Adding CPU and memory resources to a virtual machine is a simple process. The process to add resources to a virtual machine is to power down the virtual machine, increase the number of vCPUs or the amount of memory, and power on the virtual machine again.
In vSphere 4.0, two new features, CPU hot add and memory hot plug, were introduced to allow for virtual machine vCPUs and virtual machine memory to be increased without requiring that the virtual machine be powered off. CPU hot add and memory hot plug must first be enabled on the virtual machine, which does require it to be powered off. Once it is enabled, however, CPU and memory resources may be added dynamically; powering off the virtual machine is not necessary.