Difference between revisions of "Managing Xen using the xm Command-line Tool"

Latest revision as of 20:14, 27 October 2016

In previous chapters we have covered the steps necessary to install and configure Xen and Xen based guest operating systems. This chapter is dedicated to explaining the xm tool, and how it can be used to manage guest operating systems from the command-line. If you prefer to use the graphical virt-manager tool to administer your Xen configuration see Managing and Monitoring Fedora based Xen Guest Systems.

xm Command-line vs xm Shell

The xm options covered in this chapter may be invoked individually as command-line arguments to xm or used with xm in shell mode.

To use the options as command-line arguments, use them at a Terminal command prompt as shown in the following example:

su - 
xm list

To run commands in the xm shell, run the following command:

[root@fedora7 ~]# xm shell
The Xen Master. Type "help" for a list of functions.
xm>

At the xm> prompt enter the options you wish to run, for example:

su -
[root@fedora7 ~]# xm shell
The Xen Master. Type "help" for a list of functions.
xm> list
Name                                      ID   Mem VCPUs      State   Time(s)
Domain-0                                   0   384     1     r-----    455.1
XenFed                                     2   305     1     -b----      7.7
myFedoraXen                                    300     1                 0.0
myXenGuest                                     300     1                 0.0
xm> 
xm> start XenFed
xm> suspend XenFed
xm> restore 

Listing Guest System Status

The status of the host and guest systems may be viewed at any time using the list option of the xm tool. For example:

su - 
xm list

The above command will display output containing a line for the host system and a line for each guest similar to the following:

Name                                      ID   Mem VCPUs      State   Time(s)
Domain-0                                   0   389     1     r-----   1414.9
XenFed                                         305     1               349.9
myFedoraXen                                    300     1                 0.0
myXenGuest                                 6   300     1     -b----     10.6

• r - running - The domain is currently running and healthy

• b - blocked - The domain is blocked, and not running or runnable. This can be caused because the domain is waiting on IO (a traditional wait state) or has gone to sleep because there was nothing else for it to do.

• p - paused - The domain has been paused, typically as a result of the administrator running the xm pause command. When in a paused state the domain will still consume allocated resources like memory, but will not be eligible for scheduling by the Xen hypervisor.

• s - shutdown - The guest has requested to be shutdown, rebooted or suspended, and the domain is in the process of being destroyed in response.

• c - crashed - The domain has crashed. Usually this state can only occur if the domain has been configured not to restart on crash.

• d - dying - The domain is in process of dying, but hasn't completely shutdown or crashed.

Starting a Xen Guest System

A guest operating system can be started using the xm tool combined with the start option followed by the name of the guest operating system to be launched. For example:

su -
xm start myGuestOS

Connecting to a Running Xen Guest System

Once the guest operating system has started, a connection to the guest may be established using either the vncviewer tool or the virt-manager console. To use virt-manager, select Applications->System Tools->Virtual Machine Manager, select the desired system and click Open.

To connect using vncviewer enter the following command in Terminal window:

vncviewer

When prompted for a server enter localhost:5900. A VNC window will subsequently appear containing the running guest system.

Shutting Down a Guest System

The shutdown option of the xm tool is used to shutdown a guest operating system:

xm shutdown guestName

where guestName is the name of the guest system, to be shutdown.

Note that the shutdown option allows the guest operating system to perform an orderly shutdown when it receives the shutdown instruction. To instantly stop a guest operating system the destroy option may be used (with all the attendant risks of filesystem damage and data loss):

xm destroy myGuestOS

Pausing and Resuming a Guest System

A guest system can be paused and resumed using the xm tool's pause and restore options. For example, to pause a specific system named myXenGuest:

xm pause myXenGuest

Similarly, to resume the paused system:

xm resume myXenGuest

Note that a paused session will be lost if the host system is rebooted. Also, be aware that a paused system continues to reside in memory. To save a session such that it no longer takes up memory and can be restored to its exact state after a reboot, it is necessary to either suspend and resume or save and restore the guest.

Suspending and Resuming a Guest OS

A running guest operating system can be suspended and resumed using the xm utility. When suspended, the current status of the guest operating system is written to disk and removed from system memory. A suspended system may subsequently be restored at any time (including after a host system reboot):

To suspend a guest OS named myGuestOS:

xm suspend myGuestOS

To restore a suspended guest OS:

xm resume myGuestOS

Saving and Restoring Xen Guest Systems

Saving and restoring of a Xen guest operating system is similar to suspending with the exception that the file used to contain the suspended operating system memory image can be specified by the user:

To save a guest:

xm save myGuestOS path_to_save_file
<pre>

To restore a saved guest operating system session:

<pre>
xm restore path_to_save_file

Rebooting a Guest System

To reboot a guest operating system:

xm reboot myGuestOS

Configuring the Memory Assigned to a Xen Guest OS

To configure the memory assigned to a guest OS, use the mem-set option of the xm command. For example, the following command reduces the memory allocated to a guest system named myGuestOS to 256Mb:

xm mem-set myGuestOS 256

Note that acceptable memory settings must fall within the memory available to the current Domain. This may be increased using the mem-max option to xm.

Migrating a Domain to a Different Host

The migrate option allows a Xen managed domain to be migrated to a different physical server.

In order to use migrate, Xend must already be running on other host machine, and must be running the same version of Xen as the local host system. In addition, the remote host system must have the migration TCP port open and accepting connections from the source host. Finally, there must be sufficient resources for the domain to run (memory, disk space, etc).

xm migrate domainName host

Optional flags available with this command are:

-l, --live           Use live migration.
-p=portnum, --port=portnum
                     Use specified port for migration.
-r=MBIT, --resource=MBIT
                     Set level of resource usage for migration.