Technically speaking, VMFS vs. RDM isn't give you much performance boost but just marginal only and if its for file server I would use RDM especially you're already have RDM attached to it. If you want, you can use P2V Converter to automatically convert RDM drives to VMDK during the conversion. If you need to keep RDM, then just P2V the server without the drive and then map the RDM to the newly P2V'ed machine and keep it that way. If you need MSCS features, then RDM is required for this purpose, but some people prefer to manage everything under VMFS for flexibility. Read performance details here http://www.virtualization.info/2008/02/benchmarks-vmware-vmfs-vs-raw-disk.html and http://www.vmware.com/files/pdf/vmfs_rdm_perf.pdf
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VMware vExpert 2009
iGeek Systems Inc.
VMware, Citrix, Microsoft Consultant
This is a topic that every other days pops up on the forum.
My opinion is that this isn't (or shouldn't be) so much a performance debate as the differences between the two are supposed to be negligible. If I look at the architecture of the hypervisor (see first picture in this post for example: http://it20.info/blogs/main/archive/2007/06/17/25.aspx) I would lead to the conclusion that the vast majority of the virtualization overhead happens within the stack that runs on the server. The fact that at the very end of the chain you have a VMFS file system (by the way optimized for managing large files) or a raw partitions .... well I am not sure how much difference it could make.
So in my opinion you might want to use RDMs for tactical reasons (i.e. MSCS type of things as Stefan was mentioning or other scenarios where you need to have an "intimate relationship" with the storage). For everything else, as you pointed out, the value of encapsulation by far exceed the little perf gain you might get out of RDMs.
Again, this is juts my opinion.
As mentioned earlier in this post performance is not something you should keep in consideration in case of a file server. If the VM was running a very high IO application i would have suggested RDM not to say that a VMFS would have been an bad option<depends on the scenario>.
Now the things which you should consider are:
1> What storage is the file server using right now.. if its on the same fabric probably it is easier for you to advertise those LUNS on the host mount them as RDMs. This would help if you are upgrading the file server from 03 to 08 as convertor would not be an option.
2>How big is your file server on storage level. If its larger than 1-2TB whats the fun creating a VMFS volume to just hold that file server. RDM would be a better way to go.
3> Would you be using MSCS?? If yes you would have to use RDM's in that case. You would get better flexibility in case of 2008 as it can support a bigger basic disk for the cluster.
Hope these points help. let me know if you have further questions.
How about in using Paravirtualized Controller in Windows Server 2008 (not in R2 as it was hangs as i tried.) ? i heard that it can boost up your speed.
But I'm not sure if Paravirtual can be combined with the RDM.
In this scenario, you probably want RDMs because you can simply convert the OS volumes and swing the data drives over. This will save significant time and provide the same performance. You'll also want to ensure your file share permissions are restored after conversion. This is because the drive letters would ultimately be removed and re-added.
Great, thanks for the reply now I understandthe big picture of using RDM.