36 Virtual machine administration using QEMU monitor #

Tip: No monitor console for libvirt

You can access the monitor console only if you started the virtual machine directly with the qemu-system-ARCH command and are viewing its graphical output in a native QEMU window.

If you started the virtual machine with libvirt (for example, using virt-manager) and are viewing its output via VNC or Spice sessions, you cannot access the monitor console directly. You can, however, send the monitor command to the virtual machine via virsh:

# virsh qemu-monitor-command COMMAND

The way you access the monitor console depends on which display device you use to view the output of a virtual machine. Find more details about displays in Section 35.3.2.2, “Display options”. For example, to view the monitor while the -display gtk option is in use, press Ctrl–Alt–2. Similarly, when the -nographic option is in use, you can switch to the monitor console by pressing the following key combination: Ctrl–AC.

To get help while using the console, use help or ?. To get help for a specific command, use help COMMAND.

To get information about the guest system, use info. If used without any option, the list of possible options is printed. Options determine which part of the system will be analyzed:

To change the VNC password, use the change vnc password command and enter the new password:

(qemu) change vnc password
Password: ********
(qemu)

To add a new disk while the guest is running (hotplug), use the drive_add and device_add commands. First define a new drive to be added as a device to bus 0:

(qemu) drive_add 0 if=none,file=/tmp/test.img,format=raw,id=disk1
OK

You can confirm your new device by querying the block subsystem:

(qemu) info block
[...]
disk1: removable=1 locked=0 tray-open=0 file=/tmp/test.img ro=0 drv=raw \
encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0

After the new drive is defined, it needs to be connected to a device so that the guest can see it. The typical device would be a virtio-blk-pci or scsi-disk. To get the full list of available values, run:

(qemu) device_add ?
name "VGA", bus PCI
name "usb-storage", bus usb-bus
[...]
name "virtio-blk-pci", bus virtio-bus

Now add the device

(qemu) device_add virtio-blk-pci,drive=disk1,id=myvirtio1

and confirm with

(qemu) info pci
[...]
Bus  0, device   4, function 0:
    SCSI controller: PCI device 1af4:1001
      IRQ 0.
      BAR0: I/O at 0xffffffffffffffff [0x003e].
      BAR1: 32 bit memory at 0xffffffffffffffff [0x00000ffe].
      id "myvirtio1"

Tip

Devices added with the device_add command can be removed from the guest with device_del. Enter help device_del on the QEMU monitor command line for more information.

To release the device or file connected to the removable media device, use the eject DEVICE command. Use the optional -f to force ejection.

To change removable media (like CD-ROMs), use the change DEVICE command. The name of the removable media can be determined using the info block command:

(qemu) info block
ide1-cd0: type=cdrom removable=1 locked=0 file=/dev/sr0 ro=1 drv=host_device
(qemu) change ide1-cd0 /path/to/image

It is possible to use the monitor console to emulate keyboard and mouse input if necessary. For example, if your graphical user interface intercepts some key combinations at low level (such as Ctrl–Alt–F1 in X Window), you can still enter them using the sendkey KEYS:

sendkey ctrl-alt-f1

To list the key names used in the KEYS option, enter sendkey and press →|.

To control the mouse, the following commands can be used:

If the virtual machine was started with the -balloon virtio option (the paravirtualized balloon device is therefore enabled), you can change the available memory dynamically. For more information about enabling the balloon device, see Section 34.1, “Basic installation with qemu-system-ARCH”.

To get information about the balloon device in the monitor console and to determine whether the device is enabled, use the info balloon command:

(qemu) info balloon

If the balloon device is enabled, use the balloon MEMORY_IN_MB command to set the requested amount of memory:

(qemu) balloon 400

To save the content of the virtual machine memory to a disk or console output, use the following commands:

Managing snapshots in QEMU monitor is not supported by SUSE yet. The information found in this section may be helpful in specific cases.

