Xen
Encyclopedia
Xen (ˈzɛn) is a virtual-machine monitor
providing services that allow multiple computer operating system
s to execute on the same computer hardware
concurrently.
The University of Cambridge Computer Laboratory
developed the first versions of Xen. Since 2010, the Xen community develops and maintains Xen as free software
, licensed under the GNU General Public License
(GPLv2). Xen is available for the IA-32
, x86-64
, Itanium
and ARM
computer architectures.
In Xen systems the Xen hypervisor
is the lowest and most privileged software layer. This layer supports one or more guest operating systems, scheduled on the physical CPUs. The first guest operating system, called in Xen terminology domain 0 (dom0) is executed automatically when the hypervisor boots and receives special management privileges and direct access to all physical hardware by default. The system administrator can log into dom0 in order to manage any additional guest operating systems, called user domains (domU) in Xen terminology.
The dom0 domain is typically a modified version of Linux
, NetBSD
or Solaris. User domains may either be unmodified copies of open-source or proprietary operating systems, such as Microsoft Windows
, if the host processor supports x86 virtualization
, e.g., Intel VT-x and AMD-V), or modified, para-virtualized operating system with special drivers that support enhanced Xen features.
, 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 occurred in 2003. Citrix Systems
acquired XenSource, Inc in October 2007 and subsequently renamed Xensource's products under the Citrix brand:
Subsequently the product lines have been renamed XenServer (Free), Essentials for XenServer Enterprise, and Essentials for XenServer Platinum.
On 22 October 2007, Citrix Systems
completed its acquisition of XenSource, and the Xen project moved to http://www.xen.org/. This move had started some time previously, and made public the existence of the Xen Project Advisory Board (Xen AB), which had members from Citrix, IBM
, Intel, Hewlett-Packard
, Novell
, Red Hat
, Sun Microsystems
and Oracle
. The Xen AB charter assigns it 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 Xen AB will have sole responsibility for the requirements of licensing.
On 21 October 2009, Citrix further announced their, now commercial, applications of XenServer would be fully open-source and made freely available to the public under the name Xen Cloud Platform (XCP). Simon Crosby, CTO of Virtualization and Management division at Citrix, stated: "XenServer is 100% free, and also shortly fully open sourced. There is no revenue from it at all."
The product lines were renamed in Spring 2010 :
s) often operate on mainframes
and large servers running IBM
, HP
, and other systems. Internet hosting service
companies increasingly use hypervisors to provide virtual private server
s. Amazon EC2
, Fujitsu Global Cloud Platform
, Linode
, and Rackspace Cloud use Xen as the VM hypervisor.
Server virtualization can provide benefits such as:
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 development (including the development of operating systems): running the new system as a guest avoids the need to reboot the physical computer whenever a bug occurs. Sandboxed
guest systems can also help in computer-security research, allowing study of 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 run as a virtualization platform, such as Citrix XenServer Enterprise Edition (formerly XenSource's XenEnterprise), or embedded within the host operating system. As an example of the latter configuration, note the inclusion of Xen in:
In 2007, XenSource announced that Dell would incorporate Citrix XenServer OEM Edition as an embedded hypervisor installed in flash memory as an option on all PowerEdge servers, early in 2008. By May 2010 however, Citrix and their OEM partners (including Dell) decided to retire the flash memory embedded OEM XenServer editions. Only the XenServer 5.6 Retail Edition is available for PowerEdge servers from Dell. Customers currently running the Dell OEM XenServer 5.5 edition (or earlier) can upgrade to preserve their virtual machines' configuration data.
XenSource has also started developing a compatibility layer for Windows Server 2008 hypervisor, so that systems modified to run as Xen guests will also function on the 2008 hypervisor.
, meaning that guests run a modified operating system using a special hypercall ABI
instead of certain architectural features. Through paravirtualization, Xen can achieve high performance even on its host architecture (x86) which has a reputation for non-cooperation with traditional virtualization techniques.
On x86, the Xen host kernel code runs in Ring 0
, while the hosted domains run in Ring 1
or in Ring 3
.
This development allows the virtualization of proprietary operating systems (such as Microsoft Windows), since the guest system's kernel does not require modification when the host runs on Intel VT-x or AMD-V hardware.
Hardware-assisted virtualization (HVM) 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, with its "hardware" accesses under complete control of the hypervisor.
Xen-HVM has device emulation based on the QEMU
project to provide I/O virtualization to the virtual machines. The system emulates hardware 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.
Xen 3.0.2 supports the following unmodified guests:
or newer processors, x86-64
based systems, as well as on IA-64 and PowerPC
. Xen supports up to 64-way symmetric multiprocessing
.
Xen boots from a bootloader such as GNU GRUB
, and then usually loads a modified host operating system into the host domain (dom0). , most Linux distributions had included Xen packages to interact with the Xen hypervisor and start additional domains, but because Xen was not accepted into the mainline Linux kernel and installation required several kernel patches, some distros such as Red Hat Enterprise Linux 6 and Ubuntu 8.10 dropped out-of-the-box support for dom0 in subsequent releases. With the inclusion of the most significant parts of Xen in the Linux 2.6.37 mainline kernel in early 2011, several distributions are again considering dom0 support. Version 3.0 of the Linux kernel supports dom0 and domU in the mainline kernel.
as a guest operating system unmodified if the host machine's processor supports hardware virtualization
provided by Intel VT-x (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 — made possible by Microsoft
's Academic Licensing Program. The terms of this license do not allow the publication of this port, although documentation of the experience appears in the original Xen SOSP paper.
James Harper and the Xen open-source community have started developing GPL'd Paravirtualisation drivers for Windows. These provide front-end drivers for the Xen block and network devices, and allow much higher disk and network performance for Windows systems running in HVM mode. Without these drivers all disk and network traffic has to be processed through QEMU-DM.
Novell
's PlateSpin
Orchestrate also manages Xen virtual machines for Xen shipping in SUSE
Linux Enterprise Server.
Hypervisor
In computing, a hypervisor, also called virtual machine manager , is one of many hardware virtualization techniques that allow multiple operating systems, termed guests, to run concurrently on a host computer. It is so named because it is conceptually one level higher than a supervisory program...
providing services that allow multiple computer operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s to execute on the same computer hardware
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...
concurrently.
The University of Cambridge Computer Laboratory
University of Cambridge Computer Laboratory
The Computer Laboratory is the computer science department of the University of Cambridge. As of 2007, it employs 35 academic staff, 25 support staff, 35 affiliated research staff, and about 155 research students...
developed the first versions of Xen. Since 2010, the Xen community develops and maintains Xen as free software
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
, licensed under the GNU General Public License
GNU General Public License
The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
(GPLv2). Xen is available for the IA-32
IA-32
IA-32 , also known as x86-32, i386 or x86, is the CISC instruction-set architecture of Intel's most commercially successful microprocessors, and was first implemented in the Intel 80386 as a 32-bit extension of x86 architecture...
, x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
, Itanium
Itanium
Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems...
and ARM
ARM architecture
ARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced...
computer architectures.
In Xen systems the Xen hypervisor
Hypervisor
In computing, a hypervisor, also called virtual machine manager , is one of many hardware virtualization techniques that allow multiple operating systems, termed guests, to run concurrently on a host computer. It is so named because it is conceptually one level higher than a supervisory program...
is the lowest and most privileged software layer. This layer supports one or more guest operating systems, scheduled on the physical CPUs. The first guest operating system, called in Xen terminology domain 0 (dom0) is executed automatically when the hypervisor boots and receives special management privileges and direct access to all physical hardware by default. The system administrator can log into dom0 in order to manage any additional guest operating systems, called user domains (domU) in Xen terminology.
The dom0 domain is typically a modified version of Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, NetBSD
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
or Solaris. User domains may either be unmodified copies of open-source or proprietary operating systems, such as Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, if the host processor supports x86 virtualization
X86 virtualization
In computing, x86 virtualization is the facility that allows multiple operating systems to simultaneously share x86 processor resources in a safe and efficient manner, a facility generically known as hardware virtualization...
, e.g., Intel VT-x and AMD-V), or modified, para-virtualized operating system with special drivers that support enhanced Xen features.
History
Xen originated as a research project at the University of CambridgeUniversity of Cambridge
The University of Cambridge is a public research university located in Cambridge, United Kingdom. It is the second-oldest university in both the United Kingdom and the English-speaking world , and the seventh-oldest globally...
, led by Ian Pratt, senior lecturer
Lecturer
Lecturer is an academic rank. In the United Kingdom, lecturer is a position at a university or similar institution, often held by academics in their early career stages, who lead research groups and supervise research students, as well as teach...
at Cambridge and founder of XenSource, Inc. This company now supports the development of the open source
Open-source software
Open-source software is computer software that is available in source code form: the source code and certain other rights normally reserved for copyright holders are provided under a software license that permits users to study, change, improve and at times also to distribute the software.Open...
project and also sells enterprise versions of the software. The first public release of Xen occurred in 2003. Citrix Systems
Citrix Systems
Citrix Systems, Inc. is a multinational corporation founded in 1989, that provides server and desktop virtualization, networking, software-as-a-service , and cloud computing technologies, including Xen open source products....
acquired XenSource, Inc in October 2007 and subsequently renamed Xensource's products under the Citrix brand:
- XenExpress became "XenServer Express Edition" and "XenServer OEM Edition" (embedded hypervisor)
- XenServer became "XenServer Standard Edition"
- XenEnterprise became "XenServer Enterprise Edition"
Subsequently the product lines have been renamed XenServer (Free), Essentials for XenServer Enterprise, and Essentials for XenServer Platinum.
On 22 October 2007, Citrix Systems
Citrix Systems
Citrix Systems, Inc. is a multinational corporation founded in 1989, that provides server and desktop virtualization, networking, software-as-a-service , and cloud computing technologies, including Xen open source products....
completed its acquisition of XenSource, and the Xen project moved to http://www.xen.org/. This move had started some time previously, and made public the existence of the Xen Project Advisory Board (Xen AB), which had members from Citrix, IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
, Intel, Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
, Novell
Novell
Novell, Inc. is a multinational software and services company. It is a wholly owned subsidiary of The Attachmate Group. It specializes in network operating systems, such as Novell NetWare; systems management solutions, such as Novell ZENworks; and collaboration solutions, such as Novell Groupwise...
, Red Hat
Red Hat
Red Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide....
, Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
and Oracle
Oracle Corporation
Oracle Corporation is an American multinational computer technology corporation that specializes in developing and marketing hardware systems and enterprise software products – particularly database management systems...
. The Xen AB charter assigns it 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
Hypervisor
In computing, a hypervisor, also called virtual machine manager , is one of many hardware virtualization techniques that allow multiple operating systems, termed guests, to run concurrently on a host computer. It is so named because it is conceptually one level higher than a supervisory program...
; the Xen AB will have sole responsibility for the requirements of licensing.
On 21 October 2009, Citrix further announced their, now commercial, applications of XenServer would be fully open-source and made freely available to the public under the name Xen Cloud Platform (XCP). Simon Crosby, CTO of Virtualization and Management division at Citrix, stated: "XenServer is 100% free, and also shortly fully open sourced. There is no revenue from it at all."
The product lines were renamed in Spring 2010 :
- XenServer [Free edition]
- XenServer Advanced Edition
- XenServer Enterprise Edition
- XenServer Platinum Edition
Release history
| Version | Release date | Notes |
|---|---|---|
| 1.0 | 2003-10-02 | |
| 2.0 | 2004-11-05 | |
| 3.0 | 2005-12-05 |
The releases up to 3.0.4 also added:
|
| 3.1 | 2007-05-18 | |
| 3.2 | 2008-01-17 | PCI passthrough and ACPI S3 standby mode for the host system. |
| 3.3 | 2008-08-24 | Improvements for the PCI passthrough and the power management. |
| 3.4 | 2009-05-18 | Contains a first version of the "Xen Client Initiative", shortly XCI. |
| 4.0 | 2010-04-07 | Makes it possible to use a dom0 Linux kernel, which has been implemented by using PVOps. A Linux kernel of version 2.6.31 has been modified for this purpose, because the official Linux kernel actually doesn't support the usage as dom0 kernel (date July 2010). |
| 4.1 | 2011-03-25 | Some of the improvements: Support for more than 255 processors, better stability. Linux kernel v2.6.37 and onward support usage as dom0 kernel . |
Uses
Virtual machine monitors (also known as hypervisorHypervisor
In computing, a hypervisor, also called virtual machine manager , is one of many hardware virtualization techniques that allow multiple operating systems, termed guests, to run concurrently on a host computer. It is so named because it is conceptually one level higher than a supervisory program...
s) often operate on mainframes
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
and large servers running IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
, HP
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
, and other systems. Internet hosting service
Internet hosting service
An Internet hosting service is a service that runs Internet servers, allowing organizations and individuals to serve content to the Internet. There are various levels of service and various kinds of services offered....
companies increasingly use hypervisors to provide virtual private server
Virtual private server
Virtual private server is a term used by internet hosting services to refer to a virtual machine. The term is used for emphasizing that the virtual machine, although running in software on the same physical computer as other customers' virtual machines, is functionally equivalent to a separate...
s. Amazon EC2
Amazon Elastic Compute Cloud
Amazon Elastic Compute Cloud is a central part of Amazon.com's cloud computing platform, Amazon Web Services . EC2 allows users to rent virtual computers on which to run their own computer applications...
, Fujitsu Global Cloud Platform
Fujitsu Global Cloud Platform
Fujitsu Global Cloud Platform is a Fujitsu cloud computing platform that aims to deliver standardized enterprise-class public cloud services globally....
, Linode
Linode
Linode is a web application hosting and cloud computing provider based in Galloway Township, New Jersey. Linode allows system administrators and software developers to rent virtual servers on which to run their own computer applications...
, and Rackspace Cloud use Xen as the VM hypervisor.
Server virtualization can provide benefits such as:
- consolidation
- increased utilization
- rapid provisioning
- dynamic fault tolerance against software failures (through rapid bootstrapping or rebooting)
- hardware fault tolerance (through migration of a virtual machine to different hardware)
- the ability to securely separate virtual operating systems
- 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
Load balancing (computing)
Load balancing is a computer networking methodology to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid...
and the avoidance of downtime.
Virtualization also has benefits when working on development (including the development of operating systems): running the new system as a guest avoids the need to reboot the physical computer whenever a bug occurs. Sandboxed
Sandbox (computer security)
In computer security, a sandbox is a security mechanism for separating running programs. It is often used to execute untested code, or untrusted programs from unverified third-parties, suppliers, untrusted users and untrusted websites....
guest systems can also help in computer-security research, allowing study of the effects of some virus
Computer virus
A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...
or worm
Computer worm
A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus, it does not need to attach...
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 run as a virtualization platform, such as Citrix XenServer Enterprise Edition (formerly XenSource's XenEnterprise), or embedded within the host operating system. As an example of the latter configuration, note the inclusion of Xen in:
- NovellNovellNovell, Inc. is a multinational software and services company. It is a wholly owned subsidiary of The Attachmate Group. It specializes in network operating systems, such as Novell NetWare; systems management solutions, such as Novell ZENworks; and collaboration solutions, such as Novell Groupwise...
's SUSE Linux Enterprise 10 distribution - Red HatRed HatRed Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide....
's RHEL 5/Fedora 7 - Sun MicrosystemsSun MicrosystemsSun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
' Solaris - DebianDebianDebian is a computer operating system composed of software packages released as free and open source software primarily under the GNU General Public License along with other free software licenses. Debian GNU/Linux, which includes the GNU OS tools and Linux kernel, is a popular and influential...
's Etch release
In 2007, XenSource announced that Dell would incorporate Citrix XenServer OEM Edition as an embedded hypervisor installed in flash memory as an option on all PowerEdge servers, early in 2008. By May 2010 however, Citrix and their OEM partners (including Dell) decided to retire the flash memory embedded OEM XenServer editions. Only the XenServer 5.6 Retail Edition is available for PowerEdge servers from Dell. Customers currently running the Dell OEM XenServer 5.5 edition (or earlier) can upgrade to preserve their virtual machines' configuration data.
XenSource has also started developing a compatibility layer for Windows Server 2008 hypervisor, so that systems modified to run as Xen guests will also function on the 2008 hypervisor.
Paravirtualization, requiring porting of guest systems
On most CPUs, Xen uses a form of virtualization known as paravirtualizationParavirtualization
In computing, paravirtualization is a virtualization technique that presents a software interface to virtual machines that is similar but not identical to that of the underlying hardware....
, meaning that guests run a modified operating system using a special hypercall ABI
Application binary interface
In computer software, an application binary interface describes the low-level interface between an application program and the operating system or another application.- Description :...
instead of certain architectural features. Through paravirtualization, Xen can achieve high performance even on its host architecture (x86) which has a reputation for non-cooperation with traditional virtualization techniques.
On x86, the Xen host kernel code runs in Ring 0
Ring (computer security)
In computer science, hierarchical protection domains, often called protection rings, are a mechanism to protect data and functionality from faults and malicious behaviour . This approach is diametrically opposite to that of capability-based security.Computer operating systems provide different...
, while the hosted domains run in Ring 1
Ring (computer security)
In computer science, hierarchical protection domains, often called protection rings, are a mechanism to protect data and functionality from faults and malicious behaviour . This approach is diametrically opposite to that of capability-based security.Computer operating systems provide different...
or in Ring 3
Ring (computer security)
In computer science, hierarchical protection domains, often called protection rings, are a mechanism to protect data and functionality from faults and malicious behaviour . This approach is diametrically opposite to that of capability-based security.Computer operating systems provide different...
.
Hardware-assisted virtualization, allowing for unmodified guests
Both Intel and AMD have contributed modifications to Xen to support their respective Intel VT-x and AMD-V architecture extensions. Though these technologies differ quite substantially in their implementation and instruction sets, Xen manages them via a common abstraction layer, enabling unmodified guest operating systems to run within Xen virtual machines, starting with Xen 3.0.This development allows the virtualization of proprietary operating systems (such as Microsoft Windows), since the guest system's kernel does not require modification when the host runs on Intel VT-x or AMD-V hardware.
Hardware-assisted virtualization (HVM) 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, with its "hardware" accesses under complete control of the hypervisor.
Xen-HVM has device emulation based on the QEMU
QEMU
QEMU is a processor emulator that relies on dynamic binary translation to achieve a reasonable speed while being easy to port on new host CPU architectures....
project to provide I/O virtualization to the virtual machines. The system emulates hardware 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
Physical Address Extension
In computing, Physical Address Extension is a feature to allow x86 processors to access a physical address space larger than 4 gigabytes....
, and somewhat limited ACPI
Advanced Configuration and Power Interface
In computing, the Advanced Configuration and Power Interface specification provides an open standard for device configuration and power management by the operating system....
and APIC
Advanced Programmable Interrupt Controller
In computing, an Advanced Programmable Interrupt Controller is a more complex Programmable Interrupt Controller than Intel's original types such as the 8259A...
support and no SCSI emulation.
Xen 3.0.2 supports the following unmodified guests:
- certain versions of Microsoft Windows (including Windows XP)
- Linux.
Virtual machine migration
Administrators can "live migrate" Xen virtual machines between physical hosts across a LAN without loss of availability. During this procedure, the LAN iteratively copies the memory of the virtual machine to the destination without stopping its execution. The process requires a stoppage of around 60–300 ms to perform final synchronization before the virtual machine begins executing at its final destination, providing an illusion of seamless migration. Similar technology can serve to suspend running virtual machines to disk and switch to another virtual machine, resuming the first virtual machine at a later date.Host: Unix-like systems
Xen under Linux runs on x86, with Pentium IIPentium II
The Pentium II brand refers to Intel's sixth-generation microarchitecture and x86-compatible microprocessors introduced on May 7, 1997. Containing 7.5 million transistors, the Pentium II featured an improved version of the first P6-generation core of the Pentium Pro, which contained 5.5 million...
or newer processors, x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
based systems, as well as on IA-64 and PowerPC
PowerPC
PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...
