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Chapter 26. Managing guests with virsh

virsh is a command line interface tool for managing guests and the hypervisor.
The virsh tool is built on the libvirt management API and operates as an alternative to the xm command and the graphical guest Manager (virt-manager). virsh can be used in read-only mode by unprivileged users. You can use virsh to execute scripts for the guest machines.
virsh command quick reference

The following tables provide a quick reference for all virsh command line options.

Table 26.1. Guest management commands

Command Description
help Prints basic help information.
list Lists all guests.
dumpxml Outputs the XML configuration file for the guest.
create Creates a guest from an XML configuration file and starts the new guest.
start Starts an inactive guest.
destroy Forces a guest to stop.
define Outputs an XML configuration file for a guest.
domid Displays the guest's ID.
domuuid Displays the guest's UUID.
dominfo Displays guest information.
domname Displays the guest's name.
domstate Displays the state of a guest.
quit Quits the interactive terminal.
reboot Reboots a guest.
restore Restores a previously saved guest stored in a file.
resume Resumes a paused guest.
save Save the present state of a guest to a file.
shutdown Gracefully shuts down a guest.
suspend Pauses a guest.
undefine Deletes all files associated with a guest.
migrate Migrates a guest to another host.
The following virsh command options manage guest and hypervisor resources:

Table 26.2. Resource management options

Command Description
setmem Sets the allocated memory for a guest.
setmaxmem Sets maximum memory limit for the hypervisor.
setvcpus Changes number of virtual CPUs assigned to a guest. Note that this feature is unsupported in Red Hat Enterprise Linux 5.
vcpuinfo Displays virtual CPU information about a guest.
vcpupin Controls the virtual CPU affinity of a guest.
domblkstat Displays block device statistics for a running guest.
domifstat Displays network interface statistics for a running guest.
attach-device Attach a device to a guest, using a device definition in an XML file.
attach-disk Attaches a new disk device to a guest.
attach-interface Attaches a new network interface to a guest.
detach-device Detach a device from a guest, takes the same kind of XML descriptions as command attach-device.
detach-disk Detach a disk device from a guest.
detach-interface Detach a network interface from a guest.
domxml-from-native Convert from native guest configuration format to domain XML format. See the virsh man page for more details.
domxml-to-native Convert from domain XML format to native guest configuration format. See the virsh man page for more details.
These are miscellaneous virsh options:

Table 26.3. Miscellaneous options

Command Description
version Displays the version of virsh
nodeinfo Outputs information about the hypervisor
Connecting to the hypervisor

Connect to a hypervisor session with virsh: 

# virsh connect {hostname OR URL}
Where <name> is the machine name of the hypervisor. To initiate a read-only connection, append the above command with -readonly.
Creating a virtual machine XML dump (configuration file)

Output a guest's XML configuration file with virsh: 

# virsh dumpxml {domain-id, domain-name or domain-uuid}
This command outputs the guest's XML configuration file to standard out (stdout). You can save the data by piping the output to a file. An example of piping the output to a file called guest.xml: 
# virsh dumpxml GuestID > guest.xml
This file guest.xml can recreate the guest (see Editing a guest's configuration file. You can edit this XML configuration file to configure additional devices or to deploy additional guests. See Section 34.1, “Using XML configuration files with virsh” for more information on modifying files created with virsh dumpxml.
An example of virsh dumpxml output: 
# virsh dumpxml r5b2-mySQL01
<domain type='xen' id='13'>
    <name>r5b2-mySQL01</name>
    <uuid>4a4c59a7ee3fc78196e4288f2862f011</uuid>
    <bootloader>/usr/bin/pygrub</bootloader>
    <os>
        <type>linux</type>
        <kernel>/var/lib/libvirt/vmlinuz.2dgnU_</kernel>
	<initrd>/var/lib/libvirt/initrd.UQafMw</initrd>
        <cmdline>ro root=/dev/VolGroup00/LogVol00 rhgb quiet</cmdline>
    </os>
    <memory>512000</memory>
    <vcpu>1</vcpu>
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>restart</on_reboot>
    <on_crash>restart</on_crash>
    <devices>
        <interface type='bridge'>
            <source bridge='xenbr0'/>
            <mac address='00:16:3e:49:1d:11'/>
            <script path='vif-bridge'/>
        </interface>
        <graphics type='vnc' port='5900'/>
        <console tty='/dev/pts/4'/>
    </devices>
</domain>
Creating a guest from a configuration file

Guests can be created from XML configuration files. You can copy existing XML from previously created guests or use the dumpxml option (see Creating a virtual machine XML dump (configuration file)). To create a guest with virsh from an XML file: 

