public at the time. In 1988, as a result, an alternative release was developed to be good enough as "Super Mario Bros. 2" for western players (and in the future released in Japan as "Super Mario USA"). This would play a key part in forming the current look of Luigi. The game was a conversion of "", with the art changed to be characters and places from the "Mario" series. In this release, the character of "Mama", who could jump the highest, was good enough for the model for Luigi. This caused his taller, thinner look, combined with his Mario-design outfit and green color. There were appearances of Luigi being taller than Mario before - for example, in the 1988 Famicom Disk System game "" and before that, in a very limited 1986 anime "" (though in the anime he had a yellow shirt and blue overalls). Licensed art for "Super Mario Bros. 3" and "Super Mario World "presents Luigi with this new look. Nintendo of Japan would not change his art differences to his look within the game until the 1992 game "Super Mario Kart". The appearance of Luigi from the "Yume Kōjō: Doki Doki Panic" version of "Super Mario Bros. 2" has been used ever since, even for games made again in which he was a palette swap to start with.
Much like his appearance, Luigi's has changed over the years. In "Mario Kart 64", which voiced many characters for the first time, some characters had two different voices in relation to the region of the game. North American and European forms present a deep voice for Luigi, by Charles Martinet, who also voices Mario, Wario, Waluigi, and Toadsworth. The Japanese form uses a high, falsetto voice, by (then French translator at Nintendo) Julien Bardakoff. All forms of "Mario Party" present the high-pitched clips of Bardakoff from "Mario Kart 64". Luigi kept this higher voice in "Mario Party 2". In "Mario Golf", "Mario Tennis", and "Mario Party 3" his voice returned to normal. Since then, in all games but "" and "Super Smash Bros. Melee", Luigi has always had a medium-pitched voice. In "", Luigi's voice was the same high voice from the Japanese "Mario Kart 64". In "Super Smash Bros." and "Super Smash Bros. Melee", Luigi's voice is based on Mario's voice (with clips taken from "Super Mario 64"), with a higher pitch. In "Super Smash Bros. Brawl", he has his own voice clips instead of voice clips taken from Mario.
Playable only in two-player:
Playable in one-player or two-player:
RAID is an acronym that stands for Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks. RAID is a term used in computing. With RAID, several hard disks are made into one logical disk. There are different ways this can be done. Each of the methods that puts the hard disks together has some benefits and drawbacks over using the drives as single disks, independent of each other. The main reasons why RAID is used are:
It is not possible to achieve all of these goals at the same time, so choices need to be made.
There are also some bad things:
Most of the work on RAID is based on a paper written in 1988.
Companies have used RAID systems to store their data since the technology was made. There are different ways in which RAID systems can be made. Since its discovery, the cost of building a RAID system has come down a lot. For this reason, even some computers and appliances that are used at home have some RAID functions. Such systems can be used to store music or movies, for example.
A hard disk is a part of a computer. Normal hard disks use magnetism to store information. When hard disks are used, they are available to the operating system. In Microsoft Windows, each hard disk will get a drive letter (starting with C:, A: or B: are reserved for floppy drives). Unix and Linux-like operating systems have a single-rooted directory tree. This means that people who use the computers sometimes do not know where the information is stored.(To be fair, a lot of Windows users don't know where their data is stored as well.)
In computing, the hard disks (which are hardware, and can be touched) are sometimes called "physical drives" or "physical disks". What the operating system shows the user is sometimes called "logical disk". A physical drive can be split into different sections, called disk partitions. Usually, each disk partition contains one file system. The operating system will show each partition like a logical disk.
Therefore, to the user, both the setup with many physical disks and the setup with many logical disks will look the same. The user cannot decide if a "logical disk" is the same as a physical disk, or if it simply is a part of the disk. Storage Area Networks (SANs) completely change this view. All that is visible of a SAN is a number of logical disks.