. Xen supports up to 64-way symmetric multiprocessing
Symmetric multiprocessing
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...
.
Xen boots from a bootloader such as GNU GRUB
GNU GRUB
GNU GRUB is a boot loader package from the GNU Project. GRUB is the reference implementation of the Multiboot Specification, which provides a user the choice to boot one of multiple operating systems installed on a computer or select a specific kernel configuration available on a particular...
, and then usually loads a modified host operating system into the host domain (dom0). , most Linux distributions had included Xen packages to interact with the Xen hypervisor and start additional domains, but because Xen was not accepted into the mainline Linux kernel and installation required several kernel patches, some distros such as Red Hat Enterprise Linux 6 and Ubuntu 8.10 dropped out-of-the-box support for dom0 in subsequent releases. With the inclusion of the most significant parts of Xen in the Linux 2.6.37 mainline kernel in early 2011, several distributions are again considering dom0 support. Version 3.0 of the Linux kernel supports dom0 and domU in the mainline kernel.
- Novell's SUSE Linux Enterprise ServerSUSE Linux Enterprise ServerSUSE Linux Enterprise Server is a Linux distribution supplied by SUSE and targeted at the business market. It is targeted for servers, mainframes, and workstations but can be installed on desktop computers for testing as well. New major versions are released at an interval of 3-4 years, while...
release 10 pioneered the first commercial implementation of Xen in this form. - Red Hat Enterprise LinuxRed Hat Enterprise LinuxRed Hat Enterprise Linux is a Linux-based operating system developed by Red Hat and targeted toward the commercial market. Red Hat Enterprise Linux is released in server versions for x86, x86-64, Itanium, PowerPC and IBM System z, and desktop versions for x86 and x86-64...
5 also offers support for Xen, but in RHEL 6 there is no support for dom0. - FedoraFedora (operating system)Fedora is a RPM-based, general purpose collection of software, including an operating system based on the Linux kernel, developed by the community-supported Fedora Project and sponsored by Red Hat...
has host support for Xen from the FC4 to Fedora 8 releases, but Fedora doesn't support dom0. - Ubuntu included Xen packages from Ubuntu 8.04 (Hardy Heron), supporting Xen versions 3.2 and 3.1. Note however that Ubuntu doesn't include or support a dom0-capable kernel from Intrepid Ibex (Ubuntu 8.10) onward.
- DebianDebianDebian is a computer operating system composed of software packages released as free and open source software primarily under the GNU General Public License along with other free software licenses. Debian GNU/Linux, which includes the GNU OS tools and Linux kernel, is a popular and influential...
includes Xen 4.0.1 in its stable release 6.0 (codename "Squeeze"), and Xen 3.2.1 in its oldstable release 5.0 (codename "Lenny"). - GentooGentoo LinuxGentoo Linux is a computer operating system built on top of the Linux kernel and based on the Portage package management system. It is distributed as free and open source software. Unlike a conventional software distribution, the user compiles the source code locally according to their chosen...
and Arch LinuxArch LinuxArch Linux is an independently developed, Linux-based operating system for i686 and x86-64 computers. It is composed predominantly of free and open source software, and supports community involvement....
both have packages available to support Xen. - openSUSEOpenSUSEopenSUSE is a general purpose operating system built on top of the Linux kernel, developed by the community-supported openSUSE Project and sponsored by SUSE...
10.x and 11.x includes Xen 3.1 support. YaST and virt-man offer graphical VM management. - OpenSolarisOpenSolarisOpenSolaris was an open source computer operating system based on Solaris created by Sun Microsystems. It was also the name of the project initiated by Sun to build a developer and user community around the software...
-based distributions can function as dom0 and domU from Nevada build 75 onwards. support for OpenBSDOpenBSDOpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley. It was forked from NetBSD by project leader Theo de Raadt in late 1995...
self-hosting required the correction of a lock-up bug before its release. - NetBSDNetBSDNetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
3.x. includes host support for Xen 2, with host support for Xen 3.0 available from NetBSD 4.0.
