Xen
From Wikipedia, the free encyclopedia
 Xen running NetBSD and three Linux distributions |
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| Developed by | The Xen Project, XenSource, Inc. |
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| Latest release | 3.3.1 / 2009-01-05 |
| Operating system | Linux, and other Unix-like, *BSD, OpenSolaris as both hosts and guests; Microsoft Windows as the guest OS. |
| Type | Virtual machine monitor |
| License | GPL |
| Website | http://www.xen.org |
Xen is a 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 concurrently. Xen was initially created by the University of Cambridge Computer Laboratory and is now developed and maintained by the Xen community as free software, licensed under the GNU General Public License (GPL2).
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 can log into dom0 in order to manage 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 x86 virtualization (e.g., Intel VT or AMD-V).[2]
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[edit] History
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 "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, Sun Microsystems and Oracle. 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.
[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.
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
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[edit] Paravirtualization, requiring porting of guest systems
In many CPUs, Xen uses paravirtualization, means that the guest OS must be modified inorder to use a special hypercall ABI instead of certain architectural features. Through paravirtualization, it achieves high performance even on its host architecture (x86) which is not possible with traditional virtualization techniques.[5][6]
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 Intel VT (formerly Vanderpool) architecture extensions.[7] Similarly AMD has contributed support for their AMD-V (formerly Pacifica) 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 Intel VT 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.[8]
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] 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.
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 8.04. Hardy Heron, includes Xen version 3.2 and 3.1. Instructions can be found at https://help.ubuntu.com/community/Xen
Debian includes Xen 3.2.1 in its stable release 5.0, Lenny, and has Xen 3.0.3 packages in its oldstable 4.0 release (codenamed Etch).
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]
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 but discontinued
- FreeBSD (Limited, experimental support for Xen 3 in 8-CURRENT [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
Xen version 3.0 introduced the capability to run Microsoft Windows as a guest operating system unmodified if the processor supports hardware virtualization provided by Intel VT (formerly codenamed Vanderpool) or AMD-V (formerly codenamed Pacifica).
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.[9]
[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 Xen Tools, 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.
[edit] Commercial implementations
[edit] See also
- Kernel-based Virtual Machine
- Comparison of platform virtual machines
- x86 virtualization for information on Intel VT and AMD-V
- Hyper-V
[edit] References
- ^ Xen 3.0 User Manual
- ^ Xen OS Compatibility
- ^ "Citrix Systems » Citrix Completes Acquisition of XenSource". Citrix Systems. 2007-07-12. http://www.citrix.com/English/NE/news/news.asp?newsID=683171.
- ^ Microsoft Press Release, 16 July 2006
- ^ Robin and Irvine, "Analysis of the Intel Pentium's Ability to Support a Secure Virtual Machine Monitor", 9th Usenix Security Symposium, 2000
- ^ Gil Neiger, Amy Santoni, Felix Leung, Dion Rodgers, Rich Uhlig. Intel® Virtualization Technology: Software-only virtualization with the IA-32 and Itanium® architectures, Intel® Technology Journal, Volume 10 Issue 03, August 2006.
- ^ Extending Xen with Intel Virtualization Technology, intel.com
- ^ Demystifying Xen HVM
- ^ Xen and the Art of Virtualization, University of Cambridge SOSP'03 paper
[edit] External links
- The Official Xen.org Project Site
- University of Cambridge research page for Xen
- University of Cambridge, Technical Report Number 553, January 2003. Early report outlining the architecture of Xen.
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