An excellent post that is very informative on VMware Raw Device Mapping (RDM) volumes that explains what they are, the different between physical and virtual compatibility, when to use which and when not to use RDM’s at all:

http://www.virtuallifestyle.nl/2010/01/recommended-detailed-material-on-rdms/

So it’s basically a great post on everything you’ve wondered on RDM volumes.

I really appreciated this tidbit:

• The maximum size of an RDM is 2TB minus 512 bytes.
• Use the Microsoft iSCSI initiator and MPIO inside the VM when you need more than 2TB on a disk.

This is how I came across the above article. I was trying to figure out which method was better for presenting a large disk to a virtual machine – RDM vs iSCSI inside the VM. The above answered my question as it was a 3TB volume!

Also, here’s a great whitepaper from VMware discussing the performance differences between VMFS, RDM’s, etc. especially since this has changed quite a bit from ESX 3.5 to now (vSphere 4.x). The whitepaper is entitled “Comparison of Storage Protocol Performance in VMware vSphere 4″:
http://www.vmware.com/files/pdf/perf_vsphere_storage_protocols.pdf