Last night on twitter there was a discussion about RDM vs. VMFS performance. I have always told everybody that RDM did not have an performance advantage over a VMDK file on a VMFS volume. I have refered people to the following document by VMware: Performance Characteristics of VMFS and RDM but I have never done my own testing until now 🙂
My setup is ESXi 4.1 build 260247 with 4gb fibre channel HBA connected to a SUN Storagetek 2540 SAN. I did the test on a 10GB LUN located on fibre channel disks. The test was done with Iometer on a virtual machine running Windows 2003 SP2. I performed all tests on the 10GB LUN as either: Physical RDM, Virtual RDM, Lazy Zeroed VMDK on a VMFS and Eager Zeroed VMDK on a VMFS.
I configured Iometer with settings from the unoffical storage performance thread on VMware communites and ran two tests: Maxthroughput and Real life. Here are my results:
Conclusion: VMFS and RDM have similar performance. Don’t choose RDM for performance.