virt-image

virt-image(1) relies on an XML descriptor to create virtual machines from virtual machine images. In general, a virtual machine image consists of the XML descriptor (usually in a file image.xml) and a number of files for the virtual machine's disks.

In the following explanation of the structure of the image descriptor, mandatory XML elements are marked as element, whereas optional elements are marked as element.

All file names in the image descriptor are relative to the location of the descriptor itself. Generally, disk files are either kept in the same directory as the image descriptor, or in a subdirectory.

The image descriptor contains information on the requirements a guest has on the host platform through one or more the /image/domain/boot descriptors (see section "BOOT"). The image can only be used if at least one of the boot descriptors is suitable for the host platform; a boot descriptor is suitable if:

• The CPU architecture of the boot descriptor, given by the boot/guest/arch element, is supported by the host
• The host supports a guest with the features requested in the boot/guest/features element, such as providing an APIC, or having ACPI turned off

If a suitable boot descriptor is found, the guest is created and booted according to the information about booting the OS from the boot/os element and with the disks specified in the boot/drive element. If more than one suitable boot descriptor is found, one of them is chosen based on a heuristic, generally preferring paravirtualized guests over full virtualized ones, though this is an implementation detail of the tool creating the virtual machine.

The image descriptor consists of three sections, all contained in the toplevel image element:

General metadata about the image

A number of elements like label, name, and description that give some simple information about the image. The name must be a string suitable as a name for the virtual machine, the label is a short human-readable string suitable for display in graphical UI's, and the description should be a longer, free-form description of the purpose of the image. The name is mandatory.

Virtual machine attributes

The domain element contains instructions on how to boot the image, and device attributes such as the number of virtual CPU's and the size of the memory. (see section "DOMAIN")

Storage layout

The storage element lists the files to back the virtual machine's disks and some information about their format and use. (see section "STORAGE")

The domain element contains one or more boot descriptors (see section "BOOT") and a devices element. The Devices element lists the recommended number of virtual CPU's in the vcpu element and the recommended amount of memory in kB in the memory element. It also indicates whether the virtual machine should have a network interface through the interface element and whether the virtual machine has a graphical interface through the graphics element.

The storage element lists the disk image files that are part of the virtual machine image in a list of one or more disk elements. Each disk element can contain the following attributes:

• the file attribute giving the name of the disk file
• an optional id attribute. The name given with that attribute is used to reference the disk from the drive element of a boot descriptor. If the id attribute is missing, it defaults to the file attribute.
• the use attribute indicating whether the disk file is a "system", "user", or "scratch" disk. The use attribute differentiates disk files so that an update based on replacing disk files can replace "system" disks, but leave "user" disks untouched.

  Generally, "system" disks contain application code, "user" disks contain the application's data, and "scratch" disks contain temporary state that can be erased between runs of the guest.

  The virtual machine image must contain files for all "system" disks, and may contain files for the "user" and "scratch" disks. If the latter are not part of the image, they are initialized as empty files when a guest is created, with the size given by the size attribute.

• the size attribute giving the size of the disk in MB.
• the format attribute giving the format of the disk file. Currently, this can be one of: "raw" "iso", "qcow", "qcow2", or "vmdk".

The image descriptor below can be used to create a virtual machine running the System Rescue CD ("http://www.sysresccd.org/") Besides the descriptor, you only need the ISO image from the System Rescue CD website.

    <?xml version="1.0" encoding="UTF-8"?>
    <image>
      <name>sysresccd</name>
      <domain>
        <boot type="hvm">
          <guest>
            <arch>i686</arch>
          </guest>
          <os>
            <loader dev="cdrom"/>
          </os>
          <drive disk="root.raw" target="hda"/>
          <drive disk="sysresc"/>
        </boot>
        <devices>
          <vcpu>1</vcpu>
          <memory>262144</memory>
          <interface/>
          <graphics/>
        </devices>
      </domain>
      <storage>
        <disk file="root.raw" use="scratch" size="100" format="raw"/>
        <disk id="sysresc" file="isos/systemrescuecd.iso"
              use="system" format="iso"/>
      </storage>
    </image>

To create a virtual machine, save the above XML in image.xml and run:

    # virt-image --vnc image.xml

Written by David Lutterkort. See the AUTHORS file in the source distribution for the complete list of credits.

Please see "http://virt-manager.org/page/BugReporting"

Copyright (C) 2006-2012 Red Hat, Inc, and various contributors. This is free software. You may redistribute copies of it under the terms of the GNU General Public License "http://www.gnu.org/licenses/gpl.html". There is NO WARRANTY, to the extent permitted by law.

virt-image(1), virt-install(1), the project website "http://virt-manager.org", the Relax-NG grammar for image XML "image.rng"