A Look Inside Wikipedia's Infrastructure
 

The site started as a Perl CGI script running on single server in 2001. Wikipedia now has 200 application servers, 20 database servers and 70 servers dedicated to Squid cache servers.

Wikipedia is powered by the MediaWiki software, which was originally written to run Wikipedia and is now an open source project. MediaWiki uses PHP running on a MySQL database. Mituzas said MySQL instances range from 200 to 300 gigabytes. In addition to Squid, Wikipedia uses Memcached and the Linux Virtual Server load balancer. Wikipedia also uses database sharding to set up master-slave relationships between databases.

http://www.datacenterknowledge.com/a...structure.html