Abstract
The Qemu Guest Agent permits access to a VM via a virtual serial socket. This has the advantage that the VM can be accessed via the VM node (hypervisor) without network connection or remote desktop protocol.
This is a requirement to be able to take disk-only-snapshots without downtime by issueing a disk-freeze (shadow-copy) within the guest prior to taking the snapshot.
Furthermore it is useful for orchestrating CPU and memory hotplugging:
- guest OS must be told to enable CPU/memory after hot-plug
- guest OS must release a CPU/memory prior to hot-unplug
Libvirt
The XML must be amended by the following XML snippet in the devices section:
<channel type="unix"> <source mode="bind"/> <target type="virtio" name="org.qemu.guest_agent.0"/> </channel>
This will create a new virtual serial device within the VM and a new socket under code>/var/lib/libvirt/qemu/channel/target/</code> (make sure that this directory is protected!) named
/var/lib/libvirt/qemu/channel/target/${VMNAME}.org.qemu.guest_agent.0.
Please note: since libvirt will automatically listen on that socket one can not use a tool like qemu-ga-client to anything on it, everything has to go via libvirt.
VM
In the VM one must install and start the qemu-guest-agent.
On Gentoo this means:
emerge qemu-guest-agent /etc/init.d/qemu-guest-agentqemu-guest-agent rc-update add qemu-guest-agent default
Test
Shutdown a VM via qemu-ga
The advantage of using qemu-ga instead of the ACPI-based mechanism to shutdown a VM is that you get a confirmation if the qemu-ga was able to issue shutdown -P within the VM whereas with ACPI you won't even know whether the guest OS has received the event.
virsh shutdown --mode agent $VMNAME
Example:
~ # virsh shutdown --mode agent bc58f697-1f21-4613-9f4d-469cdaff0621Domain bc58f697-1f21-4613-9f4d-469cdaff0621 is being shutdown
Ping qemu-ga via libvirt
virsh qemu-agent-command $VMNAME'{"execute":"guest-ping"}'
Example:
~ # virsh qemu-agent-command bc58f697-1f21-4613-9f4d-469cdaff0621 '{"execute":"guest-ping"}'
{"return":{}}