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A mirror in computing is a direct copy of a data set. On the Internet, a mirror site is an exact copy of another Internet site. Mirror sites are most commonly used to provide multiple sources of the same information, and are of particular value as a way of providing reliable access to large downloads. Mirroring is a type of file synchronization.

Mirroring can occur locally or remotely. Locally means that a server has a second hard drive that stores data. A remote mirror means that a remote server contains an exact duplicate of the data. The second drive is called a mirrored drive. Data is written to the original drive when a write request is issued. Data is then copied to the mirrored drive, providing a mirror image of the primary drive. If one of the hard drives fails, all data is protected from loss. Mirroring is one level of the RAID standard.

A live mirror is automatically updated as soon as the original is changed.

Reasons

Mirroring of sites occurs for a variety of reasons:

  • To protect data from failure, usually in hardware.
  • To preserve a website or page, especially when it is closed or is about to be closed.
  • To allow faster downloads for users at a specific geographical location. For example, a U.S. server could be mirrored in Japan, allowing Japanese Internet users to download content faster from the local Japanese server than from the original American one. This may be viewed as caching on a worldwide scale.
  • To counteract censorship and promote freedom of information. For example, an activist might post pictures on a website of a company conducting illegal activities or make available information on secret government activity and be litigated for such. Other internet users will make the content in question available on other servers when the legal action results in the cancellation of ISP or DNS services for the original activist.
  • To provide access to otherwise unavailable information. For example, when the popular Google search engine was banned in 2002 by the People's Republic of China, the mirror elgooG was used as a way of effectively circumventing the ban.
  • To preserve historic content. Financial constraints and/or bandwidth prevent the maintainers of a server from keeping older and unsupported content available to users who still may desire them - a mirror may be made to prevent this content from disappearing.
  • To balance load. If one server is extremely popular a mirror may help relieve this load: for example if a Linux distribution is released as an ISO image onto the distribution developer's own server, this server may become overloaded with demand. Alternative download points allow the total number of download requests to be spread among several servers, maintaining the availability of the distribution. Metalink is frequently used for automatic load balancing by listing all mirrors.
  • As a temporary measure to counterbalance a sudden, temporary increase in traffic. For example, Slashdotted websites will often be mirrored by a few slashdot posters until the article is pushed off the front page.
  • To increase a site's ranking in a search engine by placing hyperlinks from each mirror to every other mirror (a technique known as link farming). This is viewed as unethical by most search engine administrators and websurfers.
  • Rarely, as a form of plagiarism; this is, however, usually pointless, as a website popular enough to be worth plagiarizing will quickly discover the copy as soon as one of their many readers stumbles onto the plagiarized site.
  • As a form of raising advertising revenue. Wikipedia is probably the best example of material released under the GNU Free Documentation License which is then duplicated by other companies which, unlike Wikipedia, then attempt to generate money from advertising, etc. An example of this is the television article, which is mirrored at
http://omniknow.com/common/wiki.php?in=en&term=Television
 
http://www.startlearningnow.com/TV.htm
 
http://www.yourart.com/research/encyclopedia.cgi?subject=/television
 
http://www.internet-encyclopedia.org/wiki.php?title=Television
 
http://www.everybase.com/Television
 
http://www.nationmaster.com/encyclopedia/Television
 
  • To serve as a method of circumventing firewalls.

Examples

A good example of mirroring is the well-known SourceForge.net website. The basis of the Sourceforge concept is, primarily, the hosting of open-source software projects, but secondarily the use of many different locations to achieve one goal: to maintain download availability to the user. Many innovative computer projects host their sites and software on SourceForge, which provides mirrors in several states and countries, from Dublin, Ireland to Tokyo, Japan.

Examples of even larger mirrored networks include those of the Debian and FreeBSD software projects. The encyclopedia Wikipedia is mirrored at numerous locations.

Programs

There are numerous offline browsers that provide automated mirroring of entire sites. Some are oriented towards personal use, which allows browsing from a local copy this means an initial waiting time but much improved load time for those pages once they're mirrored.

Other programs are intended to be used by public mirror maintainers.

  • rsync
  • CVSup

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