Virtual Machine snapshots are snapshots of the complete virtual machine including the state of CPU, RAM, and the content of all writable disks. To use virtual machine snapshots, you must have at least one non-removable and writable block device using the qcow2 disk image format.

Snapshots are helpful when you need to save your virtual machine in a particular state. For example, after you have configured network services on a virtualized server and want to quickly start the virtual machine in the same state that was saved last. You can also create a snapshot after the virtual machine has been powered off to create a backup state before you try something experimental and possibly make VM Guest unstable. This section introduces the former case, while the latter is described in Section 34.2.3, “Managing snapshots of virtual machines with qemu-img”.

The following commands are available for managing snapshots in QEMU monitor:

The following commands are available for suspending and resuming virtual machines:

The live migration process allows to transmit any virtual machine from one host system to another host system without any interruption in availability. It is possible to change hosts permanently or only during maintenance.

The requirements for live migration:

Important: Support status

The postcopy mode is not yet supported in SUSE Linux Enterprise Server. It is released as a technology preview only. For more information about postcopy, see http://wiki.qemu.org/Features/PostCopyLiveMigration.

More recommendations can be found at the following Web site: http://www.linux-kvm.org/page/Migration

The live migration process has the following steps:

QMP is a JSON-based protocol that allows applications—such as libvirt—to communicate with a running QEMU instance. There are several ways you can access the QEMU monitor using QMP commands.

> sudo qemu-system-x86_64 [...] \
-chardev stdio,id=mon0 \
-mon chardev=mon0,mode=control,pretty=on

<- {
    "QMP": {
        "version": {
            "qemu": {
                "micro": 0,
                "minor": 0,
                "major": 2
            },
            "package": ""
        },
        "capabilities": [
        ]
    }
}

-> { "execute": "qmp_capabilities" }
<- {
    "return": {
    }
}

->{ "execute": "eject", "arguments": { "device": "ide1-cd0" } }
<- {
    "timestamp": {
        "seconds": 1410353381,
        "microseconds": 763480
    },
    "event": "DEVICE_TRAY_MOVED",
    "data": {
        "device": "ide1-cd0",
        "tray-open": true
    }
}
{
    "return": {
    }
}

> sudo qemu-system-x86_64 [...] \
-chardev socket,id=mon0,host=localhost,port=4444,server,nowait \
-mon chardev=mon0,mode=control,pretty=on

> telnet localhost 4444
Trying ::1...
Connected to localhost.
Escape character is '^]'.
<- {
    "QMP": {
        "version": {
            "qemu": {
                "micro": 0,
                "minor": 0,
                "major": 2
            },
            "package": ""
        },
        "capabilities": [
        ]
    }
}

> sudo qemu-system-x86_64 [...] \
-chardev stdio,id=mon0 -mon chardev=mon0,mode=readline \
-chardev socket,id=mon1,host=localhost,port=4444,server,nowait \
  -mon chardev=mon1,mode=control,pretty=on

> sudo qemu-system-x86_64 [...] \
-qmp unix:/tmp/qmp-sock,server --monitor stdio

QEMU waiting for connection on: unix:./qmp-sock,server

> sudo nc -U /tmp/qmp-sock
<- {"QMP": {"version": {"qemu": {"micro": 0, "minor": 0, "major": 2} [...]

36.11.4 Access QMP via libvirt's virsh command #

 

If you run your virtual machines under libvirt (see Part II, “Managing virtual machines with libvirt”), you can communicate with its running guests by running the virsh qemu-monitor-command:

> sudo virsh qemu-monitor-command vm_guest1 \
--pretty '{"execute":"query-kvm"}'
<- {
    "return": {
        "enabled": true,
        "present": true
    },
    "id": "libvirt-8"
}

In the above example, we ran the simple command query-kvm which checks if the host is capable of running KVM and if KVM is enabled.

Tip: Generating human-readable output

To use the standard human-readable output format of QEMU instead of the JSON format, use the --hmp option:

> sudo virsh qemu-monitor-command vm_guest1 --hmp "query-kvm"