So far a method for switching backup servers in to assume the IP address of a lame server has been found and a way to source services from otherwise lame servers has been explored. However if we are trying to back up a service that provides a large amount of relatively dynamic data and the service goes down we still do not have an adequate solution.
An example of such a service is a HTTP server. It is not necessarily practical to keep multiple copies of a web site on different hosts due to the dynamic nature of most sites and the cost in terms of disk space. A solution that enables a backup server to access the content of a service such as HTTP when the main server goes down is to have the content situated on a third server and mounted via NFS.
If the NFS server is set up such that it does nothing but serve NFS it should be quite stable and a low risk single point of failure. Additionally, by placing the NFS server on a physically separate network or on a different segment of the LAN and giving servers that use it a second network card there is no issue relating to extra data on the network.
Therefore the content for the service can be accessed regardless of whether the main server or the backup server is in operation. In the case of an HTTP server for, which this solution is particularly well suited, this means the web site should remain accessible.