Users' Manual :Xen v3.0
DISCLAIMER: This documentation is always under active development and as such there may be mistakes and omissions -- watch out for these and please report any you find to the developers' mailing list, xen-devel@lists.xensource.com. The latest version is always available on-line. Contributions of material, suggestions and corrections are welcome.Xen is Copyright ©2002-2005, University of Cambridge, UK, XenSource Inc., IBM Corp., Hewlett-Packard Co., Intel Corp., AMD Inc., and others. All rights reserved.
Xen is an open-source project. Most portions of Xen are licensed for copying under the terms of the GNU General Public License, version 2. Other portions are licensed under the terms of the GNU Lesser General Public License, the Zope Public License 2.0, or under ``BSD-style'' licenses. Please refer to the COPYING file for details.
Contents
[*]
[*]1. Introduction
[*]1.1 Usage Scenarios
[*]1.2 Operating System Support
[*]1.3 Hardware Support
[*]1.4 Structure of a Xen-Based System
[*]1.5 History
[*]1.6 What's New
[*]1 Installation
[*]2. Basic Installation
[*]2.1 Prerequisites
[*]2.2 Installing from Binary Tarball
[*]2.3 Installing from RPMs
[*]2.4 Installing from Source
[*]2.5 Configuration
[*]2.6 Booting Xen
[*]3. Booting a Xen System
[*]3.1 Booting Domain0
[*]3.2 Booting Guest Domains
[*]3.3 Starting / Stopping Domains Automatically
[*]2 Configuration and Management
[*]4. Domain Management Tools
[*]4.1 Xend
[*]4.2 Xm
[*]5. Domain Configuration
[*]5.1 Configuration Files
[*]5.2 Network Configuration
[*]6. Storage and File System Management
[*]6.1 Exporting Physical Devices as VBDs
[*]6.2 Using File-backed VBDs
[*]6.3 Using LVM-backed VBDs
[*]6.4 Using NFS Root
[*]7. CPU Management
[*]8. Migrating Domains
[*]8.1 Domain Save and Restore
[*]8.2 Migration and Live Migration
[*]9. Securing Xen
[*]9.1 Xen Security Considerations
[*]9.2 Security Scenarios
[*]3 Reference
[*]10. Build and Boot Options
[*]10.1 Top-level Configuration Options
[*]10.2 Xen Build Options
[*]10.3 Xen Boot Options
[*]10.4 XenLinux Boot Options
[*]11. Further Support
[*]11.1 Other Documentation
[*]11.2 Online References
[*]11.3 Mailing Lists
[*]A. Glossary of Terms
=10000 =10000 1.1
1. Introduction
Xen is an open-source para-virtualizing virtual machine monitor (VMM), or ``hypervisor'', for the x86 processor architecture. Xen can securely execute multiple virtual machines on a single physical system with close-to-native performance. Xen facilitates enterprise-grade functionality, including:
[*]Virtual machines with performance close to native hardware.
[*]Live migration of running virtual machines between physical hosts.
[*]Up to 32 virtual CPUs per guest virtual machine, with VCPU hotplug.
[*]x86/32, x86/32 with PAE, and x86/64 platform support.
[*]Intel Virtualization Technology (VT-x) for unmodified guest operating systems (including Microsoft Windows).
[*]Excellent hardware support (supports almost all Linux device drivers).
1.1 Usage Scenarios
Usage scenarios for Xen include:
Server Consolidation.
Move multiple servers onto a single physical host with performance and fault isolation provided at the virtual machine boundaries.
Hardware Independence.
Allow legacy applications and operating systems to exploit new hardware.
Multiple OS configurations.
Run multiple operating systems simultaneously, for development or testing purposes.
Kernel Development.
Test and debug kernel modifications in a sand-boxed virtual machine -- no need for a separate test machine.
Cluster Computing.
Management at VM granularity provides more flexibility than separately managing each physical host, but better control and isolation than single-system image solutions, particularly by using live migration for load balancing.
Hardware support for custom OSes.
Allow development of new OSes while benefiting from the wide-ranging hardware support of existing OSes such as Linux.
1.2 Operating System Support
Para-virtualization permits very high performance virtualization, even on architectures like x86 that are traditionally very hard to virtualize.
This approach requires operating systems to be ported to run on Xen. Porting an OS to run on Xen is similar to supporting a new hardware platform, however the process is simplified because the para-virtual machine architecture is very similar to the underlying native hardware. Even though operating system kernels must explicitly support Xen, a key feature is that user space applications and libraries do not require modification.
With hardware CPU virtualization as provided by Intel VT and AMD Pacifica technology, the ability to run an unmodified guest OS kernel is available. No porting of the OS is required, although some additional driver support is necessary within Xen itself. Unlike traditional full virtualization hypervisors, which suffer a tremendous performance overhead, the combination of Xen and VT or Xen and Pacifica technology complement one another to offer superb performance for para-virtualized guest operating systems and full support for unmodified guests running natively on the processor. Full support for VT and Pacifica chipsets will appear in early 2006.
Paravirtualized Xen support is available for increasingly many operating systems: currently, mature Linux support is available and included in the standard distribution. Other OS ports--including NetBSD, FreeBSD and Solaris x86 v10--are nearing completion.
1.3 Hardware Support
Xen currently runs on the x86 architecture, requiring a ``P6'' or newer processor (e.g. Pentium Pro, Celeron, Pentium II, Pentium III, Pentium IV, Xeon, AMD Athlon, AMD Duron). Multiprocessor machines are supported, and there is support for HyperThreading (SMT). In addition, ports to IA64 and Power architectures are in progress.
The default 32-bit Xen supports up to 4GB of memory. However Xen 3.0 adds support for Intel's Physical Addressing Extensions (PAE), which enable x86/32 machines to address up to 64 GB of physical memory. Xen 3.0 also supports x86/64 platforms such as Intel EM64T and AMD Opteron which can currently address up to 1TB of physical memory.
Xen offloads most of the hardware support issues to the guest OS running in the Domain 0 management virtual machine. Xen itself contains only the code required to detect and start secondary processors, set up interrupt routing, and perform PCI bus enumeration. Device drivers run within a privileged guest OS rather than within Xen itself. This approach provides compatibility with the majority of device hardware supported by Linux. The default XenLinux build contains support for most server-class network and disk hardware, but you can add support for other hardware by configuring your XenLinux kernel in the normal way.
1.4 Structure of a Xen-Based System
A Xen system has multiple layers, the lowest and most privileged of which is Xen itself.
