--- name: "virt-image" section: "5" platform: "linux" package: "virtinst" version: "1:1.0.1-5" release: "Debian 8 (Jessie)" platforms: ["linux"] url: "https://new.carta.tech/virt-image/5" --- # virt-image(5) Format of the virtual image XML descriptor ## NAME virt-image - Format of the virtual image XML descriptor ## DESCRIPTION [*virt-image*(1)](/virt-image/1.md) 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*. Omitting a togglable feature tag turns it off. - 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 `"pygrub"` 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. ``` sysresccd i686 1 262144 ``` To create a virtual machine, save the above XML in *image.xml* and run: ``` # virt-image --vnc image.xml ``` ## BUGS Please see `"http://virt-manager.org/page/BugReporting"` ## COPYRIGHT Copyright (C) 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. ## SEE ALSO [*virt-image*(1)](/virt-image/1.md), [*virt-install*(1)](/virt-install/1.md), the project website `"http://virt-manager.org"`, the Relax-NG grammar for image XML `"image.rng"`