Skip to main content

KVM/Libvirt

Most of the OCF's hosts are virtual machines. Our virtual machines are all powered by QEMU/KVM and managed by libvirt on the hypervisors.

VM disks are stored as LVM volumes on the hypervisors, typically under /dev/vg/<vm-name>.

Administration​

How do I view which VMs are on a hypervisor?​

SSH into the hypervisor, then run sudo virsh list --all:

kpengboy@hal:~$ sudo virsh list --all
 Id    Name                           State
----------------------------------------------------
 1     fallingrocks                   running
 3     limniceruption                 running
 4     spriggs                        running
 5     dev-flood                      running
 11    dev-whiteout                   running
 15    dev-death                      running
 18    dev-tsunami                    running
 -     dev-anthrax                    shut off
 -     dev-pestilence                 shut off
 -     dev-werewolves                 shut off
 -     zombies                        shut off

How do I turn on a VM?​

On the hypervisor, run sudo virsh start <vm-name>.

How do I turn off a VM?​

You can SSH into the VM and run the shutdown command, or you can run sudo virsh shutdown <vm-name> on the hypervisor which hosts it.

If it's a public-facing VM (e.g. tsunami), remember to give a positive amount of time to the shutdown command, so users have adequate warning.

How do I make a VM automatically turn on when the hypervisor boots?​

On the hypervisor, run sudo virsh autostart <vm-name>.

You can list which VMs are set to autostart with sudo virsh list --all --autostart.

Firestorm is set to autostart because it must be running in order for any staff to log in (other than by using the root account). Other VMs are not set to autostart because if they start before LDAP and Kerberos are available, logins may not necessarily work properly.

Is there a GUI for all of this?​

You can run virt-manager on the hypervisors to graphically start and stop VMs, view their virtual monitors, and do many other similar things. To access virt-manager, SSH into the hypervisor with X forwarding (-X) enabled. Then, on the hypervisor, run sudo XAUTHORITY=~/.Xauthority virt-manager.

If you just want to see a single VM's virtual monitor​

You can use the virt-viewer tool. Replace virt-manager in the above command with virt-viewer <vm-name>.

Alternatively, you can directly connect to the VM's VNC display. This procedure gives much better performance than X forwarding, especially outside of the OCF network. To do this, first run sudo virsh vncdisplay <vm-name> on the hypervisor, and record what it prints out. Then, from your local computer, run vncviewer -via <hypervisor> <output of virsh vncdisplay>. So for instance, if virsh vncdisplay printed out 127.0.0.1:10, and the hypervisor were jaws, then you would run vncviewer -via jaws 127.0.0.1:10 (or localhost:10).

How do I create a VM?​

See Creating new hosts.

How do I delete a VM?​

On the hypervisor:

  1. Shutdown the VM.
  2. Run sudo virsh undefine <vm-name>.
  3. Backup the VM's disk (e.g. by renaming the LVM volume to vg/<vm-name>.old) or delete it. You may want to also dump the contents of the disk to a file, compressing it, and placing that file in /opt/backups/live/misc/servers on the server which contains backups (which is hal at the time of this writing). You may also want to save the VM's XML definition by running sudo virsh dumpxml [vm-name] > [vm-name].xml and placing it in the same aforementioned directory.

How do I move a VM from one host to another?​

Use the migrate-vm script.

Oh no, I've got a VM with broken networking. How can I access it?​

You can open virt-manager as described above, open the VM's display, and then log in there.

Alternatively, if you don't want to bother with opening up a GUI, you can often access the VM using its serial console. Run sudo virsh console <vm-name> on the hypervisor. This will connect you to the VM's TTY listening on its simulated serial port.

You may have to initially hit enter for the VM to (re-)print the login prompt. When you're done, make sure to log out. Then use Ctrl+] to exit the virsh console.

This method of accessing the VM only works when there is a getty process listening on the serial port. All of our VMs start such a process automatically, but only after boot has mostly finished. Therefore, the serial console probably won't work if you're trying to diagnose boot problems.

How do I edit my VM's RAM size or CPU count?​

On the hypervisor, run sudo virsh edit <vm-name> to edit the VM's XML definition.

To query and modify virtual hardware state for your vm, use the following commands, for RAM:

virsh dommemstat <vm-name> [[--config] [--live] | [--current]]
virsh setmaxmem <vm-name> <size> [[--config] [--live] | [--current]]
virsh memtune <vm-name> [...]

and for vCPUs:

virsh vcpuinfo <vm-name> [...]
virsh vcpucount <vm-name>
virsh setvcpus <vm-name> <count> [...] [--guest] [--hotpluggable]

How do I edit my VM's disk size?​

Find the lvm vg

virsh dumpxml <vm> | grep -C3 path

On the hypervisor,

virsh dominfo <vm>
virsh domblkinfo <vm>
lvextend /dev/vg/<vm> -L20G
virsh blockresize <vm> /dev/vg/<vm> 20G

On the vm,

fdisk /dev/vda        # if needed, very carefully
partprobe /dev/vda1
resize2fs /dev/vda1

If you'd like to shrink a partition on-line, see Unix SE.