# virsh create configuration_file.xml
Editing a guest's configuration file

Instead of using the dumpxml option (see Creating a virtual machine XML dump (configuration file)), guests can be edited either while they run or while they are offline. The virsh edit command provides this functionality. For example, to edit the guest named softwaretesting: 

# virsh edit softwaretesting
This opens a text editor. The default text editor is the $EDITOR shell parameter (set to vi by default).
Suspending a guest

Suspend a guest with virsh: 

# virsh suspend {domain-id, domain-name or domain-uuid}
When a guest is in a suspended state, it consumes system RAM but not processor resources. Disk and network I/O does not occur while the guest is suspended. This operation is immediate and the guest can be restarted with the resume (Resuming a guest) option.
Resuming a guest

Restore a suspended guest with virsh using the resume option: 

# virsh resume {domain-id, domain-name or domain-uuid}
This operation is immediate and the guest parameters are preserved for suspend and resume operations.
Save a guest

Save the current state of a guest to a file using the virsh command: 

# virsh save {domain-name, domain-id or domain-uuid} filename
This stops the guest you specify and saves the data to a file, which may take some time given the amount of memory in use by your guest. You can restore the state of the guest with the restore (Restore a guest) option. Save is similar to pause, instead of just pausing a guest the present state of the guest is saved.
Restore a guest

Restore a guest previously saved with the virsh save command (Save a guest) using virsh: 

# virsh restore filename
This restarts the saved guest, which may take some time. The guest's name and UUID are preserved but are allocated for a new id.
Shut down a guest

Shut down a guest using the virsh command: 

# virsh shutdown {domain-id, domain-name or domain-uuid}
You can control the behavior of the rebooting guest by modifying the on_shutdown parameter in the guest's configuration file.
Rebooting a guest

Reboot a guest using virsh command: 

#virsh reboot {domain-id, domain-name or domain-uuid}
You can control the behavior of the rebooting guest by modifying the on_reboot element in the guest's configuration file.
Forcing a guest to stop

Force a guest to stop with the virsh command: 

# virsh destroy {domain-id, domain-name or domain-uuid}
This command does an immediate ungraceful shutdown and stops the specified guest. Using virsh destroy can corrupt guest file systems . Use the destroy option only when the guest is unresponsive. For para-virtualized guests, use the shutdown option(Shut down a guest) instead.
Getting the domain ID of a guest

To get the domain ID of a guest: 

# virsh domid {domain-name or domain-uuid}
Getting the domain name of a guest

To get the domain name of a guest: 

# virsh domname {domain-id or domain-uuid}
Getting the UUID of a guest

To get the Universally Unique Identifier (UUID) for a guest: 

# virsh domuuid {domain-id or domain-name}
An example of virsh domuuid output: 
# virsh domuuid r5b2-mySQL01
4a4c59a7-ee3f-c781-96e4-288f2862f011
Displaying guest Information

Using virsh with the guest's domain ID, domain name or UUID you can display information on the specified guest: 

# virsh dominfo {domain-id, domain-name or domain-uuid}
This is an example of virsh dominfo output: 
# virsh dominfo r5b2-mySQL01
id:             13
name:           r5b2-mysql01
uuid:           4a4c59a7-ee3f-c781-96e4-288f2862f011
os type:      	linux
state:          blocked
cpu(s):         1
cpu time:     	11.0s
max memory:     512000 kb
used memory:    512000 kb
Displaying host information

To display information about the host: 

# virsh nodeinfo
An example of virsh nodeinfo output: 
# virsh nodeinfo
CPU model                    x86_64
CPU (s)                      8
CPU frequency                2895 Mhz
CPU socket(s)                2      
Core(s) per socket           2
Threads per core:            2
Numa cell(s)                 1
Memory size:                 1046528 kb
This displays the node information and the machines that support the virtualization process.
Displaying the guests

To display the guest list and their current states with virsh: 

