...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 recover easily. Also, if your virtual host server dies, you can mount the SAN volume on a different server fairly quickly and bring the virtual machines back up.
Bare metal virtualisation and host-based storage (metal/host)
Personally, I think this scenario is the hardest to manage. In this case, you're running VMware ESX server on the bare metal and storing the individual virtual machines on local disks. You have the same risks and recovery limitations inherent in the host/host model, but you do not get the benefit of backing up the individual virtual machine files without using third-party software. Your best option for this scenario is to individually back up each virtual machine as if it were a physical server. That is, use NTbackup or install your backup software's agent.
Bare metal virtualisation and shared storage (metal/shared)
This is, by far, the most desirable — and the most expensive — way to implement a robust virtualised environment in your organisation. In this scenario, you get all the great benefits of the host/shared method (snapshot capability and so on) but, with VMware ESX, you also get incredible features such as VMotion, VMware HA (High Availability) and VMware DRS (Distributed Resource Scheduler). VMotion allows you to seamlessly move a running virtual machine between ESX hosts. HA watches your ESX hosts and, upon detection of a host failure, automatically moves guests from the failed host to other servers in the cluster. DRS automatically moves virtual machines between suitable ESX hosts based on the resource requirements of the virtual machine. DRS also provides a "maintenance mode" option whereby, with minimal effort, you can force DRS to move all guests from a host node so you can perform maintenance on that node.
All the features require shared storage in the form of a supported Fibre Channel or iSCSI SAN or NAS device.
Summary
A robust, scalable, highly-available virtualised environment can save an organisation significant time and money, but you need to plan your virtualisation storage strategy carefully and decide which approach is best for your company.
