VMware VMFS Vs RDM ( Raw Device Mapping )
Posted by craig 
- on August 22nd, 2008 in Virtualization | 3 Comments »
Recently I had read a couple of article regarding the performance caparison chart from VMware, Netapps and some of the forum communities, I do really find out the real performance is much different with the technical white paper that I read before this.
As for the today, more users are actually deployed the mission critical and high I/O servers on the virtualization environment, but we do see some I/O bottle neck which cause by the storage performance always. VMDK do provide flexibility from management perspective, but it does sacrifice the performance you may require for your databases, files transfers and disk performance. I had run a couple of test with real case scenerio instead of I/O meter that been always use widely, and here is the summarize result I would like to share.
In disk perfomance, we always split it to 2 categories as sequential and random I/O. in sequential mode, you will see the huge different while you try to perform the file transfer locally or through network. My test environment is running with SAN storage from fiber channel with same LUN size and raid group which are created from the Storage Level. The only differences is VMFS Vs Raw.
Raid Group design 7+1 raid 5 configuration and run on MetaLun configuration
Each LUN size is 300GB
Performance monitoring tools = Virtual Center Performance Chart
VM Test Machine = 4 Vcpu, 8GB Memory
Operating System = SLES 10 x32, x64 ; Windows Server 2003 x32, x64
Sequential : RDM is out perform VS VMFS as it able to achieve > 2 times higher through put during the file transfer locally on the VM
Random I/O : The Raw Device Mapping is still out perform the VMFS and getting the similar through put with sequential file transfer. Multi session with random database query is been executed in the test
for NFS file transfer from VMFS to VMFS, I do see the bottle neck happen much more earlier than RDM.
Highest I/O rate for RDM = 180MB/s as during sequential files copy and DB query
Highest I/O rate for VMDK = 100MB/s during sequential files copy and DB query
Tags: I/O, nfs, Raw, Raw Device Mapping, RDM, Storage, Virtualization, VMDK, VMFS, VMware, Windows
3 Responses
[...] are looking for performance as well. Normally we assign a RAW Device for that, you can connect a RAW up to maximum 2TB disk space if required. Check VMware VMFS vs [...]
So, if we are not using SAN but local disk, has sense to make mv with RDM hard disk instead VDMK if we are finding more performance?
Thanks
it depend how many local drives you have. At the end of the day, the performance are very subjective to the numbers of Disk, raid group configuration and etc.