When it comes to server consolidation or building a development environment, virtualisation is at the forefront of making these efforts successful. A sound enterprise-grade virtualisation strategy includes careful planning. This includes appropriate selection of the components that will make up the virtualisation infrastructure beyond the host servers. A critical component to your strategy must include appropriate shared storage in order to take advantage of the high-end features of virtualisation software.

The type of storage you choose for virtualisation is dependent on your specific needs, as well as on the type of virtualisation you choose to use. For the purposes of this article, I'll be focusing on only two main types of virtualisation: host-based -- running your virtual machines under software that is installed on top of a host operating system, such as VMware Server or Virtual Server 2005; the second type is bare metal -- running VMware ESX Server, for example.

With both types of virtualisation, you can choose either host-based or shared storage, but you should be aware of some of the limitations imposed if you decide to stick with host-based storage rather than a SAN or supported NAS device. The host-based virtualisation software is not SAN-aware, but you can still run your virtual machines from a SAN or fast NAS unit by using the underlying operating system's SAN connectivity. You won't get the full benefits that you would get from running your virtual machines from a SAN, using bare-metal virtualisation, but it remains an option to consider. I'll go into more detail on each of these setups below.

Host-based virtualisation and host-based storage (host/host)
This scenario is the simplest to grasp and the easiest to implement. Here, everything is local -- the virtualisation software, the virtual machines, and the storage. Further, since the virtualisation software runs on either Linux or Windows, you're probably already familiar with managing the environment. However, in this scenario, your virtual machines are afforded very little real-time protection.

If your host server dies, all of the virtual machines will go with it. Sure, you can back up the virtual machine files and restore them in the event of a failure, but the process will be somewhat time consuming and, depending on how you handle the backup, you may lose some data.

Host-based virtualisation and shared storage (host/shared)
If you have a SAN or a supported NAS device, and you want host-based virtualisation (due to cost, perhaps), this is your best option. In this scenario, the virtual machines still run on top of the host operating system, but you can store the virtual machine configuration and data files on the SAN. While the virtualisation software itself is not SAN-aware, you can still take advantage of the capabilities offered by your SAN to keep your environment more highly available. For example, using this method allows you to make use of your SAN's snapshot capability meaning that, if a virtual machine is corrupted, you can easily recover. Further, if your virtual host server dies, you can fairly quickly mount the SAN volume on a different server and bring the virtual machines back up.