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Virtualization of operating systems is used in many different computing areas. It finds its applications in server consolidation, energy saving efforts, or the ability to run older software on new hardware, for example. This chapter introduces and explains the components and technologies you need to understand to set up and manage a Xen-based virtualization environment.
The basic components of a Xen-based virtualization environment are the Xen hypervisor, the Domain0, any number of other VM Guests, and the tools, commands, and configuration files that let you manage virtualization. Collectively, the physical computer running all these components is referred to as a VM Host Server because together these components form a platform for hosting virtual machines.
The Xen hypervisor, sometimes referred to generically as a virtual machine monitor, is an open-source software program that coordinates the low-level interaction between virtual machines and physical hardware.
The virtual machine host environment, also referred to as Domain0 or controlling domain, is comprised of several components, such as:
The SUSE Linux operating system, which gives the administrator a graphical and command line environment to manage the virtual machine host components and its virtual machines.
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The term “Domain0” refers to a special domain that provides the management environment. This may be run either in graphical or in command line mode. |
The xend daemon (xend), which stores configuration information about each virtual machine and controls how virtual machines are created and managed.
A modified version of QEMU, which is an open-source software program that emulates a full computer system, including a processor and various peripherals. It provides the ability to host operating systems in full virtualization mode.
A Xen-based virtual machine, also referred to as a
VM Guest
or
DomU
consists of the following components:
At least one virtual disk that contains a bootable operating system. The virtual disk can be based on a file, partition, volume, or other type of block device.
Virtual machine configuration information, which can be modified by exporting a text-based configuration file from xend or through Virtual Machine Manager.
A number of network devices, connected to the virtual network provided by the controlling domain.
There is a combination of GUI tools, commands, and configuration files to help you manage and customize your virtualization environment.
Guest operating systems are hosted on virtual machines in either full virtualization mode or paravirtual mode. Each virtualization mode has advantages and disadvantages.
Full virtualization mode lets virtual machines run unmodified operating systems, such as Windows* Server 2003 and Windows XP, but requires the computer running as the VM Host Server to support hardware-assisted virtualization technology, such as AMD* Virtualization or Intel* Virtualization Technology.
Some guest operating systems hosted in full virtualization mode, can be configured to run the Novell* Virtual Machine Drivers instead of drivers originating from the operating system. Running virtual machine drivers improves performance dramatically on guest operating systems, such as Windows XP and Windows Server 2003. For more information, see Chapter 13, Virtual Machine Drivers.
Paravirtual mode does not require the host computer to support hardware-assisted virtualization technology, but does require the guest operating system to be modified for the virtualization environment. Typically, operating systems running in paravirtual mode enjoy better performance than those requiring full virtualization mode.
Operating systems currently modified to run in paravirtual mode are referred to as paravirtualized operating systems and include SUSE Linux Enterprise Server 11 and NetWare® 6.5 SP8.
The following graphic depicts a virtual machine host with four virtual machines. The Xen hypervisor is shown as running directly on the physical hardware platform. Note, that the controlling domain is also just a virtual machine, although it has several additional management tasks compared to all other virtual machines.
On the left, the virtual machine host’s Domain0 is shown running the SUSE Linux operating system. The two virtual machines shown in the middle are running paravirtualized operating systems. The virtual machine on the right shows a fully virtual machine running an unmodified operating system, such as Windows Server 2003 or Windows XP.
After you install the virtualization components and reboot the computer, the GRUB boot loader menu displays a Xen menu option. Selecting the Xen menu option loads the Xen hypervisor and starts the Domain0 running the SUSE Linux operating system.
Running on Domain0, the SUSE Linux operating system displays the installed text console or desktop environment, such as GNOME or KDE. The terminals of VM Guest systems are displayed in their own window inside the controlling Domain0 when opened.