Ubuntu Manpage: virt-image - Format of the virtual image XML descriptor

Provided by: virtinst_0.600.4-3ubuntu2_all

NAME

       virt-image - Format of the virtual image XML descriptor

DESCRIPTION

       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.

HOST MATCHING

       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.

STRUCTURE

       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")

DOMAIN

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.

BOOT
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 assigned devices.

STORAGE

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".

EXAMPLE

       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

AUTHOR

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

BUGS

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

COPYRIGHT

       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.