A server farm or server cluster is a collection of computer servers - usually maintained by an organization to supply server functionality far beyond the capability of a single machine. Server farms often consist of thousands of computers which require a large amount of power to run and to keep cool. At the optimum performance level, a server farm has enormous costs associated with it, both financially and environmentally. Server farms often have backup servers, which can take over the function of primary servers in the event of a primary-server failure. Server farms are typically collocated with the network switches and/or routers which enable communication between the different parts of the cluster and the users of the cluster. Server farmers typically mount the computers, routers, power supplies, and related electronics on 19-inch racks in a server room or data center.