In the computer, data is organised in the form of bits and bytes. In most systems, 8 bits make up a byte. Computer memory uses electricity to store the data, hard disks use magnetism. Therefore, when data is "written" on a disk, the electric signal is converted into a magnetic one. When data is read from disk, the conversion is done in the other direction: An electrical signal is made from the polarity of a magnetic field.
A RAID array joins two or more hard disks so that they make a logical disk. There are different reasons why this is done. The most common ones are:
RAID is done by using special hardware or software on the computer. The joined hard disks will then look like one hard disk to the user. Most RAID levels increase the redundancy. This means that they store the data more often, or they store information on how to reconstruct the data. This allows for a number of disks to fail without the data being lost. When the failed disk is replaced, the data it should contain will be copied or rebuilt from the other disks of the system. This can take a long time. The time it takes depends on different factors, like the size of the array.
One of the reasons why many companies are using RAID is that the data in the array can simply be used. Those using the data need not be aware they are using RAID at all. When a failure occurred and the array is recovering, access to the data will be slower. Accessing the data during this time will also slow down the recovery process, but this is still much faster than not being able to work with the data at all.
Depending on the RAID level however, disks may not fail while the new disk is being prepared for use. A disk failing at that time will result in losing all the data in the array.
The different ways to join disks are called "RAID levels". A bigger number for the level is not necessarily better. Different RAID levels have different purposes. Some RAID levels need special disks and special controllers.
In 1978, a man called Norman Ken Ouchi, who worked at IBM, made a suggestion describing the plans for what would later become RAID 5. The plans also described something similar to RAID 1, as well as the protection of a part of RAID 4.
Workers at the University of Berkeley helped to plan out research in 1987. They were trying to make it possible for RAID technology to recognize two hard drives instead of one. They found that when RAID technology had two hard drives, it had much better storage than with only one hard drive. However, it crashed much more often.
In 1988, the different types of RAID (1 to 5), were written about by David Patterson, Garth Gibson and Randy Katz in their article, called "A Case for Redundant Arrays of Inexpensive Disks (RAID)". This article was the first to call the new technology RAID and the name became official.
RAID uses a few basic ideas, which were described in the article "RAID: High-Performance, Reliable Secondary Storage" by Peter Chen and others, published in 1994.
Caching is a technology that also has its uses in RAID systems. There are different kinds of caches that are used in RAID systems:
In modern systems, a write request is shown as done when the data has been written to the cache. This does not mean that the data has been written to the disk. Requests from the cache are not necessarily handled in the same order that they were written to the cache. This makes it possible that, if the system fails, sometimes some data has not been written to the disk involved. For this reason, many systems have a cache that is backed by a battery.
When talking about a mirror, this is a very simple idea. Instead of the data being in only one place, there are several copies of the data. These copies usually are on different hard disks (or disk partitions). If there are two copies, one of them can fail without the data being affected (as it still is on the other copy). Mirroring can also give a boost when reading data. It will always be taken from the fastest disk that responds. Writing data is slower though, because all disks need to be updated.
With striping, the data is split into different parts. These parts then end up on different disks (or disk partitions). This means that writing data is faster, as it can be done in parallel. This does not mean that there will not be faults, as each block of data is only found on one disk.
It is possible to calculate different kinds of checksums. Some methods of calculating checksums allow finding a mistake. Most RAID levels that use redundancy can do this. Some methods are more difficult to do, but they allow to not only detect the error, but to fix it.
Many of the ways to have RAID support something is called a "hot spare". A hot spare is an empty disk that is not used in normal operation. When a disk fails, data can directly be copied onto the hot spare disk. That way, the failed disk needs to be replaced by a new empty drive to become the hot spare.
RAID works by spreading the data over several disks. Two of the terms often used in this context are "stripe size" and "chunk size".
The "chunk size" is the smallest data block that is written to a single disk of the array. The "stripe size" is the size of a block of data that will be spread over all disks. That way, with four disks, and a stripe size of 64 kilobytes (kB), 16 kB will be written to each disk. The chunk size in this example is therefore 16 kB. Making the stripe size bigger will mean a faster data transfer rate, but also a bigger maximum latency. In this case, this is the time needed to get a block of data.
