One VMDK per VMFS?
it is always administratively easier to have multiple virtual machine disks per VMFS volume.
However there are certain cases where having one VMDK per VMFS might be ideal.
"Multiple heavily used virtual machines concurrently accessing the same VMFS volume, or multiple VMFS volumes backed by the same LUNs, can cause decreased storage performance."
How much space to allocate if you use a one to one ratio?
A good rule of thumb is to leave at least 20% free for snapshots.
You could allocate the entire lun to the vmdk but then you don’t have any space for snaps.
Example senario
In an example that came up on the forums, a member wanted to use this setup for their SQL server setting up 1 vmdk per vmfs volume.
Given the potential for high IO on an SQL server it can make sense to do so.
Pasted from <http://communities.vmware.com/message/2121272#2121272>
Why not use Raw disks?
Well you can! And many people do.
VMware recommends VMFS for most virtual disk storage, because VMFS provides much more flexibility and in almost all cases performs just as well as RDMs. RDM and VMFS can provide similar transaction throughput. In most of the tests reported in “Performance Characterization of VMFS and RDM Using a SAN,” the I/O performance difference between RDM and VMFS configurations was insignificant. You should decide which storage configuration to use based on other aspects of your deployment. For example, you can move virtual machines using VMFS more easily than those using RDM, and VMFS provides greater flexibility.
Then When to use a raw disk?
RDM may be desirable in a small number of cases, such as when using MSCS-based failover clustering for SQL Server.
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