Xen may host multiple guest operating systems, each of which is executed within a secure virtual machine. In Xen terminology, a domain. Domains are scheduled by Xen to make effective use of the available physical CPUs. Each guest OS manages its own applications. This management includes the responsibility of scheduling each application within the time allotted to the VM by Xen.
The first domain, domain 0, is created automatically when the system boots and has special management privileges. Domain 0 builds other domains and manages their virtual devices. It also performs administrative tasks such as suspending, resuming and migrating other virtual machines.
Within domain 0, a process called xend runs to manage the system. Xend is responsible for managing virtual machines and providing access to their consoles. Commands are issued to xend over an HTTP interface, via a command-line tool.
1.5 History
Xen was originally developed by the Systems Research Group at the University of Cambridge Computer Laboratory as part of the XenoServers project, funded by the UK-EPSRC.
XenoServers aim to provide a ``public infrastructure for global distributed computing''. Xen plays a key part in that, allowing one to efficiently partition a single machine to enable multiple independent clients to run their operating systems and applications in an environment. This environment provides protection, resource isolation and accounting. The project web page contains further information along with pointers to papers and technical reports: http://www.cl.cam.ac.uk/xeno
Xen has grown into a fully-fledged project in its own right, enabling us to investigate interesting research issues regarding the best techniques for virtualizing resources such as the CPU, memory, disk and network. Project contributors now include XenSource, Intel, IBM, HP, AMD, Novell, RedHat.
Xen was first described in a paper presented at SOSP in 20031.1, and the first public release (1.0) was made that October. Since then, Xen has significantly matured and is now used in production scenarios on many sites.
1.6 What's New
Xen 3.0.0 offers:
[*]Support for up to 32-way SMP guest operating systems
[*]Intel (Physical Addressing Extensions) PAE to support 32-bit servers with more than 4GB physical memory
[*]x86/64 support (Intel EM64T, AMD Opteron)
[*]Intel VT-x support to enable the running of unmodified guest operating systems (Windows XP/2003, Legacy Linux)
[*]Enhanced control tools
[*]Improved ACPI support
[*]AGP/DRM graphics
Xen 3.0 features greatly enhanced hardware support, configuration flexibility, usability and a larger complement of supported operating systems. This latest release takes Xen a step closer to being the definitive open source solution for virtualization.
1 Installation
2. Basic Installation
The Xen distribution includes three main components: Xen itself, ports of Linux and NetBSD to run on Xen, and the userspace tools required to manage a Xen-based system. This chapter describes how to install the Xen 3.0 distribution from source. Alternatively, there may be pre-built packages available as part of your operating system distribution.
2.1 Prerequisites
The following is a full list of prerequisites. Items marked `http://tx.downloads.xensource.com/downloads/docs/user/img2.png' are required by the xend control tools, and hence required if you want to run more than one virtual machine; items marked `*' are only required if you wish to build from source.
$$
A working Linux distribution using the GRUB bootloader and running on a P6-class or newer CPU.
http://tx.downloads.xensource.com/downloads/docs/user/img2.png
The iproute2 package.
http://tx.downloads.xensource.com/downloads/docs/user/img2.png
The Linux bridge-utils2.1 (e.g., /sbin/brctl)
http://tx.downloads.xensource.com/downloads/docs/user/img2.png
The Linux hotplug system2.2 (e.g., /sbin/hotplug and related scripts). On newer distributions, this is included alongside the Linux udev system2.3.
*
Build tools (gcc v3.2.x or v3.3.x, binutils, GNU make).
*
Development installation of zlib (e.g., zlib-dev).
*
Development installation of Python v2.2 or later (e.g., python-dev).
*
LATEX and transfig are required to build the documentation.
Once you have satisfied these prerequisites, you can now install either a binary or source distribution of Xen.
2.2 Installing from Binary Tarball
Pre-built tarballs are available for download from the XenSource downloads page:
http://www.xensource.com/downloads/
Once you've downloaded the tarball, simply unpack and install:
# tar zxvf xen-3.0-install.tgz
# cd xen-3.0-install
# sh ./install.sh
Once you've installed the binaries you need to configure your system as described in Section 2.5.
2.3 Installing from RPMs
Pre-built RPMs are available for download from the XenSource downloads page:
http://www.xensource.com/downloads/
Once you've downloaded the RPMs, you typically install them via the RPM commands:
# rpm -iv rpmname
See the instructions and the Release Notes for each RPM set referenced at:
http://www.xensource.com/downloads/.
2.4 Installing from Source
This section describes how to obtain, build and install Xen from source.
2.4.1 Obtaining the Source
The Xen source tree is available as either a compressed source tarball or as a clone of our master Mercurial repository.
Obtaining the Source Tarball
Stable versions and daily snapshots of the Xen source tree are available from the Xen download page:
http://www.xensource.com/downloads/
Obtaining the source via Mercurial
The source tree may also be obtained via the public Mercurial repository at:
http://xenbits.xensource.com See the instructions and the Getting Started Guide referenced at:
http://www.xensource.com/downloads/
2.4.2 Building from Source
The top-level Xen Makefile includes a target ``world'' that will do the following:
[*]Build Xen.
[*]Build the control tools, including xend.
[*]Download (if necessary) and unpack the Linux 2.6 source code, and patch it for use with Xen.
[*]Build a Linux kernel to use in domain 0 and a smaller unprivileged kernel, which can be used for unprivileged virtual machines.
After the build has completed you should have a top-level directory called dist/ in which all resulting targets will be placed. Of particular interest are the two XenLinux kernel images, one with a ``-xen0'' extension which contains hardware device drivers and drivers for Xen's virtual devices, and one with a ``-xenU'' extension that just contains the virtual ones. These are found in dist/install/boot/ along with the image for Xen itself and the configuration files used during the build.
To customize the set of kernels built you need to edit the top-level Makefile. Look for the line:
KERNELS ?= linux-2.6-xen0 linux-2.6-xenU
You can edit this line to include any set of operating system kernels which have configurations in the top-level buildconfigs/ directory.
2.4.3 Custom Kernels
If you wish to build a customized XenLinux kernel (e.g. to support additional devices or enable distribution-required features), you can use the standard Linux configuration mechanisms, specifying that the architecture being built for is xen, e.g:
# cd linux-2.6.12-xen0
# make ARCH=xen xconfig
# cd ..
# make
You can also copy an existing Linux configuration (.config) into e.g. linux-2.6.12-xen0 and execute:
# make ARCH=xen oldconfig
You may be prompted with some Xen-specific options. We advise accepting the defaults for these options.
Note that the only difference between the two types of Linux kernels that are built is the configuration file used for each. The ``U'' suffixed (unprivileged) versions don't contain any of the physical hardware device drivers, leading to a 30% reduction in size; hence you may prefer these for your non-privileged domains. The ``0'' suffixed privileged versions can be used to boot the system, as well as in driver domains and unprivileged domains.