Many controllers (and also software) can put disks together in the following way: Take the first disk, till it ends, then they take the second, and so on. In that way, several smaller disks look like a larger one. This is not really RAID, as there is no redundancy. Also, spanning can combine disks where RAID 0 cannot do anything. Generally, this is called "just a bunch of disks" (JBOD).
This is like a distant relative of RAID because the logical drive is made of different physical drives. Concatenation is sometimes used to turn several small drives into one larger useful drive. This can not be done with RAID 0. For example, JBOD could combine 3 GB, 15 GB, 5.5 GB, and 12 GB drives into a logical drive at 35.5 GB, which is often more useful than the drives alone.
In the diagram to the right, data are concatenated from the end of disk 0 (block A63) to the beginning of disk 1 (block A64); end of disk 1 (block A91) to the beginning of disk 2 (block A92).
If RAID 0 were used, then disk 0 and disk 2 would be truncated to 28 blocks, the size of the smallest disk in the array (disk 1) for a total size of 84 blocks.
Some RAID controllers use JBOD to talk about working on drives without RAID features. Each drive shows up separately in the operating system. This JBOD is not the same as concatenation.
Many Linux systems use the terms "linear mode" or "append mode". The Mac OS X 10.4 implementation — called a "Concatenated Disk Set" — does not leave the user with any usable data on the remaining drives if one drive fails in a concatenated disk set, although the disks otherwise operate as described above.
Concatenation is one of the uses of the Logical Volume Manager in Linux. It can be used to create virtual drives.
Most modern hard disks have a standard called Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T). SMART allows to monitor certain things on a hard disk drive. Certain controllers allow to replace a single hard disk even before it fails, for example because S.M.A.R.T or another disk test reports too many correctable errors. To do this, the controller will copy all the data onto a hot spare drive. After this, the disk can be replaced by another (which will simply become the new hot spare).
The setup of the disks and how they use the techniques above affects the performance and reliability of the system. When more disks are used, one of the disks is more likely to fail. Because of this, mechanisms have to be built to be able to find and fix errors. This makes the whole system more reliable, as it is able to survive and repair the failure.
RAID 0 is not really RAID because it is not redundant. With RAID 0, disks are simply put together to make a large disk. This is called "striping". When one disk fails, the whole array fails. Therefore, RAID 0 is rarely used for important data, but reading and writing data from the disk can be faster with striping because each disk reads part of the file at the same time.
With RAID 0, disk blocks that come after one another are usually placed on different disks. For this reason, all disks used by a RAID 0 should be the same size.
RAID 0 is often used for Swapspace on Linux or Unix-like operating systems.
With RAID 1, two disks are put together. Both hold the same data, one is "mirroring" the other. This is easy, fast configuration whether implemented with a hardware controller or by software.
RAID Level 5 is what is probably used most of the time. At least three hard disks are needed to build a RAID 5 storage array. Each block of data will be stored in three different places. Two of these places will store the block as it is, the third will store a checksum. This checksum is a special case of a Reed-Solomon code that only uses bitwise addition. Usually, it is calculated using the XOR method. Since this method is symmetric, one lost data block can be rebuilt from the other data block and the checksum. For each block, a different disk will hold the parity block which holds the checksum. This is done to increase redundancy. Any disk can fail. Overall, there will be one disk holding the checksums, so the total usable capacity will be that of all disks except for one. The size of the resulting logical disk will be the size of all disks together, except for one disk which holds parity information.
Of course this is slower than RAID level 1, since on every write, all disks need to be read to calculate and update the parity information. The read performance of RAID 5 is almost as good as RAID 0 for the same number of disks. Except for the parity blocks, the distribution of data over the drives follows the same pattern as RAID 0. The reason RAID 5 is slightly slower is that the disks must skip over the parity blocks.
