A logical partition (LPAR) is a part of the firmware of IBM System Z Mainframes, virtualized as a separate computer. In effect, a physical machine are often partitioned into multiple logical partitions, each hosting a separate instance of an OS.

Each LPAR functions independently, it can be any mainframe operating system or installation planners can elect to share I/O devices across several LPAR. Hence the operating systems in each LPAR can be separate and have their operator console. This ensures no effect on all the LPARs if a system in one LPAR crashes.

One or more system processors can be assigned by the system admin for the exclusive use of an LPAR. Also, the administrator can allow all processors to be used on some or all LPARs. The firmware provides a dispatcher to share the processors among the selected LPARs. The administrator can decide the number of concurrent processors executing in each LPAR. Also, can provide weightings for various LPARs; for instance , specifying that LPAR1 should receive twice the maximum amount processor time as LPAR2.

We can say there is no difference between, for example, three separate mainframes running z/OS (and sharing most of their I/O configuration) and three LPARs on the same mainframe doing the same thing. With minor exceptions z/OS, the operators, and applications cannot detect the difference.

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