virtualization is required by default on ibm z systems. first layer virtualization is provided by the processor resource and system manager (pr/sm) to deploy one or more logical partitions (lpars). each lpar supports a variety of operating systems. a hypervisor called z/vm can also be run as the second layer virtualization in lpars to create as many virtual machines (vms) as there are resources assigned to the lpars to support them. the first layer of ibm z virtualization (pr/sm) allows a z machine to run a limited number of lpars (up to 80 on the ibm z13). these can be considered virtual "bare metal" servers because pr/sm allows cpus to be dedicated to individual lpars. z/vm lpars allocated within pr/sm lpars can run a very large number of virtual machines as long as there are adequate cpu, memory, and i/o resources configured with the system for the desired performance, capacity, and throughput.