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If a virtual disk is configured as an RDM the scsi commands are processed directly by the Guest O.S however if its configured as VMDK the VMkernel layer has to do all the processing.
Refer to this thread for a detailed discussion about using RDM or VMDK http://communities.vmware.com/thread/83541
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VCP3,VCP4,HP UX CSA
There is no difference between vmdk and raw in terms of performance.
1) Truly dynamic - you can migrate it anywhere
2) You can hot-extend it without downtime
1) Once you lost VMFS - you lost data also
2) Can not be used for p-v clusters with shared storage
1) You can connect LUN to any VM or physical machine and get access to data
2) The only solution for p-v shared storage cluster
1) It's not that dynamic as vmdk
MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert