I never knew this:

Using SCP/CP to place the VMDK on the VMFS volume can cause poor disk performance for the VM, vmfkstools properly allocates the space to prevent this.

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vmkfstools allocates all space at once and then copies the data, a “normal” copy (cp, scp, …) allocates the space while copying.
This behavior can lead to VMDK fragmentation and therefore to poor disk performance.

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Fascinating

From here http://communities.vmware.com/message/704200