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{{for|the type of disk partition|Disk partitioning#Extended partition}}
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A '''logical partition''' ('''LPAR''') is a subset of a computer's hardware resources, [[Platform virtualization|virtualized]] as a separate computer. In effect, a physical machine can be partitioned into multiple logical partitions, each hosting a separate instance of an [[operating system]].<ref name="Singh">{{cite web |url=http://www.redbooks.ibm.com/redpieces/pdfs/sg247803.pdf |title=Security on the Mainframe |date=2009-12-02 |
==History==
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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 [[IBM i|iSeries]] and [[IBM pSeries|pSeries]] servers in 1999 and 2001, respectively,<ref>{{cite web |url=http://www.ibm.com/developerworks/aix/library/au-aix-vioserver-v2/index.html |title=POWER5 Virtualization: How to set up the Virtual I/O Server |
In first part of 2010 year, [[Fujitsu]] announced availability of its [[x86 64]] PRIMEQUEST line of servers,<ref>[http://www.fujitsu.com/global/news/pr/archives/month/2010/20100331-01.html Fujitsu Upgrades Lineup of PRIMEQUEST Mission-Critical Servers]</ref> which support LPARs.
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Logical partitioning divides hardware resources. Two LPARs may access [[memory (computers)|memory]] from a common memory chip, provided that the ranges of addresses directly accessible to each do not overlap. It is possible for one partition to control memory managed by a second partition indirectly by communicating with a process on the partition with direct access, which acts as an intermediary. [[CPU]]s may be dedicated to a single LPAR or shared. While on Amdahl's MDF (Multiple Domain Facility) it was possible to configure an LPAR with both shared and dedicated CPUs, this is no longer possible with any mainframes currently in the market.
On IBM mainframes, LPARs are managed by the [[PR/SM]] facility or a related, optional, simplified facility called Dynamic Partition Manager (DPM). All 64-bit IBM mainframes, except for the first generation 64-bit models (z900 and z800), operate exclusively in LPAR mode, even when there is only one partition on a machine. Multiple LPARs running z/OS can form a [[Sysplex]] or [[IBM Parallel Sysplex|Parallel Sysplex]], whether on one machine or spread across multiple machines.<ref name="redbooks.ibm.com">{{cite web |url=http://www.redbooks.ibm.com/abstracts/sg247803.html |title=Security on the Mainframe |date=2009-12-02 |
On IBM [[IBM pSeries|System p]] [[IBM Power Systems|POWER hardware]], LPARs are managed by PHYP (the POWER [[Hypervisor#Unix and Linux servers|Hypervisor]]).<ref>https://www.ibm.com/support/knowledgecenter/POWER6/iphb2/iphb2hypervisor.htm{{dead link|fix-attempted=yes|date=September 2020}}</ref> PHYP acts as a virtual switch between the LPARs and also handles the virtual [[SCSI]] traffic between LPARs. [[Micro-Partitioning]] supports 10 times as many LPARs as processors with fractional allocations. It was introduced with the [[POWER5]] processor. All IBM [[POWER5]], [[POWER6]], and successor systems may be partitioned. Note that a full system partition may be defined where all resources are consumed by a single partition. System P servers with PowerVM enabled allow LPARs with shared CPUs to delegate their unused cycles into the shared pool. Dedicated processors are not available for sharing. Unused cycles become available for other partitions and are governed by the parameters specified when the LPAR is defined. Changes to a running partition can be made dynamically up to the maximum value set, and down to the minimum value set in the active profile. The changing of resource allocations without restart of the logical partition is called [[Dynamic Logical Partitioning|dynamic logical partitioning]]. IBM PowerVM is the licensed/purchased feature that enables the virtualization features on p4, 5, 6, 7, and subsequent series servers.<ref>{{cite web |url=http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&appname=Demo&htmlfid=897/ENUS207-269 |title=IBM System p Virtualization — The most complete virtualization offering for UNIX and Linux |date=2007-11-06 |
Exploiting [[Intel vPro]] (i.e. [[Non-uniform memory access]]), there are also implementations of Logical Partitioning based on [[Xeon#Haswell-based_Xeon|Intel Xeon]] e.g. by [[Hitachi Data Systems]].<ref name="HDS">{{cite web |url=https://www.hds.com/en-us/pdf/white-paper/hitachi-white-paper-high-performance-nested-virtualization-with-hitachi-lpar.pdf |title=High-Performance Nested Virtualization With Hitachi Logical Partitioning Feature |date=2014-09-01 |
LPARs (with sufficient certification) safely allow combining multiple test, development, quality assurance, and production work on the same server, offering advantages such as lower costs, faster deployment, and more convenience. IBM mainframe LPARs are Common Criteria [[Evaluation Assurance Level#EAL5: Semiformally Designed and Tested|EAL 5+]] certifiable, equivalent to physically unconnected servers, so they support the highest security requirements, including military use. Nearly all IBM mainframes run with multiple LPARs with the [[IBM System z9]] and [[IBM System z10]] supporting up to 60 LPARs and later models up to 85.<ref name="redbooks.ibm.com"/>
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