# virsh list
Other options available include:
the --inactive option to list inactive guests (that is, guests that have been defined but are not currently active), and
the --all option lists all guests. For example: 
# virsh list --all
 Id Name                 State
----------------------------------
  0 Domain-0             running
  1 Domain202            paused
  2 Domain010            inactive
  3 Domain9600           crashed
The output from virsh list is categorized as one of the six states (listed below).
  • The running state refers to guests which are currently active on a CPU.
  • Guests listed as blocked are blocked, and are not running or runnable. This is caused by a guest waiting on I/O (a traditional wait state) or guests in a sleep mode.
  • The paused state lists domains that are paused. This occurs if an administrator uses the pause button in virt-manager, xm pause or virsh suspend. When a guest is paused it consumes memory and other resources but it is ineligible for scheduling and CPU resources from the hypervisor.
  • The shutdown state is for guests in the process of shutting down. The guest is sent a shutdown signal and should be in the process of stopping its operations gracefully. This may not work with all guest operating systems; some operating systems do not respond to these signals.
  • Domains in the dying state are in is in process of dying, which is a state where the domain has not completely shut-down or crashed.
  • crashed guests have failed while running and are no longer running. This state can only occur if the guest has been configured not to restart on crash.
Displaying virtual CPU information

To display virtual CPU information from a guest with virsh: 

# virsh vcpuinfo {domain-id, domain-name or domain-uuid}
An example of virsh vcpuinfo output: 
# virsh vcpuinfo r5b2-mySQL01
VCPU:           0
CPU:            0
State:          blocked
CPU time:       0.0s
CPU Affinity:   yy
Configuring virtual CPU affinity

To configure the affinity of virtual CPUs with physical CPUs: 

# virsh vcpupin domain-id vcpu cpulist
The domain-id parameter is the guest's ID number or name.
The vcpu parameter denotes the number of virtualized CPUs allocated to the guest.The vcpu parameter must be provided.
The cpulist parameter is a list of physical CPU identifier numbers separated by commas. The cpulist parameter determines which physical CPUs the VCPUs can run on.
Configuring virtual CPU count

To modify the number of CPUs assigned to a guest with virsh: 

# virsh setvcpus {domain-name, domain-id or domain-uuid} count
The new count value cannot exceed the count above the amount specified when the guest was created.

Important

This feature is unsupported and known not to work in Red Hat Enterprise Linux 5.
Configuring memory allocation

To modify a guest's memory allocation with virsh : 

# virsh setmem {domain-id or domain-name} count
You must specify the count in kilobytes. The new count value cannot exceed the amount you specified when you created the guest. Values lower than 64 MB are unlikely to work with most guest operating systems. A higher maximum memory value does not affect an active guests. If the new value is lower the available memory will shrink and the guest may crash.
Displaying guest block device information

Use virsh domblkstat to display block device statistics for a running guest. 

# virsh domblkstat GuestName block-device
Displaying guest network device information

Use virsh domifstat to display network interface statistics for a running guest. 

# virsh domifstat GuestName interface-device
Migrating guests with virsh

A guest can be migrated to another host with virsh. Migrate domain to another host. Add --live for live migration. The migrate command accepts parameters in the following format: 

# virsh migrate --live GuestName DestinationURL
The --live parameter is optional. Add the --live parameter for live migrations.
The GuestName parameter represents the name of the guest which you want to migrate.
The DestinationURL parameter is the URL or hostname of the destination system. The destination system requires:
  • the same version of Red Hat Enterprise Linux,
  • the same hypervisor (KVM or Xen), and
  • the libvirt service must be started.
Once the command is entered you will be prompted for the root password of the destination system.
Managing virtual networks

This section covers managing virtual networks with the virsh command. To list virtual networks: 

# virsh net-list
This command generates output similar to: 
# virsh net-list
Name                 State      Autostart
-----------------------------------------
default              active     yes      
vnet1	             active     yes      
vnet2	             active     yes
To view network information for a specific virtual network: 
# virsh net-dumpxml NetworkName
This displays information about a specified virtual network in XML format: 
# virsh net-dumpxml vnet1
<network>
  <name>vnet1</name>
  <uuid>98361b46-1581-acb7-1643-85a412626e70</uuid>
  <forward dev='eth0'/>
  <bridge name='vnet0' stp='on' forwardDelay='0' />
  <ip address='192.168.100.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.100.128' end='192.168.100.254' />
    </dhcp>
  </ip>
</network>
Other virsh commands used in managing virtual networks are:
  • virsh net-autostart network-name — Autostart a network specified as network-name.
  • virsh net-create XMLfile — generates and starts a new network using an existing XML file.
  • virsh net-define XMLfile — generates a new network device from an existing XML file without starting it.
  • virsh net-destroy network-name — destroy a network specified as network-name.
  • virsh net-name networkUUID — convert a specified networkUUID to a network name.
  • virsh net-uuid network-name — convert a specified network-name to a network UUID.
  • virsh net-start nameOfInactiveNetwork — starts an inactive network.
  • virsh net-undefine nameOfInactiveNetwork — removes the definition of an inactive network.