One of the main benefits of a SAN is that if you have homogeneous hardware servers, you can use free Xen to employ hot-failover for Virtual Machines. I've seen a lot of hype over the power of the Xen Server's abilities on this front, requiring a shared SAN.

This can be creeping death for the unwary DBA.

SAN gives fast, redundany, off-server, snapshottable shared storage. (See NetAPP)

+ XEN can give us the ability to have super-fast redundant failover using the SAN

+ SQL Server can be installed on a Virtual Machine

= Put the above together, and you have what looks like a 'good' redundant system.

Sadly, from tests with SQL on XEN+SAN, the I/O throughput is surprisingly bad. You must over-supply your servers with RAM to cope with the poor I/O, and you must provide more processor cores.

Running a production Server in the above way may seem like a good choice, but the cost is prohibitive, and the performance for the high-IO system disappointing. This becomes doubly-true if your sys admins see the expensive NetAPP SAN with all that unused space, and throw up a few more VMs, also doing I/O using the same disk set. Caveat NASor 😉