A RAID 5 with a failed disk will continue to work. It is in "degraded mode". A degraded RAID 5 can be very slow. For this reason an additional disk is often added. This is called "hot spare" disk. If a disk fails, the data can be directly rebuilt onto the extra disk. RAID 5 can also be done in software quite easily.
Mainly because of performance problems of failed RAID 5 arrays, some database experts have formed a group called BAARF—the "Battle Against Any Raid Five".
If the system fails while there are active writes, the parity of a stripe may become inconsistent with the data. If this is not repaired before a disk or block fails, data loss may occur. An incorrect parity will be used to reconstruct the missing block in that stripe. This problem is sometimes known as the "write hole". Battery-backed caches and similar techniques are commonly used to reduce the chance for this to occur.
This was used with very large computers. Special expensive disks and a special controller are needed to use RAID Level 2. The data is distributed at the bit-level (all other levels use byte-level actions). Special calculations are done. Data is split up into static sequences of bits. 8 data bits and 2 parity bits are put together. Then a Hamming code is calculated. The fragments of the Hamming code are then distributed over the different disks.
RAID 2 is the only RAID level that can "repair" errors, the other RAID levels can only detect them. When they find that the information needed does not make sense, they will simply rebuild it. This is done with calculations, using information on the other disks. If that information is missing or wrong, they cannot do much. Because it uses Hamming codes, RAID 2 can find out which piece of the information is wrong, and correct only that piece.
RAID 2 needs at least 10 disks to work. Because of its complexity and its need for very expensive and special hardware, RAID 2 is no longer used very much.
Raid Level 3 is much like RAID Level 0. An additional disk is added to store parity information. This is done by bitwise addition of the value of a block on the other disks. The parity information is stored on a separate (dedicated) disk. This is not good, because if the parity disk crashes, the parity information is lost.
RAID Level 3 is usually done with at least 3 disks. A two-disk setup is identical to a RAID Level 0.
This is very similar to RAID 3, except that the parity information is calculated over larger blocks, and not single bytes. This is like RAID 5. At least three disks are needed for a RAID 4 array.
RAID level 6 was not an original RAID level. It adds an additional parity block to a RAID 5 array. It needs at least four disks (two disks for the capacity, two disks for redundancy). RAID 5 can be seen as a special case of a Reed-Solomon code. RAID 5 is a special case, though, it only needs addition in the Galois field "GF(2)". This is easy to do with XORs. RAID 6 extends these calculations. It is no longer a special case, and all of the calculations need to be done. With RAID 6, an extra checksum (called polynomial) is used, usually of "GF (2)". With this approach it is possible to protect against any number of failed disks. RAID 6 is for the case of using two checksums to protect against the loss of two disks.
Like with RAID 5, parity and data are on different disks for each block. The two parity blocks are also located on different disks.
There are different ways to do RAID 6. They are different in their write performance, and in how much calculations are needed. Being able to do faster writes usually means more calculations are needed.
RAID 6 is slower than RAID 5, but it allows the RAID to continue with any two disks failed. RAID 6 is becoming popular because it allows an array to be rebuilt after a single-drive failure even if one of the remaining disks has one or more bad sectors.
RAID 6 uses two parity blocks. These are calculated in a special way over a polynomial. "Double parity" RAID (also called "diagonal parity" RAID) uses a different polynomial for each of these parity blocks. Recently, the industry association that defined RAID said that double parity RAID is a different form of RAID 6.
RAID-DP is another way of having double parity.
RAID 1.5 (not to be confused with RAID 15, which is different) is a proprietary RAID implementation. Like RAID 1, it only uses two disks, but it does both striping and mirroring (similar to RAID 10). Most things are done in hardware.
RAID 5E, RAID 5EE and RAID 6E (with the added "E" for "Enhanced") generally refer to different types of RAID 5 or RAID 6 with a hot spare. With these implementations, the hot spare drive is not a physical drive. Rather, it exists in the form of free space on the disks. This increases performance, but it means that a hot spare cannot be shared between different arrays. The scheme was introduced by IBM ServeRAID around 2001.
