Quoting from VMTS:
The VMFS-2 metadata manager serializes operations performed on different hosts that require metadata updates. This is a standard file system practice that protects a shared resource (the metadata) from being modified simultaneously by multiple hosts. Here, normal data I/O is not affected—only operations that originate from different hosts and that require a metadata update are serialized. Typically, these operations occur infrequently and performance impact is not significant.
cp , however, can change this dynamic grow (in blocks of 10Kb) as data is appended to them. Growing a copied file is an operation that requires a lock on the VMFS-2 metadata.
When you have a cp in action you may experience a degradation in I/O performance due to increased VMFS-2 metadata contention. Other symptoms may include the inability to change power states or modify a virtual machine.
The extent of any performance degradation depends on several factors, including the number of ESX Server hosts with virtual machines that use the VMFS-2 volume, and the intensity and pattern of I/O activity in the virtual machines with REDO log files and the number of concurrent cp operation.
The only way to do a file copy inside a VMFS volume is to use the undocumented vmkfstools option:
vmkfstools -e /vmfs/vmfsname/target.vmdk -d vmfs /vmfs/vmfsname/source.vmdk