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.