Xen Virtual Machine Overview

From openSUSE

Xen (pronounced zen) is an open source GPL project managed by XenSource. This is the primary virtualization technology that openSuse and Novell is supporting. You can find more general information here and technical information here.

Xen Virtual Machine Modes

The VM Server hosts virtual machines running operating systems in one of two modes: fully virtual or paravirtual.

• Fully virtual: Complete emulation of all hardware devices. Although it requires special computer hardware, most operating systems can run in fully virtual mode because the VMM emulates all computer devices to fool the operating system into thinking it has exclusive access to an entire computer. This complete emulation of computer hardware demands more CPU resources from the VM Server. As a result, an operating system running in full virtualization mode runs slower.

• Paravirtual: Selective emulation of hardware devices. A paravirtualized operating system can run in paravirtual mode, which does not require complete emulation and therefore requires less management overhead. For example, VM-aware operating systems do not require an emulated graphics card, so the VM Server does not need to emulate video data. As a result, an operating system running in paravirtual mode demands fewer CPU resources and has better performance. It also requires no special computer hardware.

Virtual Machine Monitor

The virtual machine monitor (VMM) runs between the server hardware and the SUSE Linux operating system kernel. When the computer boots, the VMM loads first and then starts the VM Server in privileged mode, which means that the VM Server has the ability to create and control virtual machines and has direct access to the computer hardware. The VM Server is configured with native device drivers that match the actual devices in the computer. For example, if the computer has a physcial e1000 network card, the VM Server is configured to load and run the SUSE Linux device driver for the e1000.

• Virtual machine monitor (VMM) and hypervisor refer to the software layer developed and maintained by the Xen open source community that provides VM Server functionality.

Defining a VM

Virtual machines are defined and the images stored on the VM Server. The definitions are stored in a configuration file located at /etc/xen/vm/vm_name. The configuration file defines the virtual resources, such as CPU, memory, network card, and block devices, the operating system sees when it is installed and booted on the virtual machine.

VM Device Drivers

In both full virtualization and paravirtual modes, a VM’s operating system uses device drivers to interact with the VMM. In full virtualization mode, the operating system uses its native OS device drivers for a standard set of emulated devices, such as an AMD PCNet or NE2000 network card, an IDE disk drive, and a VGA graphics card. In paravirtual mode, the VM-aware operating systems include special device drivers (called Xen drivers) to communicate through the VMM and VM Server to the physical devices in the computer.

For example, if a VM’s operating system running in full-virtualization mode needs to save a file on its virtual 20-GB disk drive, the operating system passes its request through the device driver to the VMM. The VMM understands which portion of the 500-GB physical disk the VM has access to and passes instructions to the VM Server. The VM Server accesses the disk drive and writes the file to the pre-defined location on the 500-GB disk. Depending on your computing needs and available computer resources, any number of VMs can be created and can simultaneously run on the VM Server. The operating system of each VM interacts independently with the VMM and VM Server platform to consume virtual or emulated CPU, memory, block device, and network resources.

VMs can be viewed and managed from the VM Server desktop.

This page is part of:

• An Introduction to Virtualization
• Xen Virtual Machine Overview
• How to Install a Xen VM Server
• How to add a Xen VM Guest
• How to Install a VM's OS from CD
• How to Install a VM's OS from ISO
• How to Install a VM's OS from Installation Source
• How to Install a VM's OS from Disk Image
• How to Create an OS Disk Image for a VM
• How to Manage VMs
• How to Install an Xen VM Server
• How to Add a VM