NodeWorks - Category Description for Encyclopedia: XE: XEN

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, 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.

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.