Unix-like systems as guests
Guest systems can run fully virtualized (which requires hardware support) or paravirtualized (which requires a modified guest operating system). The following systems have patches allowing them to operate as paravirtualized Xen guests:- LinuxLinux kernelThe Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
, paravirtualization integrated in 2.6.23, patches for other versions exist - MinixMinixMINIX is a Unix-like computer operating system based on a microkernel architecture created by Andrew S. Tanenbaum for educational purposes; MINIX also inspired the creation of the Linux kernel....
- Plan 9 from Bell LabsPlan 9 from Bell LabsPlan 9 from Bell Labs is a distributed operating system. It was developed primarily for research purposes as the successor to Unix by the Computing Sciences Research Center at Bell Labs between the mid-1980s and 2002...
- NetBSDNetBSDNetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
(NetBSD 2.0 has support for Xen 1.2, NetBSD 3.0 has support for Xen 2.0, NetBSD 3.1 supports Xen 3.0, NetBSD 5.0 features Xen 3.3) - OpenBSDOpenBSDOpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley. It was forked from NetBSD by project leader Theo de Raadt in late 1995...
, announced in a, now dead, blog entry The support has since been discontinued. - FreeBSDFreeBSDFreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
(Limited, experimental support for Xen 3 in 8-CURRENT http://wiki.freebsd.org/FreeBSD/Xen) - OpenSolarisOpenSolarisOpenSolaris was an open source computer operating system based on Solaris created by Sun Microsystems. It was also the name of the project initiated by Sun to build a developer and user community around the software...
(See The Xen Community On OpenSolaris) - NetWare (at Brainshare 2005, NovellNovellNovell, Inc. is a multinational software and services company. It is a wholly owned subsidiary of The Attachmate Group. It specializes in network operating systems, such as Novell NetWare; systems management solutions, such as Novell ZENworks; and collaboration solutions, such as Novell Groupwise...
showed a port that can run as a Xen guest) - GNU/Hurd/Mach (gnumach-1-branch-Xen-branch)
- OZONEOZONEOZONE is an object-oriented operating system written in the C programming language. Almost everything in the kernel is an object to which can be assigned logical names....
(has support for Xen v1.2)
Microsoft Windows systems as guests
Xen version 3.0 introduced the capability to run Microsoft WindowsMicrosoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
as a guest operating system unmodified if the host machine's processor supports hardware virtualization
X86 virtualization
In computing, x86 virtualization is the facility that allows multiple operating systems to simultaneously share x86 processor resources in a safe and efficient manner, a facility generically known as hardware virtualization...
provided by Intel VT-x (formerly codenamed Vanderpool) or AMD-V (formerly codenamed Pacifica).
During the development of Xen 1.x, Microsoft Research
Microsoft Research
Microsoft Research is the research division of Microsoft created in 1991 for developing various computer science ideas and integrating them into Microsoft products. It currently employs Turing Award winners C.A.R. Hoare, Butler Lampson, and Charles P...
, along with the University of Cambridge Operating System group, developed a port of Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
to Xen — made possible by Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
's Academic Licensing Program. The terms of this license do not allow the publication of this port, although documentation of the experience appears in the original Xen SOSP paper.
James Harper and the Xen open-source community have started developing GPL'd Paravirtualisation drivers for Windows. These provide front-end drivers for the Xen block and network devices, and allow much higher disk and network performance for Windows systems running in HVM mode. Without these drivers all disk and network traffic has to be processed through QEMU-DM.
Xen Management Consoles
Third-party developers have built a number of tools (known as Xen Management Consoles) to facilitate the common tasks of administering a Xen host, such as configuring, starting, monitoring and stopping of Xen guests. Examples include:- Servarica WHMCS XenServer Module SWXSM
- SolusVM
- Xen Tools
- Ganeti
- the PerlPerlPerl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
-based MLN - the web-based HyperVMHyperVMHyperVM is a multi-tiered, multi-server, multi-virtualization software product allowing a VPS vendor to provision, manage and delegate Xen or OpenVZ based virtual private servers. HyperVM also comes integrated with Kloxo hosting control panel which means that, using HyperVM, the provider can deploy...
and FluidVM - the web-based Cloudmin
- the GUIGraphical user interfaceIn computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
applications Convirture (formerly XenManXenManXenMan is a Xen Hypervisor management tool with a graphical user interface that allows a user to perform the standard set of operations in addition to some higher level operations such as the creation of a guest domain XenMan is a Xen Hypervisor management tool with a graphical user interface that...
