Hi,
As per study by VMware, there is no ,uch difference between RDM and VMFS( Datatstore) performance. Here is some highlights.
For random reads and writes, VMFS and RDM yield similar I/O performance.
For sequential reads and writes, RDM provides slightly better I/O performance at smaller I/O block sizes,
but VMFS and RDM provide similar I/O performance for larger I/O block sizes (greater than 32KB).
CPU cycles required per I/O operation for both VMFS and RDM are similar for random reads and writes
at smaller I/O block sizes. As the I/O block size increases, CPU cycles per I/O operation increases for VMFS
compared to RDM.
For sequential reads and writes, RDM requires relatively fewer CPU cycles per I/O operation.
If you are looking for configuring specific virtual machine best performance, then you can refer VMware white paper like for Microsoft Exchange, SQL , Oracle Database.
http://communities.vmware.com/servlet/JiveServlet/previewBody/17635-102-2-22794/SQLbestpractices.pdf
For windows iscsi lun, look for requirement of VMware
Regards
Mohammed