virt-image - Format of the virtual image XML descriptor
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")
The storage element lists the files to back the virtual machine’s
disks and some information about their format and use. (see section
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.
Each boot descriptor details how the virtual machine should be started
on a certain hypervisor. The type attribute of the boot element, which
can either be "xen" or "hvm", depending on whether the boot descriptor
is for a paravirtualized Xen(tm) guest or a fully-virtualized guest.
The boot element contains three subelements:
The platform requirements of the guest
The platform requirements, contained in the guest element, consist
of the arch element and the features element. The arch element
indicates the CPU architecture the guest expects, e.g. "i686",
"x86_64", or "ppc".
The features element indicates whether certain platform features
should be on or off. Currently, the platform features are pae,
acpi, and apic. They can be turned on or off by giving a state
attribute of either "on" or "off". When a feature is mentioned in
the features element, it defaults to "on".
The details of booting the image’s operating system
The os element for fully-virtualized "hvm" guests contains a loader
element whose dev attribute indicates whether to boot off a hard
disk ("dev='hd'") or off a CD-ROM ("dev='cdrom'")
For paravirtualized guests, the os element either contains a
"<loader>pygrub</loader>" element, indicating that the guest should
be booted with pygrub, or kernel, initrd and cmdline elements. The
contents of the kernel and initrd elements are the names of the
kernel and initrd files, whereas the cmdline element contains the
command line that should be passed to the kernel on boot.
The mapping of disk files as devices into the guest
The mapping of disk files into the guest is performed by a list of
drive elements inside the boot element. Each drive element
references the name of a disk file from the "STORAGE" section
through its disk attribute and can optionally specify as what
device that disk file should appear in the guest through its target
attribute. If the target is omitted, device names are assigned in
the order in which the drive elements appear, skipping already
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
· 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
· 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", "qemu", "qemu2", 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
<?xml version="1.0" encoding="UTF-8"?>
<drive disk="root.raw" target="hda"/>
<disk file="root.raw" use="scratch" size="100" format="raw"/>
<disk id="sysresc" file="isos/systemrescuecd.iso"
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) 2006-2007 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