2.4.4 Installing Generated Binaries
The files produced by the build process are stored under the dist/install/ directory. To install them in their default locations, do:
# make install
Alternatively, users with special installation requirements may wish to install them manually by copying the files to their appropriate destinations.
The dist/install/boot directory will also contain the config files used for building the XenLinux kernels, and also versions of Xen and XenLinux kernels that contain debug symbols such as (xen-syms-3.0.0 and vmlinux-syms-2.6.12.6-xen0) which are essential for interpreting crash dumps. Retain these files as the developers may wish to see them if you post on the mailing list.
2.5 Configuration
Once you have built and installed the Xen distribution, it is simple to prepare the machine for booting and running Xen.
2.5.1 GRUB Configuration
An entry should be added to grub.conf (often found under /boot/ or /boot/grub/) to allow Xen / XenLinux to boot. This file is sometimes called menu.lst, depending on your distribution. The entry should look something like the following:
title Xen 3.0 / XenLinux 2.6
kernel /boot/xen-3.0.gz dom0_mem=262144
module /boot/vmlinuz-2.6-xen0 root=/dev/sda4 ro console=tty0
The kernel line tells GRUB where to find Xen itself and what boot parameters should be passed to it (in this case, setting the domain 0 memory allocation in kilobytes and the settings for the serial port). For more details on the various Xen boot parameters see Section 10.3.
The module line of the configuration describes the location of the XenLinux kernel that Xen should start and the parameters that should be passed to it. These are standard Linux parameters, identifying the root device and specifying it be initially mounted read only and instructing that console output be sent to the screen. Some distributions such as SuSE do not require the ro parameter.
To use an initrd, add another module line to the configuration, like:
module /boot/my_initrd.gz
When installing a new kernel, it is recommended that you do not delete existing menu options from menu.lst, as you may wish to boot your old Linux kernel in future, particularly if you have problems.
2.5.2 Serial Console (optional)
Serial console access allows you to manage, monitor, and interact with your system over a serial console. This can allow access from another nearby system via a null-modem (``LapLink'') cable or remotely via a serial concentrator.
You system's BIOS, bootloader (GRUB), Xen, Linux, and login access must each be individually configured for serial console access. It is not strictly necessary to have each component fully functional, but it can be quite useful.
For general information on serial console configuration under Linux, refer to the ``Remote Serial Console HOWTO'' at The Linux Documentation Project: http://www.tldp.org
2.5.2.1 Serial Console BIOS configuration
Enabling system serial console output neither enables nor disables serial capabilities in GRUB, Xen, or Linux, but may make remote management of your system more convenient by displaying POST and other boot messages over serial port and allowing remote BIOS configuration.
Refer to your hardware vendor's documentation for capabilities and procedures to enable BIOS serial redirection.
2.5.2.2 Serial Console GRUB configuration
Enabling GRUB serial console output neither enables nor disables Xen or Linux serial capabilities, but may made remote management of your system more convenient by displaying GRUB prompts, menus, and actions over serial port and allowing remote GRUB management.
Adding the following two lines to your GRUB configuration file, typically either /boot/grub/menu.lst or /boot/grub/grub.conf depending on your distro, will enable GRUB serial output.
serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
terminal --timeout=10 serial console
Note that when both the serial port and the local monitor and keyboard are enabled, the text ``Press any key to continue'' will appear at both. Pressing a key on one device will cause GRUB to display to that device. The other device will see no output. If no key is pressed before the timeout period expires, the system will boot to the default GRUB boot entry.
Please refer to the GRUB documentation for further information.
2.5.2.3 Serial Console Xen configuration
Enabling Xen serial console output neither enables nor disables Linux kernel output or logging in to Linux over serial port. It does however allow you to monitor and log the Xen boot process via serial console and can be very useful in debugging.
In order to configure Xen serial console output, it is necessary to add a boot option to your GRUB config; e.g. replace the previous example kernel line with:
kernel /boot/xen.gz dom0_mem=131072 com1=115200,8n1
This configures Xen to output on COM1 at 115,200 baud, 8 data bits, 1 stop bit and no parity. Modify these parameters for your environment.
One can also configure XenLinux to share the serial console; to achieve this append ``console=ttyS0'' to your module line.
2.5.2.4 Serial Console Linux configuration
Enabling Linux serial console output at boot neither enables nor disables logging in to Linux over serial port. It does however allow you to monitor and log the Linux boot process via serial console and can be very useful in debugging.
To enable Linux output at boot time, add the parameter console=ttyS0 (or ttyS1, ttyS2, etc.) to your kernel GRUB line. Under Xen, this might be:
module /vmlinuz-2.6-xen0 ro root=/dev/VolGroup00/LogVol00 \
console=ttyS0, 115200
to enable output over ttyS0 at 115200 baud.
2.5.2.5 Serial Console Login configuration
Logging in to Linux via serial console, under Xen or otherwise, requires specifying a login prompt be started on the serial port. To permit root logins over serial console, the serial port must be added to /etc/securetty.
To automatically start a login prompt over the serial port, add the line:
c:2345:respawn:/sbin/mingetty ttyS0 to /etc/inittab. Run init q to force a reload of your inttab and start getty.
To enable root logins, add ttyS0 to /etc/securetty if not already present.
Your distribution may use an alternate getty; options include getty, mgetty and agetty. Consult your distribution's documentation for further information.
2.5.3 TLS Libraries
Users of the XenLinux 2.6 kernel should disable Thread Local Storage (TLS) (e.g. by doing a mv /lib/tls /lib/tls.disabled) before attempting to boot a XenLinux kernel2.4. You can always reenable TLS by restoring the directory to its original location (i.e. mv /lib/tls.disabled /lib/tls).
The reason for this is that the current TLS implementation uses segmentation in a way that is not permissible under Xen. If TLS is not disabled, an emulation mode is used within Xen which reduces performance substantially. To ensure full performance you should install a `Xen-friendly' (nosegneg) version of the library.
2.6 Booting Xen
It should now be possible to restart the system and use Xen. Reboot and choose the new Xen option when the Grub screen appears.
What follows should look much like a conventional Linux boot. The first portion of the output comes from Xen itself, supplying low level information about itself and the underlying hardware. The last portion of the output comes from XenLinux.
You may see some error messages during the XenLinux boot. These are not necessarily anything to worry about--they may result from kernel configuration differences between your XenLinux kernel and the one you usually use.
When the boot completes, you should be able to log into your system as usual. If you are unable to log in, you should still be able to reboot with your normal Linux kernel by selecting it at the GRUB prompt.
3. Booting a Xen System
Booting the system into Xen will bring you up into the privileged management domain, Domain0. At that point you are ready to create guest domains and ``boot'' them using the xm create command.
3.1 Booting Domain0
After installation and configuration is complete, reboot the system and and choose the new Xen option when the Grub screen appears.
What follows should look much like a conventional Linux boot. The first portion of the output comes from Xen itself, supplying low level information about itself and the underlying hardware. The last portion of the output comes from XenLinux.
When the boot completes, you should be able to log into your system as usual. If you are unable to log in, you should still be able to reboot with your normal Linux kernel by selecting it at the GRUB prompt.
The first step in creating a new domain is to prepare a root filesystem for it to boot. Typically, this might be stored in a normal partition, an LVM or other volume manager partition, a disk file or on an NFS server. A simple way to do this is simply to boot from your standard OS install CD and install the distribution into another partition on your hard drive.
To start the xend control daemon, type
# xend start
If you wish the daemon to start automatically, see the instructions in Section 4.1. Once the daemon is running, you can use the xm tool to monitor and maintain the domains running on your system. This chapter provides only a brief tutorial. We provide full details of the xm tool in the next chapter.
3.2 Booting Guest Domains
3.2.1 Creating a Domain Configuration File
Before you can start an additional domain, you must create a configuration file. We provide two example files which you can use as a starting point:
[*]/etc/xen/xmexample1 is a simple template configuration file for describing a single VM.
[*]/etc/xen/xmexample2 file is a template description that is intended to be reused for multiple virtual machines. Setting the value of the vmid variable on the xm command line fills in parts of this template.
There are also a number of other examples which you may find useful. Copy one of these files and edit it as appropriate. Typical values you may wish to edit include:
kernel
Set this to the path of the kernel you compiled for use with Xen (e.g. kernel = ``/boot/vmlinuz-2.6-xenU'')
memory
Set this to the size of the domain's memory in megabytes (e.g. memory = 64)
disk
Set the first entry in this list to calculate the offset of the domain's root partition, based on the domain ID. Set the second to the location of /usr if you are sharing it between domains (e.g. disk = ['phy:your_hard_drive%d,sda1,w' % (base_partition_number + vmid), 'phy:your_usr_partition,sda6,r' ]
dhcp
Uncomment the dhcp variable, so that the domain will receive its IP address from a DHCP server (e.g. dhcp=``dhcp'')
You may also want to edit the vif variable in order to choose the MAC address of the virtual ethernet interface yourself. For example:
vif = ['mac=00:16:3E:F6:BB:B3'] If you do not set this variable, xend will automatically generate a random MAC address from the range 00:16:3E:xx:xx:xx, assigned by IEEE to XenSource as an OUI (organizationally unique identifier). XenSource Inc. gives permission for anyone to use addresses randomly allocated from this range for use by their Xen domains.
For a list of IEEE OUI assignments, see http://standards.ieee.org/regauth/oui/oui.txt
3.2.2 Booting the Guest Domain
The xm tool provides a variety of commands for managing domains. Use the create command to start new domains. Assuming you've created a configuration file myvmconf based around /etc/xen/xmexample2, to start a domain with virtual machine ID 1 you should type:
# xm create -c myvmconf vmid=1
The -c switch causes xm to turn into the domain's console after creation. The vmid=1 sets the vmid variable used in the myvmconf file.
You should see the console boot messages from the new domain appearing in the terminal in which you typed the command, culminating in a login prompt.
3.3 Starting / Stopping Domains Automatically
It is possible to have certain domains start automatically at boot time and to have dom0 wait for all running domains to shutdown before it shuts down the system.
To specify a domain is to start at boot-time, place its configuration file (or a link to it) under /etc/xen/auto/.
A Sys-V style init script for Red Hat and LSB-compliant systems is provided and will be automatically copied to /etc/init.d/ during install. You can then enable it in the appropriate way for your distribution.
For instance, on Red Hat:
# chkconfig --add xendomains
By default, this will start the boot-time domains in runlevels 3, 4 and 5.
You can also use the service command to run this script manually, e.g:
# service xendomains start
Starts all the domains with config files under /etc/xen/auto/.
# service xendomains stop
Shuts down all running Xen domains.
2 Configuration and Management
4. Domain Management Tools
This chapter summarizes the management software and tools available.
4.1 Xend
The Xen Daemon (Xend) performs system management functions related to virtual machines. It forms a central point of control for a machine and can be controlled using an HTTP-based protocol. Xend must be running in order to start and manage virtual machines.
Xend must be run as root because it needs access to privileged system management functions. A small set of commands may be issued on the xend command line:
# xend start
start xend, if not already running
# xend stop
stop xend if already running
# xend restart
restart xend if running, otherwise start it
# xend status
indicates xend status by its return code
A SysV init script called xend is provided to start xend at boot time. make install installs this script in /etc/init.d. To enable it, you have to make symbolic links in the appropriate runlevel directories or use the chkconfig tool, where available. Once xend is running, administration can be done using the xm tool.
As xend runs, events will be logged to /var/log/xend.log and /var/log/xend-debug.log. These, along with the standard syslog files, are useful when troubleshooting problems.
4.2 Xm
The xm tool is the primary tool for managing Xen from the console. The general format of an xm command line is:
# xm command
The available switches and arguments are dependent on the command chosen. The variables may be set using declarations of the form variable=value and command line declarations override any of the values in the configuration file being used, including the standard variables described above and any custom variables (for instance, the xmdefconfig file uses a vmid variable).
For online help for the commands available, type:
# xm help
This will list the most commonly used commands. The full list can be obtained using xm help --long. You can also type xm helpfor more information on a given command.
4.2.1 Basic Management Commands
One useful command is # xm list which lists all domains running in rows of the following format:
name domid memory vcpus state cputime
The meaning of each field is as follows:
name
The descriptive name of the virtual machine.
domid
The number of the domain ID this virtual machine is running in.
memory
Memory size in megabytes.
vcpus
The number of virtual CPUs this domain has.
state
Domain state consists of 5 fields:
r
running
b
blocked
p
paused
s
shutdown
c
crashed
cputime
How much CPU time (in seconds) the domain has used so far.
The xm list command also supports a long output format when the -l switch is used. This outputs the fulls details of the running domains in xend's SXP configuration format.
You can get access to the console of a particular domain using the # xm console command (e.g. # xm console myVM).
5. Domain Configuration
The following contains the syntax of the domain configuration files and description of how to further specify networking, driver domain and general scheduling behavior.
5.1 Configuration Files
Xen configuration files contain the following standard variables. Unless otherwise stated, configuration items should be enclosed in quotes: see the configuration scripts in /etc/xen/ for concrete examples.
kernel
Path to the kernel image.
ramdisk
Path to a ramdisk image (optional).
memory
Memory size in megabytes.
vcpus
The number of virtual CPUs.
console
Port to export the domain console on (default 9600 + domain ID).
nics
Number of virtual network interfaces.
vif
List of MAC addresses (random addresses are assigned if not given) and bridges to use for the domain's network interfaces, e.g.
vif = [ 'mac=00:16:3E:00:00:11, bridge=xen-br0',
'bridge=xen-br1' ]
to assign a MAC address and bridge to the first interface and assign a different bridge to the second interface, leaving xend to choose the MAC address.
disk
List of block devices to export to the domain e.g. disk = [ 'phy:hda1,sda1,r' ] exports physical device /dev/hda1 to the domain as /dev/sda1 with read-only access. Exporting a disk read-write which is currently mounted is dangerous - if you are certain you wish to do this, you can specify w! as the mode.
dhcp
Set to `dhcp' if you want to use DHCP to configure networking.
netmask
Manually configured IP netmask.
gateway
Manually configured IP gateway.
hostname
Set the hostname for the virtual machine.
root
Specify the root device parameter on the kernel command line.
nfs_server
IP address for the NFS server (if any).
nfs_root
Path of the root filesystem on the NFS server (if any).
extra
Extra string to append to the kernel command line (if any)
Additional fields are documented in the example configuration files (e.g. to configure virtual TPM functionality).
For additional flexibility, it is also possible to include Python scripting commands in configuration files. An example of this is the xmexample2 file, which uses Python code to handle the vmid variable.
5.2 Network Configuration
For many users, the default installation should work ``out of the box''. More complicated network setups, for instance with multiple Ethernet interfaces and/or existing bridging setups will require some special configuration.
The purpose of this section is to describe the mechanisms provided by xend to allow a flexible configuration for Xen's virtual networking.
5.2.1 Xen virtual network topology
Each domain network interface is connected to a virtual network interface in dom0 by a point to point link (effectively a ``virtual crossover cable''). These devices are named vif. (e.g. vif1.0 for the first interface in domain 1, vif3.1 for the second interface in domain 3).
Traffic on these virtual interfaces is handled in domain 0 using standard Linux mechanisms for bridging, routing, rate limiting, etc. Xend calls on two shell scripts to perform initial configuration of the network and configuration of new virtual interfaces. By default, these scripts configure a single bridge for all the virtual interfaces. Arbitrary routing / bridging configurations can be configured by customizing the scripts, as described in the following section.
5.2.2 Xen networking scripts
Xen's virtual networking is configured by two shell scripts (by default network-bridge and vif-bridge). These are called automatically by xend when certain events occur, with arguments to the scripts providing further contextual information. These scripts are found by default in /etc/xen/scripts. The names and locations of the scripts can be configured in /etc/xen/xend-config.sxp.
network-bridge:
This script is called whenever xend is started or stopped to respectively initialize or tear down the Xen virtual network. In the default configuration initialization creates the bridge `xen-br0' and moves eth0 onto that bridge, modifying the routing accordingly. When xend exits, it deletes the Xen bridge and removes eth0, restoring the normal IP and routing configuration.
vif-bridge:
This script is called for every domain virtual interface and can configure firewalling rules and add the vif to the appropriate bridge. By default, this adds and removes VIFs on the default Xen bridge.
Other example scripts are available (network-route and vif-route, network-nat and vif-nat). For more complex network setups (e.g. where routing is required or integrate with existing bridges) these scripts may be replaced with customized variants for your site's preferred configuration.
6. Storage and File System Management
Storage can be made available to virtual machines in a number of different ways. This chapter covers some possible configurations.
The most straightforward method is to export a physical block device (a hard drive or partition) from dom0 directly to the guest domain as a virtual block device (VBD).
Storage may also be exported from a filesystem image or a partitioned filesystem image as a file-backed VBD.
Finally, standard network storage protocols such as NBD, iSCSI, NFS, etc., can be used to provide storage to virtual machines.
6.1 Exporting Physical Devices as VBDs
One of the simplest configurations is to directly export individual partitions from domain 0 to other domains. To achieve this use the phy: specifier in your domain configuration file. For example a line like
disk = ['phy:hda3,sda1,w'] specifies that the partition /dev/hda3 in domain 0 should be exported read-write to the new domain as /dev/sda1; one could equally well export it as /dev/hda or /dev/sdb5 should one wish.
In addition to local disks and partitions, it is possible to export any device that Linux considers to be ``a disk'' in the same manner. For example, if you have iSCSI disks or GNBD volumes imported into domain 0 you can export these to other domains using the phy: disk syntax. E.g.:
disk = ['phy:vg/lvm1,sda2,w']
http://tx.downloads.xensource.com/downloads/docs/user/img3.png
Block devices should typically only be shared between domains in a read-only fashion otherwise the Linux kernel's file systems will get very confused as the file system structure may change underneath them (having the same ext3 partition mounted rw twice is a sure fire way to cause irreparable damage)! Xend will attempt to prevent you from doing this by checking that the device is not mounted read-write in domain 0, and hasn't already been exported read-write to another domain. If you want read-write sharing, export the directory to other domains via NFS from domain 0 (or use a cluster file system such as GFS or ocfs2).
6.2 Using File-backed VBDs
It is also possible to use a file in Domain 0 as the primary storage for a virtual machine. As well as being convenient, this also has the advantage that the virtual block device will be sparse -- space will only really be allocated as parts of the file are used. So if a virtual machine uses only half of its disk space then the file really takes up half of the size allocated.
For example, to create a 2GB sparse file-backed virtual block device (actually only consumes 1KB of disk):
# dd if=/dev/zero of=vm1disk bs=1k seek=2048k count=1
Make a file system in the disk file:
# mkfs -t ext3 vm1disk
(when the tool asks for confirmation, answer `y')
Populate the file system e.g. by copying from the current root:
# mount -o loop vm1disk /mnt
# cp -ax /{root,dev,var,etc,usr,bin,sbin,lib} /mnt
# mkdir /mnt/{proc,sys,home,tmp}
Tailor the file system by editing /etc/fstab, /etc/hostname, etc. Don't forget to edit the files in the mounted file system, instead of your domain 0 filesystem, e.g. you would edit /mnt/etc/fstab instead of /etc/fstab. For this example put /dev/sda1 to root in fstab.
Now unmount (this is important!):
# umount /mnt
In the configuration file set:
disk = ['file:/full/path/to/vm1disk,sda1,w']
As the virtual machine writes to its `disk', the sparse file will be filled in and consume more space up to the original 2GB.
Note that file-backed VBDs may not be appropriate for backing I/O-intensive domains. File-backed VBDs are known to experience substantial slowdowns under heavy I/O workloads, due to the I/O handling by the loopback block device used to support file-backed VBDs in dom0. Better I/O performance can be achieved by using either LVM-backed VBDs (Section 6.3) or physical devices as VBDs (Section 6.1).
Linux supports a maximum of eight file-backed VBDs across all domains by default. This limit can be statically increased by using the max_loop module parameter if CONFIG_BLK_DEV_LOOP is compiled as a module in the dom0 kernel, or by using the max_loop=n boot option if CONFIG_BLK_DEV_LOOP is compiled directly into the dom0 kernel.
6.3 Using LVM-backed VBDs
A particularly appealing solution is to use LVM volumes as backing for domain file-systems since this allows dynamic growing/shrinking of volumes as well as snapshot and other features.
To initialize a partition to support LVM volumes:
# pvcreate /dev/sda10
Create a volume group named `vg' on the physical partition:
# vgcreate vg /dev/sda10
Create a logical volume of size 4GB named `myvmdisk1':
# lvcreate -L4096M -n myvmdisk1 vg
You should now see that you have a /dev/vg/myvmdisk1 Make a filesystem, mount it and populate it, e.g.:
# mkfs -t ext3 /dev/vg/myvmdisk1
# mount /dev/vg/myvmdisk1 /mnt
# cp -ax / /mnt
# umount /mnt
Now configure your VM with the following disk configuration:
disk = [ 'phy:vg/myvmdisk1,sda1,w' ]
LVM enables you to grow the size of logical volumes, but you'll need to resize the corresponding file system to make use of the new space. Some file systems (e.g. ext3) now support online resize. See the LVM manuals for more details.
You can also use LVM for creating copy-on-write (CoW) clones of LVM volumes (known as writable persistent snapshots in LVM terminology). This facility is new in Linux 2.6.8, so isn't as stable as one might hope. In particular, using lots of CoW LVM disks consumes a lot of dom0 memory, and error conditions such as running out of disk space are not handled well. Hopefully this will improve in future.
To create two copy-on-write clones of the above file system you would use the following commands:
# lvcreate -s -L1024M -n myclonedisk1 /dev/vg/myvmdisk1
# lvcreate -s -L1024M -n myclonedisk2 /dev/vg/myvmdisk1
Each of these can grow to have 1GB of differences from the master volume. You can grow the amount of space for storing the differences using the lvextend command, e.g.:
# lvextend +100M /dev/vg/myclonedisk1
Don't let the `differences volume' ever fill up otherwise LVM gets rather confused. It may be possible to automate the growing process by using dmsetup wait to spot the volume getting full and then issue an lvextend.
In principle, it is possible to continue writing to the volume that has been cloned (the changes will not be visible to the clones), but we wouldn't recommend this: have the cloned volume as a `pristine' file system install that isn't mounted directly by any of the virtual machines.
6.4 Using NFS Root
First, populate a root filesystem in a directory on the server machine. This can be on a distinct physical machine, or simply run within a virtual machine on the same node.
Now configure the NFS server to export this filesystem over the network by adding a line to /etc/exports, for instance:
/export/vm1root 1.2.3.4/24 (rw,sync,no_root_squash)
Finally, configure the domain to use NFS root. In addition to the normal variables, you should make sure to set the following values in the domain's configuration file:
root = '/dev/nfs'
nfs_server = '2.3.4.5' # substitute IP address of server
nfs_root = '/path/to/root' # path to root FS on the server
The domain will need network access at boot time, so either statically configure an IP address using the config variables ip, netmask, gateway, hostname; or enable DHCP (dhcp='dhcp').
Note that the Linux NFS root implementation is known to have stability problems under high load (this is not a Xen-specific problem), so this configuration may not be appropriate for critical servers.
7. CPU Management
Xen allows a domain's virtual CPU(s) to be associated with one or more host CPUs. This can be used to allocate real resources among one or more guests, or to make optimal use of processor resources when utilizing dual-core, hyperthreading, or other advanced CPU technologies.
Xen enumerates physical CPUs in a `depth first' fashion. For a system with both hyperthreading and multiple cores, this would be all the hyperthreads on a given core, then all the cores on a given socket, and then all sockets. I.e. if you had a two socket, dual core, hyperthreaded Xeon the CPU order would be:
socket0
socket1
core0
core1
core0
core1
ht0
ht1
ht0
ht1
ht0
ht1
ht0
ht1
#0
#1
#2
#3
#4
#5
#6
#7
Having multiple vcpus belonging to the same domain mapped to the same physical CPU is very likely to lead to poor performance. It's better to use `vcpus-set' to hot-unplug one of the vcpus and ensure the others are pinned on different CPUs.
If you are running IO intensive tasks, its typically better to dedicate either a hyperthread or whole core to running domain 0, and hence pin other domains so that they can't use CPU 0. If your workload is mostly compute intensive, you may want to pin vcpus such that all physical CPU threads are available for guest domains.
8. Migrating Domains
8.1 Domain Save and Restore
The administrator of a Xen system may suspend a virtual machine's current state into a disk file in domain 0, allowing it to be resumed at a later time.
For example you can suspend a domain called ``VM1'' to disk using the command:
# xm save VM1 VM1.chk
This will stop the domain named ``VM1'' and save its current state into a file called VM1.chk.
To resume execution of this domain, use the xm restore command:
# xm restore VM1.chk
This will restore the state of the domain and resume its execution. The domain will carry on as before and the console may be reconnected using the xm console command, as described earlier.
8.2 Migration and Live Migration
Migration is used to transfer a domain between physical hosts. There are two varieties: regular and live migration. The former moves a virtual machine from one host to another by pausing it, copying its memory contents, and then resuming it on the destination. The latter performs the same logical functionality but without needing to pause the domain for the duration. In general when performing live migration the domain continues its usual activities and--from the user's perspective--the migration should be imperceptible.
To perform a live migration, both hosts must be running Xen / xend and the destination host must have sufficient resources (e.g. memory capacity) to accommodate the domain after the move. Furthermore we currently require both source and destination machines to be on the same L2 subnet.
Currently, there is no support for providing automatic remote access to filesystems stored on local disk when a domain is migrated. Administrators should choose an appropriate storage solution (i.e. SAN, NAS, etc.) to ensure that domain filesystems are also available on their destination node. GNBD is a good method for exporting a volume from one machine to another. iSCSI can do a similar job, but is more complex to set up.
When a domain migrates, it's MAC and IP address move with it, thus it is only possible to migrate VMs within the same layer-2 network and IP subnet. If the destination node is on a different subnet, the administrator would need to manually configure a suitable etherip or IP tunnel in the domain 0 of the remote node.
A domain may be migrated using the xm migrate command. To live migrate a domain to another machine, we would use the command:
# xm migrate --live mydomain destination.ournetwork.com
Without the -live flag, xend simply stops the domain and copies the memory image over to the new node and restarts it. Since domains can have large allocations this can be quite time consuming, even on a Gigabit network. With the -live flag xend attempts to keep the domain running while the migration is in progress, resulting in typical down times of just 60-300ms.
For now it will be necessary to reconnect to the domain's console on the new machine using the xm console command. If a migrated domain has any open network connections then they will be preserved, so SSH connections do not have this limitation.
9. Securing Xen
This chapter describes how to secure a Xen system. It describes a number of scenarios and provides a corresponding set of best practices. It begins with a section devoted to understanding the security implications of a Xen system.
9.1 Xen Security Considerations
When deploying a Xen system, one must be sure to secure the management domain (Domain-0) as much as possible. If the management domain is compromised, all other domains are also vulnerable. The following are a set of best practices for Domain-0:
[*]Run the smallest number of necessary services. The less things that are present in a management partition, the better. Remember, a service running as root in the management domain has full access to all other domains on the system.
[*]Use a firewall to restrict the traffic to the management domain. A firewall with default-reject rules will help prevent attacks on the management domain.
[*]Do not allow users to access Domain-0. The Linux kernel has been known to have local-user root exploits. If you allow normal users to access Domain-0 (even as unprivileged users) you run the risk of a kernel exploit making all of your domains vulnerable.
9.2 Security Scenarios
9.2.1 The Isolated Management Network
In this scenario, each node has two network cards in the cluster. One network card is connected to the outside world and one network card is a physically isolated management network specifically for Xen instances to use.
As long as all of the management partitions are trusted equally, this is the most secure scenario. No additional configuration is needed other than forcing Xend to bind to the management interface for relocation.
9.2.2 A Subnet Behind a Firewall
In this scenario, each node has only one network card but the entire cluster sits behind a firewall. This firewall should do at least the following:
[*]Prevent IP spoofing from outside of the subnet.
[*]Prevent access to the relocation port of any of the nodes in the cluster except from within the cluster.
The following iptables rules can be used on each node to prevent migrations to that node from outside the subnet assuming the main firewall does not do this for you:
# this command disables all access to the Xen relocation
# port:
iptables -A INPUT -p tcp --destination-port 8002 -j REJECT
# this command enables Xen relocations only from the specific
# subnet:
iptables -I INPUT -p tcp -{}-source 192.168.1.1/8 \
--destination-port 8002 -j ACCEPT
9.2.3 Nodes on an Untrusted Subnet
Migration on an untrusted subnet is not safe in current versions of Xen. It may be possible to perform migrations through a secure tunnel via an VPN or SSH. The only safe option in the absence of a secure tunnel is to disable migration completely. The easiest way to do this is with iptables:
# this command disables all access to the Xen relocation port
iptables -A INPUT -p tcp -{}-destination-port 8002 -j REJECT
3 Reference
10. Build and Boot Options
This chapter describes the build- and boot-time options which may be used to tailor your Xen system.
10.1 Top-level Configuration Options
Top-level configuration is achieved by editing one of two files: Config.mk and Makefile.
The former allows the overall build target architecture to be specified. You will typically not need to modify this unless you are cross-compiling or if you wish to build a PAE-enabled Xen system. Additional configuration options are documented in the Config.mk file.
The top-level Makefile is chiefly used to customize the set of kernels built. Look for the line:
KERNELS ?= linux-2.6-xen0 linux-2.6-xenU
Allowable options here are any kernels which have a corresponding build configuration file in the buildconfigs/ directory.
10.2 Xen Build Options
Xen provides a number of build-time options which should be set as environment variables or passed on make's command-line.
verbose=y
Enable debugging messages when Xen detects an unexpected condition. Also enables console output from all domains.
debug=y
Enable debug assertions. Implies verbose=y. (Primarily useful for tracing bugs in Xen).
debugger=y
Enable the in-Xen debugger. This can be used to debug Xen, guest OSes, and applications.
perfc=y
Enable performance counters for significant events within Xen. The counts can be reset or displayed on Xen's console via console control keys.
10.3 Xen Boot Options
These options are used to configure Xen's behaviour at runtime. They should be appended to Xen's command line, either manually or by editing grub.conf.
noreboot
Don't reboot the machine automatically on errors. This is useful to catch debug output if you aren't catching console messages via the serial line.
nosmp
Disable SMP support. This option is implied by `ignorebiostables'.
watchdog
Enable NMI watchdog which can report certain failures.
noirqbalance
Disable software IRQ balancing and affinity. This can be used on systems such as Dell 1850/2850 that have workarounds in hardware for IRQ-routing issues.
badpage=,, ...
Specify a list of pages not to be allocated for use because they contain bad bytes. For example, if your memory tester says that byte 0x12345678 is bad, you would place `badpage=0x12345' on Xen's command line.
com1=,DPS,, com2=,DPS,,
Xen supports up to two 16550-compatible serial ports. For example: `com1=9600, 8n1, 0x408, 5' maps COM1 to a 9600-baud port, 8 data bits, no parity, 1 stop bit, I/O port base 0x408, IRQ 5. If some configuration options are standard (e.g., I/O base and IRQ), then only a prefix of the full configuration string need be specified. If the baud rate is pre-configured (e.g., by the bootloader) then you can specify `auto' in place of a numeric baud rate.
console=
Specify the destination for Xen console I/O. This is a comma-separated list of, for example:
vga
Use VGA console and allow keyboard input.
com1
Use serial port com1.
com2H
Use serial port com2. Transmitted chars will have the MSB set. Received chars must have MSB set.
com2L
Use serial port com2. Transmitted chars will have the MSB cleared. Received chars must have MSB cleared. The latter two examples allow a single port to be shared by two subsystems (e.g. console and debugger). Sharing is controlled by MSB of each transmitted/received character.
sync_console
Force synchronous console output. This is useful if you system fails unexpectedly before it has sent all available output to the console. In most cases Xen will automatically enter synchronous mode when an exceptional event occurs, but this option provides a manual fallback.
conswitch=
Specify how to switch serial-console input between Xen and DOM0. The required sequence is CTRL- pressed three times. Specifying the backtick character disables switching. Thespecifies whether Xen should auto-switch input to DOM0 when it boots -- if it is `x' then auto-switching is disabled. Any other value, or omitting the character, enables auto-switching.
nmi=xxx
Specify what to do with an NMI parity or I/O error.
