The xm(1) program uses python executable config files to define
domains to create from scratch. Each of these config files needs to
contain a number of required options, and may specify many more.
Domain configuration files live in /etc/xen by default, if you store
config files anywhere else the full path to the config file must be
specified in the xm create command.
/etc/xen/auto is a special case. Domain config files in that
directory will be started automatically at system boot if the
xendomain init script is enabled. The contents of /etc/xen/auto
should be symlinks to files in /etc/xen to allow xm create to be
used without full paths.
Options are specified by name = value statements in the
xmdomain.cfg files.
OPTIONS
The following lists the most commonly used options for a domain config
file.
kernel
The kernel image for the domain. The format of the parameter is the
fully qualified path to the kernel image file,
i.e. /boot/vmlinuz-2.6.12-xenU.
ramdisk
The initial ramdisk for the domain. The format of the parameter is
the fully qualified path to the initrd, i.e. /boot/initrd.gz. On
many Linux distros you will not need a ramdisk if using the default
xen kernel.
memory
The amount of RAM, in megabytes, to allocate to the domain when it
starts. Allocating insufficient memory for a domain may produce
extremely bizarre behavior. If there isn't enough free memory left on
the machine to fulfill this request, the domain will fail to start.
Xen does not support overcommit of memory, so the total memory of all
guests (+ 64 MB needed for Xen) must be less than or equal to the
physical RAM in the machine.
name
A unique name for the domain. Attempting to create two domains with
the same name will cause an error.
root
Specifies the root device for the domain. This is required for Linux
domains, and possibly other OSes.
nics
The number of network interfaces allocated to the domain on boot. It
defaults to 1.
disk
An array of block device stanzas, in the form:
disk = [ "stanza1", "stanza2", ... ]
Each stanza has 3 terms, separated by commas,
``backend-dev,frontend-dev,mode''.
backend-dev
The device in the backend domain that will be exported to the guest
(frontend) domain. Supported formats include:
phy:device - export the physical device listed. The device can be
in symbolic form, as in sda7, or as the hex major/minor number, as in
0x301 (which is hda1).
file://path/to/file - export the file listed as a loopback device.
This will take care of the loopback setup before exporting the device.
frontend-dev
How the device should appear in the guest domain. The device can be
in symbolic form, as in sda7, or as the hex major/minor number, as in
0x301 (which is hda1).
mode
The access mode for the device. There are currently 2 valid options,
r (read-only), w (read/write).
vif
An array of virtual interface stanzas in the form:
vif = [ "stanza1", "stanza2", ... ]
Each stanza specifies a set of name = value options separated by
commas, in the form: ``name1=value1, name2=value2, ...''
OPTIONS
bridge
The network bridge to be used for this device. This is especially
needed if multiple bridges exist on the machine.
mac
The MAC address for the virtual interface. If mac is not specified,
one will be randomly chosen by xen with the 00:16:3e vendor id prefix.
ADDITIONAL OPTIONS
The following options are also supported in the config file, though
are far more rarely used.
builder
Which builder should be used to construct the domain. This defaults
to the linux if not specified, which is the builder for
paravirtualized Linux domains.
cpu
Specifies which CPU the domain should be started on, where 0 specifies
the first cpu, 1 the second, and so on. This defaults to -1, which
means Xen is free to pick which CPU to start on.
cpus
Specifies a list of CPUs on which the domains' VCPUs are allowed to
execute upon. The syntax supports ranges (0-3), and negation, ^1.
For instance:
cpus = "0-3,5,^1"
Will result in CPUs 0, 2, 3, 5 being available for use by the domain.
extra
Extra information to append to the end of the kernel parameter line.
The format is a string, the contents of which can be anything that the
kernel supports. For instance:
extra = "4"
Will cause the domain to boot to runlevel 4.
nfs_server
The IP address of the NFS server to use as the root device for the
domain. In order to do this you'll need to specify root=/dev/nfs,
and specify nfs_root.
nfs_root
The directory on the NFS server to be used as the root filesystem.
Specified as a fully qualified path, i.e. /full/path/to/root/dir.
vcpus
The number of virtual cpus to allocate to the domain. In order to use
this the xen kernel must be compiled with SMP support.
This defaults to 1, meaning running the domain as a UP.
DOMAIN SHUTDOWN OPTIONS
There are 3 options which control domain shutdown (both planned and
unplanned) under certain events. The 3 events currently captured are:
on_shutdown
Triggered on either an xm shutdown or graceful shutdown from inside
the DomU.
on_reboot
Triggered on either an xm reboot or graceful reboot from inside the
DomU.
on_crash
Triggered when a DomU goes to the crashed state for any reason.
All of them take one of 4 valid states listed below.
destroy
The domain will be cleaned up completely. No attempt at respawning
will occur. This is what a typical shutdown would look like.
restart
The domain will be restarted with the same name as the old domain.
This is what a typical reboot would look like.
preserve
The domain will not be cleaned up at all. This is often useful for
crash state domains which ensures that enough evidence is to debug the
real issue.
rename-restart
The old domain will not be cleaned up, but will be renamed so a new
domain can be restarted in it's place. The old domain will be renamed with
a suffix -1, -2, etc, and assigned a new random UUID; the new domain will
keep the original name and UUID. The old domain will release the devices that
it holds, so that the new one may take them.
EXAMPLES
The following are quick examples of ways that domains might be
configured. They should not be considered an exhaustive set.
A Loopback File as Root
kernel = "/boot/vmlinuz-2.6-xenU"
memory = 128
name = "MyLinux"
root = "/dev/hda1 ro"
disk = [ "file:/var/xen/mylinux.img,hda1,w" ]
This creates a domain called MyLinux with 128 MB of memory using a
default xen kernel, and the file /var/xen/mylinux.img loopback mounted
at hda1, which is the root filesystem.
