Xen (virtual machine monitor)
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Xen is an open source virtual machine monitor, developed by the University of Cambridge. It is intended to run up to 100 full featured OSs on a single computer. Operating systems must be explicitly modified ("ported") to run on Xen (although compatibility is maintained for user applications). This enables Xen to achieve high performance virtualization without special hardware support.
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Use
Virtual machines are often used by IBM and others on their mainframes and servers to extract the most work possible, and to secure the programs by putting them in separate virtual OSs, like a chroot jail but more so. It can also be used, not just for security or performance reason, but to run different and incompatible OSs on the same computer.
Comparison with other VMMs
- Denali uses paravirtualisation to provide high performance virtual machines on x86 computers. Denali's virtual machines support specialised minimal OSs for Internet services. The system can scale to 1000s of virtual machines. Xen's motivation differs from that of Denali in that it is intended to run a moderate number of full-featured operating systems, rather than a large number of specialised lightweight OSs.
- VMware provides virtual machines for x86 that can run unmodified PC operating systems. The technology involved in doing this is extremely complex and also incurs (sometimes significant) performance overheads. Xen trades full binary compatibility for comparative simplicity and improved performance.
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 performance penalties of ~20%), even on its host architecture (x86) which is notoriously uncooperative with traditional virtualization techniques. Unlike traditional virtual machine monitors, which provide a software-based execution environment exactly like the simulated 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. On Brainshare 2005 (http://www.novell.com/brainshare/), Novell showed a port of NetWare to Xen. A Windows XP port was carried out during the inital development of Xen, but Microsoft's licensing 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 or the Pacifica extensions (Intel's, and AMD's, respectively, extensions to natively support virtualization). Practically, this will mean that there will be a performance boost, and that you will be able to virtualize Windows without having to modify it in any way, which licensing encumbrances would prevent.
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 final destination, providing an illusion of seamless migration. Similar technology is used to suspend running virtual machines to disk and switch to another virtual machine, and resume the first virtual machine at a later date.
Platform Support
Xen currently runs on x86-based systems. Ports are currently underway to x86_64, IA64 and PPC. Ports for other platforms are also technically possible and may be available in the future.
Xen in Unix-like Systems
Novell's Suse Linux Professional 9.3, released April 15, 2005 includes, Xen version 2.0.5c. Red Hat has also announced that their Linux distribution, Fedora Core 4, would be shipping with Xen included. Xenophilia at [1] (http://cosi.clarkson.edu/xen/) is a Linux distribution that is based around Xen. The Xen demo CD at [2] (http://www.cl.cam.ac.uk/Research/SRG/netos/xen/downloads.html) is a live ISO CD running Debian Linux that enables you to try Xen on your system without installing it to the hard disk. Debian also includes Xen packages, in unstable and experimental.
NetBSD 2.0 includes support for Xen 1.2, the upcoming 3.0 release includes support for Xen 2.0.
See also
External links
- Official Home page (http://www.cl.cam.ac.uk/Research/SRG/netos/xen/)
- Design and benchmarks (http://www.cl.cam.ac.uk/netos/papers/2003-xensosp.pdf)
- An overview of migrating virtualized OSs from machine to machine (http://www.cl.cam.ac.uk/netos/papers/2005-migration-nsdi-pre.pdf)
- Press announcement of Xen's inclusion within Suse (http://www.zdnet.com.au/news/software/0,2000061733,39184207,00.htm)
- Announcement of Xen's inclusion in Fedora (http://news.zdnet.co.uk/0,39020330,39187943,00.htm)
- A comparison of Linux and NetBSD performance under Xen (http://users.piuha.net/martti/comp/xendom0/xendom0.html)de:Xen