Once VM was generally available in 1972, some companies used it to partition machines in order to run different workloads in different partitions. It turned out that for some companies, a simpler partitioning scheme would be just as useful. As a result, some fundamental virtualization concepts were included into the architecture. These concepts became the hardware hypervisor Processor Resource/Systems Manager (PR/SM) and the logical partitions (LPARs) into which PR/SM could divide a machine. A zSeries 900 or 800 mainframe can run in one of two modes: basic mode or LPAR mode. In basic mode, the entire machine is under the control of a single operating system. In LPAR mode, you can logically divide the machine into partitions so that multiple operating systems can run concurrently.
LPAR partitioning helps companies to exploit the resources of the machine better than if there were no partitioning. LPAR allows companies to have a single machine support both production and test requirements. Initially, companies would bring up the test partition only at off-peak times.
The PR/SM hypervisor of the zSeries 900 can manage up to 15 LPAR "guests." The CPU and channel paths are the two resources that can be logically shared. Central storage must be physically partitioned and assigned to each active partition.
Although the virtualization technology that is best known in the context of Linux on the mainframe is z/VM, there are some occasions where you might consider using an LPAR:
Companies can utilize their mainframe with more flexibility using LPARs than using native mode. However, LPAR provides far less flexibility and function than what is available with z/VM.