25 Managing a virtualization environment #

The xl program is a tool for managing Xen guest domains. It is part of the xen-tools package. xl is based on the LibXenlight library, and can be used for general domain management, such as domain creation, listing, pausing, or shutting down. Usually you need to be root to execute xl commands.

Note

xl can only manage running guest domains specified by their configuration file. If a guest domain is not running, you cannot manage it with xl.

Tip

To allow users to continue to have managed guest domains in the way the obsolete xm command allowed, we now recommend using libvirt's virsh and virt-manager tools. For more information, see Part II, “Managing virtual machines with libvirt”.

xl operations rely upon xenstored and xenconsoled services. Make sure you start

> systemctl start xencommons

at boot time to initialize all the daemons required by xl.

Tip: Set up a xenbr0 network bridge in the host domain

In the most common network configuration, you need to set up a bridge in the host domain named xenbr0 to have a working network for the guest domains.

The basic structure of every xl command is:

xl <subcommand> [options] domain_id

where <subcommand> is the xl command to run, domain_id is the ID number assigned to a domain or the name of the virtual machine, and OPTIONS indicates subcommand-specific options.

For a complete list of the available xl subcommands, run xl help. For each command, there is a more detailed help available that is obtained with the extra parameter --help. More information about the respective subcommands is available in the manual page of xl.

For example, the xl list --help displays all options that are available to the list command. As an example, the xl list command displays the status of all virtual machines.

> sudo xl list
Name                                 ID    Mem VCPUs        State   Time(s)
Domain-0                              0    457     2       r-----   2712.9
sles12                                7    512     1       -b----     16.3
opensuse                                   512     1                  12.9

The State information indicates if a machine is running, and in which state it is. The most common flags are r (running) and b (blocked) where blocked means it is either waiting for IO, or sleeping because there is nothing to do. For more details about the state flags, see man 1 xl.

Other useful xl commands include:

name= "sled12"
builder = "hvm"
vncviewer = 1
memory = 512
disk = [ '/var/lib/xen/images/sled12.raw,,hda', '/dev/cdrom,,hdc,cdrom' ]
vif = [ 'mac=00:16:3e:5f:48:e4,model=rtl8139,bridge=br0' ]
boot = "n"

To make a guest domain start automatically after the host system boots, follow these steps:

In the guest domain configuration file, you can define actions to be performed on a predefined set of events. For example, to tell the domain to restart itself after it is powered off, include the following line in its configuration file:

on_poweroff="restart"

A list of predefined events for a guest domain follows:

For these events, you can define one of the following actions:

The Time Stamp Counter (TSC) may be specified for each domain in the guest domain configuration file (for more information, see Section 25.1.1, “Guest domain configuration file”).

With the tsc_mode setting, you specify whether rdtsc instructions are executed “natively” (fast, but TSC-sensitive applications may sometimes run incorrectly) or emulated (always run correctly, but performance may suffer).

For background information, see https://xenbits.xen.org/docs/4.3-testing/misc/tscmode.txt.

A virtual machine’s state can be displayed by viewing the results of the xl list command, which abbreviates the state using a single character.