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In computing, 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 LPARs, each housing a separate operating system.

The technology was initially developed by IBM for mainframe architecture ESA/390 around 1990, and continued also for zSeries and System z9 architectures. However, IBM later extended the idea to non-mainframe servers, such as pSeries and iSeries, albeit with varying technical specifications. Multiple operating systems are compatible with LPARs, including z/OS, z/VM, z/VSE, z/TPF, AIX, Linux (including Linux on zSeries), and i5/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.

Logical partitioning is performed mostly on the hardware layer. Two LPARs may access memory from a common memory chip, provided that the ranges of addresses directly accessible to each do not overlap. One partition may indirectly control memory of a second partition, but only by commanding a process of the second partition to directly operate on the memory. Initially, each CPU was dedicated to a single LPAR, but with introduction of micro-partitioning one CPU can be shared between separate LPARs.

On IBM mainframes, LPARs are managed by a PR/SM facility. Modern IBM mainframes operate exclusively in LPAR mode, even when there is only one logical partition on a footprint. Multiple LPARs can form a Sysplex or Parallel Sysplex, whether on one footprint or spread across multiple footprints.

LPARs safely allow combining multiple test, development, quality assurance, and production work on the same system, offering several advantages such as lower costs, faster deployment, and more convenience. IBM mainframe LPARs are Common Criteria EAL5 certifiable, equivalent to physically separate servers with no connections, so they are appropriate for the highest security requirements, including military use. Nearly all IBM mainframes run with multiple LPARs (up to 60 in recent models).

• Dynamic Logical Partitioning (DLPAR)
• HiperSocket
• Virtualization
• Fujitsu XPAR
• SUN Dynamic Domains
• SUN LDOM