Introduction
When you have read the article about tunneling a vnc socket over ssh, you already know that I am a fan of using simple technologies like Unix sockets and ssh to access virtual machine information.
I am currently in a project to create a new virtual machine infrastructure based on Qemu/KVM. The problem we were facing is "How to shutdown a virtual machine, when the host is being shut down".
Background
Qemu/KVM has a so called monitor to control the VM. Usually this monitor is reachable from the VNC socket (or whatever you use to view the console) by pressing Ctrl-Alt-2.
This is inappropriate for automatic shutdown (I'm not in the mood to script vnc sessions currently), so there must be a better solution.
The solution
Qemu/KVM is able to redirect the monitor to a "character device". There are ways to create a character device with Qemu/KVM and then attach the monitor to it, but you can have it more easy by connecting the monitor directly to a UNIX socket:
qemu-kvm ... -monitor unix:/opt/local.ch/sys/kvm/vm/kvmtest-vm-inx01.intra.local.ch/monitor,server,nowait
This way we can connect (and control!) Qemu/KVM using [socat](http://www.dest-unreach.org/socat/]:
socat - UNIX-CONNECT:/opt/local.ch/sys/kvm/vm/kvmtest-vm-inx01.intra.local.ch/monitor
And when we can connect to it, we can also shutdown a virtual machine:
echo system_powerdown | socat - UNIX-CONNECT:/opt/local.ch/sys/kvm/vm/kvmtest-vm-inx01.intra.local.ch/monitor
Or we could reset it:
echo system_reset | socat - UNIX-CONNECT:/opt/local.ch/sys/kvm/vm/kvmtest-vm-inx01.intra.local.ch/monitor
The full implementation
The full command line for running a VM as we do it in this project looks like this:
[root@kvm-hw-inx01 kvmtest-vm-inx01.intra.local.ch]# cat start
#!/bin/sh
/usr/libexec/qemu-kvm -m 65536 \
-hda /opt/local.ch/sys/kvm/vm/kvmtest-vm-inx01.intra.local.ch/system-disk \
-vnc unix:/opt/local.ch/sys/kvm/vm/kvmtest-vm-inx01.intra.local.ch/vnc \
-boot order=nc \
-net nic,macaddr=00:16:3e:00:00:2d,vlan=200 \
-net tap,script=/opt/local.ch/sys/kvm/bin/ifup-pz,downscript=/opt/local.ch/sys/kvm/bin/ifdown,vlan=200 \
-net nic,macaddr=00:16:3e:00:00:2e,vlan=300 \
-net tap,script=/opt/local.ch/sys/kvm/bin/ifup-fz,downscript=/opt/local.ch/sys/kvm/bin/ifdown,vlan=300 \
-smp 16 \
-name kvmtest-vm-inx01.intra.local.ch \
-enable-kvm \
-monitor unix:/opt/local.ch/sys/kvm/vm/kvmtest-vm-inx01.intra.local.ch/monitor,server,nowait
As the VMs are currently not performing as well as they should, we will do some investigations into performance tuning of Qemu/KVM. So stay tuned, if you are interested in this topic.