) and Red HatRed HatRed Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide....
's Virtual Machine Manager, virt-manager. - the OpenNebulaOpenNebulaOpenNebula is an open-source cloud computing toolkit for managing heterogeneous distributed data center infrastructures. The OpenNebula toolkit manages a data center's virtual infrastructure to build private, public and hybrid IaaS clouds...
cloud management toolkit - OpenXenManager - An open source clone of Citrix's XenCenter.
- XVP - Web-based VM manager for XenServer and Xen Cloud Platform
Novell
Novell
Novell, Inc. is a multinational software and services company. It is a wholly owned subsidiary of The Attachmate Group. It specializes in network operating systems, such as Novell NetWare; systems management solutions, such as Novell ZENworks; and collaboration solutions, such as Novell Groupwise...
's PlateSpin
PlateSpin
PlateSpin is a subsidiary of Novell headquartered in Toronto, Canada, which produces tools to help manage virtualization on VMware, Microsoft Hyper-V or Citrix XenServer.- History :PlateSpin won an International Stevie Award in 2006 for best overall company....
Orchestrate also manages Xen virtual machines for Xen shipping in SUSE
SUSE Linux distributions
SUSE Linux is a computer operating system. It is built on top of the open source Linux kernel and is distributed with system and application software from other open source projects. SUSE Linux is of German origin and mainly developed in Europe. The first version appeared in early 1994, making...
Linux Enterprise Server.
Commercial implementations
- Citrix XenServer
- Oracle VMOracle VMOracle VM is the server virtualization offering from Oracle Corporation. Oracle VM is based on the open-source Xen hypervisor technology, supports Windows, Linux, and Oracle Solaris guests and includes an integrated Web based management console. Oracle VM features fully tested and certified Oracle...
- Sun xVMSun xVMSun xVM is a product group from Sun Microsystems that addresses virtualization technology on x86 platforms.-History:Sun originally announced the xVM product family in October 2007 as a broader product line...
- Virtual IronVirtual IronVirtual Iron Software, was located in Lowell, Massachusetts, sold proprietary software for virtualization and management of a virtual infrastructure...
- Thinsy Corporation
See also
- Kernel-based Virtual MachineKernel-based Virtual MachineKernel-based Virtual Machine is a virtualization infrastructure for the Linux kernel. KVM supports native virtualization on processors with hardware virtualization extensions....
(KVM) - Comparison of platform virtual machines
- Hyper-VHyper-VMicrosoft Hyper-V, codenamed Viridian and formerly known as Windows Server Virtualization, is a hypervisor-based virtualization system for x86-64 systems. A beta version of Hyper-V was shipped with certain x86-64 editions of Windows Server 2008, and the finalized version was released on June 26,...
- VMware ESXVMware ESXVMware ESX is an enterprise-level computer virtualization product offered by VMware, Inc. ESX is a component of VMware's larger offering, VMware Infrastructure, and adds management and reliability services to the core server product...
- Red Hat Enterprise VirtualizationRed Hat Enterprise VirtualizationRed Hat Enterprise Virtualization is an enterprise virtualization product produced by Red Hat.It is based on the KVM hypervisor, Red Hat Enterprise Virtualization uses the SPICE protocol and VDSM with a Windows-based centralized management server...
- Virtual disk imageVirtual disk imageA virtual disk image is a file on a physical disk, which has a well-defined, published or proprietary, format and is interpreted by a Virtual Machine Monitor as a hard disk. IT administrators and software developers administer them through offline operations using built-in or third-party tools...
External links
- Official website
- Official Citrix product site
- Step-by-step installation instructions for Xen, with a DomU capable of running Windows XP SP3 and ReactOS
- University of Cambridge research page for Xen
- University of Cambridge, Technical Report Number 553, January 2003. Early report outlining the architecture of Xen.
- Xen Guide by Tapas Mishra
- UnifiedSessionsManager - An Unofficial Xen configuration file creator