From Wikipedia, the free encyclopedia
This article is about mirror sites. For
information about hard disk mirrors, see
disk mirror.
In computing, a
mirror is an exact 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.
A live mirror is automatically updated as soon as the
original is changed.
Reasons
Mirroring of sites occurs for a variety of reasons.
- 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. See Wikipedia:Mirrors and forks.
- 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 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.
Software
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.
Free file mirroring software includes MMup by MassMirror and wget, mirror and mirrordir available
as add ons for many Linux
distributions.
See also
Wikis
Quiz
Map
Search