`nmi=fatal': Xen prints a diagnostic and then hangs.
`nmi=dom0': Inform DOM0 of the NMI.
`nmi=ignore': Ignore the NMI.
mem=xxx
Set the physical RAM address limit. Any RAM appearing beyond this physical address in the memory map will be ignored. This parameter may be specified with a B, K, M or G suffix, representing bytes, kilobytes, megabytes and gigabytes respectively. The default unit, if no suffix is specified, is kilobytes.
dom0_mem=xxx
Set the amount of memory to be allocated to domain0. In Xen 3.x the parameter may be specified with a B, K, M or G suffix, representing bytes, kilobytes, megabytes and gigabytes respectively; if no suffix is specified, the parameter defaults to kilobytes. In previous versions of Xen, suffixes were not supported and the value is always interpreted as kilobytes.
tbuf_size=xxx
Set the size of the per-cpu trace buffers, in pages (default 1). Note that the trace buffers are only enabled in debug builds. Most users can ignore this feature completely.
sched=xxx
Select the CPU scheduler Xen should use. The current possibilities are `sedf' (default) and `bvt'.
apic_verbosity=debug,verbose
Print more detailed information about local APIC and IOAPIC configuration.
lapic
Force use of local APIC even when left disabled by uniprocessor BIOS.
nolapic
Ignore local APIC in a uniprocessor system, even if enabled by the BIOS.
apic=bigsmp,default,es7000,summit
Specify NUMA platform. This can usually be probed automatically.
In addition, the following options may be specified on the Xen command line. Since domain 0 shares responsibility for booting the platform, Xen will automatically propagate these options to its command line. These options are taken from Linux's command-line syntax with unchanged semantics.
acpi=off,force,strict,ht,noirq,...
Modify how Xen (and domain 0) parses the BIOS ACPI tables.
acpi_skip_timer_override
Instruct Xen (and domain 0) to ignore timer-interrupt override instructions specified by the BIOS ACPI tables.
noapic
Instruct Xen (and domain 0) to ignore any IOAPICs that are present in the system, and instead continue to use the legacy PIC.
10.4 XenLinux Boot Options
In addition to the standard Linux kernel boot options, we support:
xencons=xxx
Specify the device node to which the Xen virtual console driver is attached. The following options are supported:
`xencons=off': disable virtual console
`xencons=tty': attach console to /dev/tty1 (tty0 at boot-time)
`xencons=ttyS': attach console to /dev/ttyS0
The default is ttyS for dom0 and tty for all other domains.
11. Further Support
If you have questions that are not answered by this manual, the sources of information listed below may be of interest to you. Note that bug reports, suggestions and contributions related to the software (or the documentation) should be sent to the Xen developers' mailing list (address below).
11.1 Other Documentation
For developers interested in porting operating systems to Xen, the Xen Interface Manual is distributed in the docs/ directory of the Xen source distribution.
11.2 Online References
The official Xen web site can be found at:
http://www.xensource.com
This contains links to the latest versions of all online documentation, including the latest version of the FAQ.
Information regarding Xen is also available at the Xen Wiki at
http://wiki.xensource.com/xenwiki/ The Xen project uses Bugzilla as its bug tracking system. You'll find the Xen Bugzilla at http://bugzilla.xensource.com/bugzilla/.
11.3 Mailing Lists
There are several mailing lists that are used to discuss Xen related topics. The most widely relevant are listed below. An official page of mailing lists and subscription information can be found at
http://lists.xensource.com/
xen-devel@lists.xensource.com
Used for development discussions and bug reports. Subscribe at:
http://lists.xensource.com/xen-devel
xen-users@lists.xensource.com
Used for installation and usage discussions and requests for help. Subscribe at:
http://lists.xensource.com/xen-users
xen-announce@lists.xensource.com
Used for announcements only. Subscribe at:
http://lists.xensource.com/xen-announce
xen-changelog@lists.xensource.com
Changelog feed from the unstable and 2.0 trees - developer oriented. Subscribe at:
http://lists.xensource.com/xen-changelog
A. Glossary of Terms
BVT
The BVT scheduler is used to give proportional fair shares of the CPU to domains.
Domain
A domain is the execution context that contains a running virtual machine. The relationship between virtual machines and domains on Xen is similar to that between programs and processes in an operating system: a virtual machine is a persistent entity that resides on disk (somewhat like a program). When it is loaded for execution, it runs in a domain. Each domain has a domain ID.
Domain 0
The first domain to be started on a Xen machine. Domain 0 is responsible for managing the system.
Domain ID
A unique identifier for a domain, analogous to a process ID in an operating system.
Full virtualization
An approach to virtualization which requires no modifications to the hosted operating system, providing the illusion of a complete system of real hardware devices.
Hypervisor
An alternative term for VMM, used because it means `beyond supervisor', since it is responsible for managing multiple `supervisor' kernels.
Live migration
A technique for moving a running virtual machine to another physical host, without stopping it or the services running on it.
Paravirtualization
An approach to virtualization which requires modifications to the operating system in order to run in a virtual machine. Xen uses paravirtualization but preserves binary compatibility for user space applications.
Shadow pagetables
A technique for hiding the layout of machine memory from a virtual machine's operating system. Used in some VMMs to provide the illusion of contiguous physical memory, in Xen this is used during live migration.
Virtual Block Device
Persistant storage available to a virtual machine, providing the abstraction of an actual block storage device. VBDs may be actual block devices, filesystem images, or remote/network storage.
Virtual Machine
The environment in which a hosted operating system runs, providing the abstraction of a dedicated machine. A virtual machine may be identical to the underlying hardware (as in full virtualization, or it may differ, as in paravirtualization).
VMM
Virtual Machine Monitor - the software that allows multiple virtual machines to be multiplexed on a single physical machine.
Xen
Xen is a paravirtualizing virtual machine monitor, developed primarily by the Systems Research Group at the University of Cambridge Computer Laboratory.
XenLinux
A name for the port of the Linux kernel that runs on Xen.
Footnotes
... 20031.1
http://www.cl.cam.ac.uk/netos/papers/2003-xensosp.pdf
... bridge-utils2.1
Available from http://bridge.sourceforge.net
... system2.2
Available from http://linux-hotplug.sourceforge.net/
... system2.3
See http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html/
... kernel2.4
If you boot without first disabling TLS, you will get a warning message during the boot process. In this case, simply perform the rename after the machine is up and then run /sbin/ldconfig to make it take effect.
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