High availability used to be expensive, requiring both specialized software and redundant hardware. Today it almost grows on trees, but you need two key ingredients: virtualization and network storage. By creating a virtual server farm across multiple physical servers, and storing all of your virtual machines on a central SAN or NAS, you can ensure that the failure of any given piece of physical hardware will not bring down your virtual environment. We recently created a highly available virtual server cluster based on the free edition of Citrix XenServer; this article outlines the process step by step. Although the obvious choice for any enterprise-grade virtualization deployment is VMware vSphere, I chose XenServer for two reasons. First, we're cheap. We don't like spending money on things that we can get for free. Second, and more important, the licensing of VMware is extremely confusing; we're never sure what exactly is required to be properly licensed. (I guess...
