Editing Hypervisor (section)

==Operating system support==
{{sources needed|section|date=September 2021}}
Several factors led to a resurgence around 2005 in the use of [[Platform virtualization|virtualization]] technology among [[Unix]], [[Linux]], and other [[Unix-like]] operating systems:<ref>{{cite web|last=Loftus|first=Jack|title=Xen virtualization quickly becoming open source 'killer app'|work=TechTarget|date=19 December 2005|access-date=26 October 2015|url=http://searchdatacenter.techtarget.com/news/1153127/Xen-virtualization-quickly-becoming-open-source-killer-app}}</ref>
* Expanding hardware capabilities, allowing each single machine to do more simultaneous work
* Efforts to control costs and to simplify management through consolidation of servers
* The need to control large [[multiprocessor]] and [[computer cluster|cluster]] installations, for example in [[server farm]]s and [[render farm]]s
* The improved security, reliability, and device independence possible from hypervisor architectures
* The ability to run complex, OS-dependent applications in different hardware or OS environments
* The ability to overprovision resources, fitting more applications onto a host

Major Unix vendors, including [[Hewlett-Packard|HP]], [[IBM]], [[Silicon Graphics|SGI]], and [[Sun Microsystems]], have been selling virtualized hardware since before 2000. These have generally been large, expensive systems (in the multimillion-dollar range at the high end), although virtualization has also been available on some low- and mid-range systems, such as IBM [[System p|pSeries]] servers, [[HP Superdome]] series machines, and [[Sun Microsystems|Sun]]/[[Oracle Corporation|Oracle]] T-series CoolThreads servers.

Although [[Solaris (operating system)|Solaris]] has always been the only guest domain OS officially supported by Sun/Oracle on their [[Logical Domains]] hypervisor, {{as of | 2006 | alt = as of late 2006}}, [[Linux]] (Ubuntu and Gentoo), and [[FreeBSD]] have been ported to run on top of the hypervisor (and can all run simultaneously on the same processor, as fully virtualized independent guest OSes). Wind River "Carrier Grade Linux" also runs on Sun's Hypervisor.<ref>{{cite press release|url=http://www.windriver.com/news/press/pr.html?ID=3881|title=Wind River To Support Sun's Breakthrough UltraSPARC T1 Multithreaded Next-Generation Processor|work=Wind River Newsroom|location=Alameda, California|date=1 November 2006|access-date=26 October 2015|archive-date=November 10, 2006|archive-url=https://web.archive.org/web/20061110175456/http://www.windriver.com/news/press/pr.html?ID=3881|url-status=dead}}</ref> Full virtualization on [[SPARC]] processors proved straightforward: since its inception in the mid-1980s Sun deliberately kept the SPARC architecture clean of artifacts that would have impeded virtualization. (Compare with virtualization on x86 processors below.)<ref>{{cite report|url=https://www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/Publications/Studies/TC_ErgA/TC-ErgA_Part1.pdf?__blob=publicationFile|title=Complementary and Alternative Technologies to Trusted Computing (TC-Erg./-A.), Part 1, A study on behalf of the German Federal Office for Information Security (BSI)|first1=Lothar|last1=Fritsch|first2=Rani|last2=Husseiki|first3=Ammar|last3=Alkassar|format=PDF|access-date=February 28, 2011|archive-date=June 7, 2020|archive-url=https://web.archive.org/web/20200607211407/https://www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/Publications/Studies/TC_ErgA/TC-ErgA_Part1.pdf?__blob=publicationFile|url-status=dead}}</ref>

HPE provides [[HP Integrity Virtual Machines]] (Integrity VM) to host multiple operating systems on their [[Itanium]] powered Integrity systems. Itanium can run [[HP-UX]], Linux, Windows and [[OpenVMS]], and these environments are also supported as virtual servers on HP's Integrity VM platform. The HP-UX operating system hosts the Integrity VM hypervisor layer that allows for multiple features of HP-UX to be taken advantage of and provides major differentiation between this platform and other commodity platforms - such as processor hotswap, memory hotswap, and dynamic kernel updates without system reboot. While it heavily leverages HP-UX, the Integrity VM hypervisor is really a hybrid that runs on bare-metal while guests are executing. Running normal HP-UX applications on an Integrity VM host is heavily discouraged,{{By whom|date=April 2010}} because Integrity VM implements its own memory management, scheduling and I/O policies that are tuned for virtual machines and are not as effective for normal applications. HPE also provides more rigid partitioning of their Integrity and HP9000 systems by way of VPAR and [[HP nPar (Hard Partitioning)|nPar]] technology, the former offering shared resource partitioning and the latter offering complete I/O and processing isolation. The flexibility of virtual server environment (VSE) has given way to its use more frequently in newer deployments.{{Citation needed|date=November 2008}}

IBM provides virtualization partition technology known as [[LPAR|logical partitioning]] (LPAR) on [[System/390]], [[zSeries]], [[pSeries]] and [[IBM AS/400]] systems. For IBM's Power Systems, the POWER Hypervisor (PHYP) is a native (bare-metal) hypervisor in firmware and provides isolation between LPARs. Processor capacity is provided to LPARs in either a dedicated fashion or on an entitlement basis where unused capacity is harvested and can be re-allocated to busy workloads. Groups of LPARs can have their processor capacity managed as if they were in a "pool" - IBM refers to this capability as Multiple Shared-Processor Pools (MSPPs) and implements it in servers with the [[POWER6]] processor. LPAR and MSPP capacity allocations can be dynamically changed. Memory is allocated to each LPAR (at LPAR initiation or dynamically) and is address-controlled by the POWER Hypervisor. For real-mode addressing by operating systems ([[AIX]], [[Linux]], [[IBM i]]), the [[IBM Power microprocessors|Power]] processors ([[POWER4]] onwards) have designed virtualization capabilities where a hardware address-offset is evaluated with the OS address-offset to arrive at the physical memory address. Input/Output (I/O) adapters can be exclusively "owned" by LPARs or shared by LPARs through an appliance partition known as the Virtual I/O Server (VIOS). The Power Hypervisor provides for high levels of reliability, availability and serviceability (RAS) by facilitating hot add/replace of multiple parts (model dependent: processors, memory, I/O adapters, blowers, power units, disks, system controllers, etc.)

Similar trends have occurred with x86/x86-64 server platforms, where [[open-source model|open-source]] projects such as [[Xen]] have led virtualization efforts. These include hypervisors built on Linux and Solaris kernels as well as custom kernels. Since these technologies span from large systems down to desktops, they are described in the next section.