Xen

From Wikipedia, the free encyclopedia

For other uses, see Xen (disambiguation).

Xen
Xen running NetBSD and three Linux distributions
Developed by	The Xen Project, XenSource, Inc.
Latest release	3.2.1 / Apr 25, 2008
OS	Linux, and other Unix-like, *BSD, OpenSolaris as both hosts and guests; Microsoft Windows as the guest OS.
Genre	Virtual machine monitor
License	GPL
Website	http://www.xen.org

Xen is a free software virtual machine monitor for IA-32, x86, x86-64, IA-64 and PowerPC 970 architectures. It allows several guest operating systems to be executed on the same computer hardware at the same time.

A Xen system is structured with the Xen hypervisor as the lowest and most privileged layer.^[1] Above this layer are one or more guest operating systems, which the hypervisor schedules across the physical CPUs. The first guest operating system, called in Xen terminology "domain 0" (dom0), is booted automatically when the hypervisor boots and given special management privileges and direct access to the physical hardware. The system administrator logs into dom0 in order to start any further guest operating systems, called "domain U" (domU) in Xen terminology.

Modified versions of Linux, NetBSD and Solaris can be used as the dom0. Several modified Unix-like operating systems may be employed as guest operating systems (domU); on certain hardware, as of Xen version 3.0, unmodified versions of Microsoft Windows and other proprietary operating systems can also be used as guests if the CPU supports Intel VT or AMD V technologies.^[2]

Xen originated as a research project at the University of Cambridge, led by Ian Pratt, senior lecturer at Cambridge and founder of XenSource, Inc. This company now supports the development of the open source project and also sells enterprise versions of the software. The first public release of Xen was made available in 2003. XenSource, Inc was acquired by Citrix Systems in October 2007. XenSource's products have subsequently been renamed under the Citrix brand: XenExpress was renamed "XenServer Express Edition" and "XenServer OEM Edition" (embedded hypervisor); XenServer was renamed "XenServer Standard Edition"; XenEnterprise was renamed "XenServer Enterprise Edition".

On 2007-10-22, Citrix Systems completed its acquisition of XenSource,^[3] and the Xen project moved to http://www.xen.org. This move had been under way for some time, and afforded the project an opportunity to make public the existence of the Xen Project Advisory Board (Xen AB), which currently has members from Citrix, IBM, Intel, Hewlett-Packard, Novell, Red Hat and Sun Microsystems. The Xen AB is chartered with oversight of the project's code management procedures, and with development of a new trademark policy for the Xen mark, which Citrix intends to freely license to all vendors and projects that implement the Xen hypervisor; the requirements for licensing will be solely the responsibility of the Xen AB.

1 Uses
2 Technology
3 Xen Management Consoles
4 Commercial implementations
5 See also
6 References
7 External links

[edit] Uses

Virtual machine monitors (also known as hypervisors) are often used by IBM, HP, and others on mainframes and large servers. They are also increasingly being used by Internet hosting service companies to provide virtual dedicated servers. The primary benefits of server virtualization are consolidation, increased utilization, an ability to rapidly provision and start a virtual machine, and increased ability to dynamically respond to faults by re-booting a virtual machine or moving a virtual machine to different hardware. Another benefit is the ability to securely separate virtual operating systems, and the ability to support legacy software as well as new OS instances on the same computer. Xen's support for virtual machine live migration from one host to another allows workload balancing and the avoidance of downtime.

Xen may also be used on personal computers that run Linux but also have Windows installed. Traditionally, such systems are used in a dual boot setup, but with Xen it is possible to start Windows "in a window" from within Linux, effectively running applications from both systems at the same time.

Virtualization also has benefits when working on operating system development: running the new system as a guest avoids the need to reboot the computer whenever a bug is encountered. Such a sandboxed guest system is also useful in computer security research, in order to study the effects of some virus or worm without the possibility of compromising the host system. Finally, hardware appliance vendors may decide to ship their appliance running several guest systems, so as to be able to execute various pieces of software that require different operating systems.

Xen can be delivered to market as a virtualization platform, such as Citrix XenServer Enterprise Edition (formerly XenSource's XenEnterprise), or embedded within the host operating system. An example of the latter configuration is the inclusion of Xen in Novell's SUSE Linux Enterprise 10 distribution, Red Hat's RHEL 5/Fedora 7, Sun Microsystems' Solaris, or Debian's Etch release. XenSource recently announced that Dell will incorporate Citrix XenServer OEM Edition as an embedded hypervisor installed in flash memory as an option on all Power Edge Servers, early in 2008.

XenSource is also developing a compatibility layer for Windows Server 2008 hypervisor, so that systems that have been modified to run as Xen guests will be able to function on the 2008 hypervisor.^[4]

[edit] Technology

[edit] Paravirtualization, requiring porting of guest systems

On most CPUs, Xen uses a form of virtualization known as paravirtualization, meaning that the guest operating system must be modified to use a special hypercall ABI instead of certain architectural features. Through paravirtualization, Xen can achieve high performance even on its host architecture (x86) which is notoriously uncooperative with traditional virtualization techniques.^[5]

On x86, the Xen host kernel code runs in Ring 0, while the hosted domains run in Ring 1 or Ring 3.

[edit] Hardware assisted virtualization, allowing for unmodified guests

Intel has contributed modifications to Xen to support their VT-x (formerly Vanderpool) architecture extensions.^[6] Similarly AMD has contributed support for their AMD-V extensions. These technologies, while differing quite substantially in their implementation and instruction sets, are managed by a common abstraction layer in Xen and enable unmodified guest operating systems to run within Xen virtual machines, starting with Xen 3.0.