This is a proprietary implementation. It adds caching to a RAID 3 or RAID 4 array.
Some Intel main boards have RAID chip that have this feature. It uses two or three disks, and then partitions them equally to form a combination of RAID 0, RAID 1, RAID 5 or RAID 1+0 levels.
This is the name for the driver that allows to do software RAID with Linux. In addition to the normal RAID levels 0-6, it also has a RAID 10 implementation. Since Kernel 2.6.9, RAID 10 is a single level. The implementation has some non-standard features.
Sun has implemented a file system called ZFS. This file system is optimised for handling large amounts of data. It includes a Logical Volume Manager. It also includes a feature called RAID-Z. It avoids the problem called "RAID 5 write hole" because it has a copy-on-write policy: It does not overwrite the data directly, but writes new data in a new location on the disk. When the write was successful, the old data is deleted. It avoids the need for read-modify-write operations for small writes, because it only writes full-stripes. Small blocks are mirrored instead of parity protected, which is possible because the file system knows the way the storage is organised. It can therefore allocate extra space if necessary. There is also "RAID-Z2" which uses two forms of parity to achieve results similar to RAID 6: the ability to survive up to two drive failures without losing data.
With RAID different disks can be put together to get a logical disk.The user will only see the logical disk. Each one of the RAID levels mentioned above has good and bad points. But RAID can also work with logical disks. That way one of the RAID levels above can be used with a set of logical disks. Many people note it by writing the numbers together. Sometimes, they write a '+' or an '&' in between. Common combinations (using two levels) are the following:
With six disks of 300 GB each, a total capacity of 1.8TB, it is possible to make a RAID 5, with 1.5 TB usable space. In that array, one disk can fail without data loss. With RAID 50, the space is reduced to 1.2 TB, but one disk of each RAID 5 can fail, plus there is a noticeable increase in performance. RAID 51 reduces the usable size to 900 GB, but allows any three drives to fail.
There are different ways to make a RAID. It can either be done with software, or with hardware.
A RAID can be made with software in two different ways. In the case of Software RAID, the disks are connected like normal hard disks. It is the computer that makes the RAID work. This means that for each access the CPU also needs to do the calculations for the RAID. The calculations for RAID 0 or RAID 1 are simple. However, the calculations for RAID 5, RAID 6, or one of the combined RAID levels can be a lot of work. In a software RAID, automatically booting from an array that failed may be a difficult thing to do. Finally, the way RAID is done in software depends on the operating system used; it is generally not possible to re-build a Software RAID array with a different operating system. Operating systems usually use hard disk partitions rather than whole hard disks to make RAID arrays.
A RAID can also be made with hardware. In this case, a special disk controller is used; this controller card hides the fact that it is doing RAID from the operating system and the user. The calculations of checksum information, and other RAID-related calculations are done on a special microchip in that controller. This makes the RAID independent of the operating system. The operating system will not see the RAID, it will see a single disk. Different manufacturers do RAID in different ways. This means that a RAID built with one hardware RAID controller cannot be rebuilt by another RAID controller of a different manufacturer. Hardware RAID controllers are often expensive to buy.
This is a mix between hardware RAID and software RAID. Hardware-assisted RAID uses a special controller chip (like hardware RAID), but this chip can not do many operations. It is only active when the system is started; as soon as the operating system is fully loaded, this configuration is like software RAID. Some motherboards have RAID functions for the disks attached; most often, these RAID functions are done as "hardware-assisted RAID". This means that special software is needed to be able to use these RAID functions and to be able to recover from a failed disk.
There are different terms that are used when talking about hardware failures:
The failure rate is how often a system fails. The "mean time to failure" (MTTF) or "mean time between failures" (MTBF) of a RAID system is the same as that of its components. A RAID system cannot protect against failures of its individual hard drives, after all. The more complicated types of RAID (anything beyond "striping" or "concatenation") can help keep the data intact even if an individual hard drive fails, though.