A logical partition, commonly called an LPAR, is a subset of computer's hardware resources, virtualized as a separate computer. In effect, a physical machine can be partitioned into multiple logical partitions, each hosting a separate operating system.
IBM developed the concept of hypervisors (virtual machines in CP-40 and CP-67) and in 1972 provided it for the S/370 as Virtual Machine Facility/370. IBM introduced the Start Interpretive Execution (SIE) instruction (designed specifically for the execution of virtual machines) as part of 370-XA architecture on the 3081, as well as VM/XA versions of VM to exploit it. PR/SM is a type-1 Hypervisor based on the CP component of VM/XA that runs directly on the machine level and allocates system resources across LPARs to share physical resources. It is a standard feature on IBM System z, System p, and System i machines.
The terms PR/SM and LPAR are often used interchangeably, including in IBM documentation. Formally, LPAR designates the logical partitioning function and mode of operation, whereas PR/SM is the commercial designation of the feature.
This technology was later developed separately by Amdahl, and Hitachi Data Systems for their implementations of the ESA/390 architecture in the mid 1980s; and continued by IBM for the System z and System i architectures. LPAR and PR/SM reconfigurations can be made without rebooting the computer, i.e., while some LPARs remain active. Reconfigurations can include changing channel path definitions and device definitions.
z/VM supports the z/Architecture HiperSockets function for high-speed TCP/IP communication among virtual machines and logical partitions (LPARs) within the same IBM zSeries server. This function uses an adaptation of the Queued-Direct Input/Output (QDIO) high-speed I/O protocol.
IBM later introduced LPARs to their midrange iSeries and pSeries servers in 1999 and 2001, respectively, albeit with varying technical specifications. Multiple operating systems are compatible with LPARs, including z/OS, z/VM, z/VSE, z/TPF, AIX, Linux, and i/OS. In storage systems, such as the IBM TotalStorage DS8000, LPARs allow for multiple virtual instances of a storage array to exist within a single physical array.
In first part of 2010 year, Fujitsu announced availability of its x86 64 PRIMEQUEST line of servers, which support LPARs.
In second part of 2011 year, Hitachi has announced availability of CB2000 and CB320 blade systems,which support LPAR on x86 64 hardware.