This has been a significant development because it allows proprietary operating systems (such as Microsoft Windows) to be virtualized since the guest system's kernel does not require modification when the host runs on VT-x or AMD-V hardware.

Hardware assisted virtualization offers new instructions to support direct calls by a paravirtualized guest/driver into the hypervisor, typically used for I/O or other so-called hypercalls. It also provides additional execution modes: "root mode" and "non-root mode". Both of these modes have Rings 0-3; the Xen host operates in root mode and has access to the real hardware, while the unmodified guest operates in Rings 0-3 of non-root mode and its "hardware" accesses are under complete control of the hypervisor.

Xen-HVM has device emulation based on the QEMU project to provide I/O virtualization to the VMs. Hardware is emulated via a patched QEMU "device manager" (qemu-dm) daemon running as a backend in dom0. This means that the virtualized machines see as hardware: a PIIX3 IDE (with some rudimentary PIIX4 capabilities), Cirrus Logic or vanilla VGA emulated video, RTL8139 or NE2000 network emulation, PAE, and somewhat limited ACPI and APIC support and no SCSI emulation.^[7]

As of Xen 3.0.2, the list of supported unmodified guests is limited to certain versions of Windows (incl. XP) and Linux.

[edit] Virtual machine migration

Xen virtual machines can be "live migrated" between physical hosts across a LAN without loss of availability. During this procedure, the memory of the virtual machine is iteratively copied to the destination without stopping its execution. Stoppage of around 60–300 ms is required to perform final synchronization 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.

[edit] Supported operating systems and hardware

An operating system that runs Xen as host is also known as domain number 0 (dom0), while a system that runs as a Xen guest is known as the unprivileged domain (domU).

[edit] Unix-like systems as hosts

Xen boots from a bootloader like GNU GRUB, and then generally loads a modified host OS into dom0.

Most Linux distributions now include Xen packages to interact with the Xen hypervisor and start additional domains.

Xen under Linux currently runs on x86, with Pentium II or newer processors, x86-64 based systems, as well as on IA-64 and PowerPC. Xen supports up to 64-way symmetric multiprocessing machines.

XenSource offers a live ISO CD running Debian GNU/Linux as well as other free Linux distributions, enabling users to try Xen on their system without installing it to the hard disk.

The first commercial implementation of Xen in this form was Novell's SUSE Linux Enterprise Server 10 release. Red Hat Enterprise Linux 5 also offers support for Xen.

Fedora has included Xen packages since its FC4 release. A quick-start document is available.

openSUSE 10.3 includes Xen 3.1 support. Graphical VM management is offered by YaST and virt-man.[1]

Ubuntu includes Xen packages since Ubuntu 6.10. The latest release, Hardy Heron, includes Xen version 3.2 and 3.1. Instructions can be found at https://help.ubuntu.com/community/Xen

Debian includes Xen 3.0.3 packages in its 4.0 release (codenamed Etch). Packages for Debian 3.1 (Sarge) can be found on http://www.backports.org/

Host support for Xen 2 is included in NetBSD 3.x. Host support for Xen 3.0 is available since NetBSD 4.0 [2].

Support for OpenBSD self-hosting is near complete, requiring the correction of a lock-up bug before its release.[3]

A Gentoo package exists for Xen in Portage. [4] Set-up instructions also exist on gentoo-wiki.com.

ArchLinux package exists for Xen in the community repository [5]

OpenSolaris based distributions can be used as dom0 and domU, from Nevada build 75 onwards.

[edit] Unix-like systems as guests

Guest systems can run fully virtualized (requires special hardware) or paravirtualized (requires guest OS code modification). The following systems have been patched so that they can operate as a paravirtualized Xen guest:

Linux, paravirtualization integrated in 2.6.23, patches for other versions exist
Minix
Plan 9 from Bell Labs
NetBSD (NetBSD 2.0 has support for Xen 1.2, NetBSD 3.0 has support for Xen 2.0, and NetBSD 3.1 supports Xen 3.0)
OpenBSD, announced here
FreeBSD (Limited) [6]
OpenSolaris (See The Xen Community On OpenSolaris)
NetWare (at Brainshare 2005, Novell showed a port that is able to run as a Xen guest)
GNU/Hurd/Mach (gnumach-1-branch-Xen-branch)
OZONE (has support for Xen v1.2)

[edit] MS Windows systems as guests

It is currently possible to run Microsoft Windows as a guest operating system unmodified, using hardware virtualization provided by Intel's Vanderpool technology (now Intel VT or IVT) or AMD's Pacifica. This is supported beginning with Xen 3.0.

During the development of Xen 1.x, Microsoft Research, along with the University of Cambridge Operating System group, developed a port of Windows XP to Xen. This was possible due to Microsoft's Academic Licensing Program. The terms of this license do not allow this port to be published, although the experience is documented in the original Xen SOSP paper.^[8]

[edit] Xen Management Consoles

A number of third-party tools (known as Xen Management Consoles) have been developed to facilitate the common tasks of administering a Xen host, such as configuring, starting, monitoring and stopping of Xen guests. Examples include the Python-based Enomalism dashboard (LGPL), Xen Tools, Google's Ganeti, the Perl-based MLN, the web-based HyperVM and FluidVM, and the GUI applications ConVirt (formerly XenMan) and Red Hat's Virtual Machine Manager, virt-manager.