In the Expanding or growing a VMFS datastore recipe, we discussed the procedure involved in increasing the size of a datastore on the same LUN backing the VMFS volume, and that was possible only if the LUN has unused free space on it or was expanded.

You can run into a situation where there is no unused space on the LUN backing the VMFS volume, but your datastore runs out of space. Fortunately, vSphere supports the spanning of a VMFS volume onto multiple LUNs. This means you can span the VMFS volume onto a new LUN so that it can use the free space on it. This process of spanning a VMFS volume onto another LUN is called extending a VMFS datastore.