The resource group clusters are running tenant workloads. A scale-up approach (increase the compute capacity of the ESXi hosts) is typically feasible only at the end of the life of the hardware components when the ESXi hosts are replaced with newer, more powerful hardware. The hardware in vSphere clusters belonging to the same provider VDC must be identical and not mixed to provide a consistent experience for the customer and for vSphere DRS efficiency. VMware recommends a horizontal scale-out approach. vCloud Director can scale its resources inside a cluster, inside a vCenter Server boundary, or outside vCenter Server boundaries.