There is no difference between vmdk and raw in terms of performance.
VMDK pros:
1) Truly dynamic - you can migrate it anywhere
2) You can hot-extend it without downtime
VMDK cons:
1) Once you lost VMFS - you lost data also
2) Can not be used for p-v clusters with shared storage
RAW pros:
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
RAW cons:
1) It's not that dynamic as vmdk
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MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert
http://blog.vadmin.ru