Applies to openSUSE Leap 15

19 Managing a Virtualization Environment

Apart from using the recommended libvirt library (Part II, “Managing Virtual Machines with libvirt), you can manage Xen guest domains with the xl tool from the command line.

19.1 XL—Xen Management Tool

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
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
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

tux > systemctl start xencommons

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

Tip
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.

tux > 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:

  • xl create creates a virtual machine from a given configuration file.

  • xl reboot reboots a virtual machine.

  • xl destroy immediately terminates a virtual machine.

  • xl block-list displays all virtual block devices attached to a virtual machine.

19.1.1 Guest Domain Configuration File

When operating domains, xl requires a domain configuration file for each domain. The default directory to store such configuration files is /etc/xen/.

A domain configuration file is a plain text file. It consists of several KEY=VALUE pairs. Some keys are mandatory, some are general and apply to any guest, and some apply only to a specific guest type (para or fully virtualized). A value can either be a "string" surrounded by single or double quotes, a number, a boolean value, or a list of several values enclosed in brackets [ value1, value2, ... ].

Example 19.1: Guest Domain Configuration File for SLED 12: /etc/xen/sled12.cfg
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 start such domain, run xl create /etc/xen/sled12.cfg.

19.2 Automatic Start of Guest Domains

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

  1. Create the domain configuration file if it does not exist, and save it in the /etc/xen/ directory, for example /etc/xen/domain_name.cfg.

  2. Make a symbolic link of the guest domain configuration file in the auto/ subdirectory.

    tux > sudo ln -s /etc/xen/domain_name.cfg /etc/xen/auto/domain_name.cfg
  3. On the next system boot, the guest domain defined in domain_name.cfg will be started.

19.3 Event Actions

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:

List of Events
on_poweroff

Specifies what should be done with the domain if it shuts itself down.

on_reboot

Action to take if the domain shuts down with a reason code requesting a reboot.

on_watchdog

Action to take if the domain shuts down because of a Xen watchdog timeout.

on_crash

Action to take if the domain crashes.

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

List of Related Actions
destroy

Destroy the domain.

restart

Destroy the domain and immediately create a new domain with the same configuration.

rename-restart

Rename the domain that terminated, and then immediately create a new domain with the same configuration as the original.

preserve

Keep the domain. It can be examined, and later destroyed with xl destroy.

coredump-destroy

Write a core dump of the domain to /var/xen/dump/NAME and then destroy the domain.

coredump-restart

Write a core dump of the domain to /var/xen/dump/NAME and then restart the domain.

19.4 Time Stamp Counter

The Time Stamp Counter (TSC) may be specified for each domain in the guest domain configuration file (for more information, see Section 19.1.1, “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).

tsc_mode=0 (default)

Use this to ensure correctness while providing the best performance possible—for more information, see https://xenbits.xen.org/docs/4.3-testing/misc/tscmode.txt.

tsc_mode=1 (always emulate)

Use this when TSC-sensitive apps are running and worst-case performance degradation is known and acceptable.

tsc_mode=2 (never emulate)

Use this when all applications running in this VM are TSC-resilient and highest performance is required.

tsc_mode=3 (PVRDTSCP)

High-TSC-frequency applications may be paravirtualized (modified) to obtain both correctness and highest performance—any unmodified applications must be TSC-resilient.

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

19.5 Saving Virtual Machines

Procedure 19.1: Save a Virtual Machine’s Current State
  1. Make sure the virtual machine to be saved is running.

  2. In the host environment, enter

    tux > sudo xl save ID STATE-FILE

    where ID is the virtual machine ID you want to save, and STATE-FILE is the name you specify for the memory state file. By default, the domain will no longer be running after you create its snapshot. Use -c to keep it running even after you create the snapshot.

19.6 Restoring Virtual Machines

Procedure 19.2: Restore a Virtual Machine’s Current State
  1. Make sure the virtual machine to be restored has not been started since you ran the save operation.

  2. In the host environment, enter

    tux > sudo xl restore STATE-FILE

    where STATE-FILE is the previously saved memory state file. By default, the domain will be running after it is restored. To pause it after the restore, use -p.

19.7 Virtual Machine States

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.

  • r - running - The virtual machine is currently running and consuming allocated resources.

  • b - blocked - The virtual machine’s processor is not running and not able to run. It is either waiting for I/O or has stopped working.

  • p - paused - The virtual machine is paused. It does not interact with the hypervisor but still maintains its allocated resources, such as memory.

  • s - shutdown - The guest operating system is in the process of being shut down, rebooted, or suspended, and the virtual machine is being stopped.

  • c - crashed - The virtual machine has crashed and is not running.

  • d - dying - The virtual machine is in the process of shutting down or crashing.

Print this page