What role does virtualization play in a network with a SAN?

Solution 1:

Virtualisation isn't a required component to use a SAN, you've got it the wrong way round. A SAN is a desirable (not required) component to get the best out of major virtualisation deployments.

A SAN is useful in virtualisation in at least two areas:

You usually need a fast, large disk store to hold virtual disk images. A SAN is a common way of achieving both of those things.
One part of large virtualisation installations such as VMWare ESX and HyperV is the ability to "move" virtual machines from one host to another, both as part of capacity planning and to improve availability in the event of a hardware failure on the host. To do this you need shared storage, and to do this properly you need shared resilient storage.

For both those cases, a SAN isn't the only way of doing things, and it might not even be the cheapest way of doing things, but it does solve the problem and usually solves them very well.

On my LAN we have both VMWare virtual hosts and directly attached file servers making use of SANs. We use whatever combination we feel will be effective for each particular problem we solve.

Solution 2:

With technologies like XEN and HyperV, if you have a SAN with multiple servers hanging off it, you can live migrate a running SAN backed virtual server from one server to another. Very handy for load balancing and doing maintenance on physical machines without any downtime.