I found another fantastic article on Disk IOPS and planning for Exchange on Nicole Allen's Blog. This is a must read!
While I was at TechEd, I had a great time talking with our customers and hearing about their experiences as Exchange administrators. One of the areas that have come up in discussion a lot has to do with planning sufficient disk throughput for the back-end exchange servers. A Lead Program Manager in the Exchange team started pestering me to write it up in a blog, since if so many customers at TechEd had questions, it followed that their might be other Exchange administrators that would be interested as well.
So....I've written up the basics of what you need to know about disk sizing for the Exchange server. I'm not claiming it's comprehensive (I'm not going to go into SAN technology details, for example, since they differ from vendor to vendor), but it should be enough to:
a) determine if you have sufficient disks
b) detect if you have a disk bottleneck
c) calculate the number of disk I/Os per second per user (also known as IOPS/user)
d) estimate how many disks you need for a new server, based on past user behavior.