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XenXen is an open source virtual machine monitor, developed by the University of Cambridge. It is intended to run up to 100 emulated OS's on a single computer, but not an unlimited number like the Denali (operating system), nor is it a full 'perfect' emulator like VMware. This feature is often used by IBM and others on their mainframes and servers to extract the most work possible, and to secure the programs being run by putting them in separate virtual OSs, somewhat like a chroot jail. It can also be used, not just for security or performance reason, to run different and incompatible OSs on the same computer. Paravirtualization with Xen Xen uses a technique called paravirtualization to achieve high performance (i. e. low performance penalties, typically around 2%, with worst-case scenarios at 8% performance penalty; this is in stark contrast to perfect emulation solutions which typically entail penalties of ~20%), even on its host architecture (x86) which is notoriously uncooperative to traditional virtualization techniques. Unlike traditional virtual machine monitors, which provide an execution environment exactly like the underlying hardware, Xen requires the porting of guest operating systems to the Xen API. Thus far, this porting has happened for NetBSD, GNU/Linux, FreeBSD and Plan 9. A Windows XP port was carried out during the inital development of Xen, but licensing encumbrances prevent its public release. Full Virtualization with Xen Intel has contributed modifications to Xen support their Vanderpool architecture extensions. This technology will enable unmodified guest operating systems to run within Xen virtual machines, if the host system supports the Vanderpool extensions. Virtual Machine Migration Xen virtual machines can be "live migrated" between physical hosts without stopping them. During this procedure, the memory of the virtual machine is iteratively copied to the destination without stopping its execution. A very brief stoppage of around 60 - 300ms is required to perform final synchronisation before the virtual machine begins executing at its destination, providing the illusion of a seamless migration. Similar technology is used to suspend running virtual machines to disk and resume them at a later date. Xen in Linux Distributions In 2005, both Red Hat and Novell announced that their Linux distros, Fedora and Suse respectively, would be shipping with Xen included. Debian Testing also includes Xen packages. See also External links
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