Frequently Asked Questions

This section collects a number of frequently asked questions concerning the administration of a StratusLab cloud infrastructure. If there are other questions that you would like to see treated here, please contact StratusLab support.

How to restart VMs after a power outage?

After the power outage the nodes hosting the virtual machines must be restarted. If this has not happened automatically, ensure that all of the nodes have rebooted.

Connect to each of the nodes to ensure that all of the iSCSI mounts are present.

# iscsiadm -m session

If the links are not correctly mounted (bad links will blink in the list), then the easiest way to recover this is simply to reboot the machine again.

Once you have verified all of the iSCSI links on all of the nodes, then you must log into the frontend machine as oneadmin. As this user, you can then force OpenNebula to restart all of the machines in the unknown state.

The commands for this (assuming that you’re logged into the frontend as root), are:

# su - oneadmin
$ for vm in `onevm list all |grep unk | awk ‘{print $1}’`; \
    do; \
      onevm restart $vm; \

After this, all of the VMs should return to the running state and again be accessible from the network. Any machines that don’t restart are probably lost and will have to be killed and a new instance created.

How to support advanced CPU flags in virtual machines?

By default, libvirt and KVM provide a generic CPU processor to virtual machines. For computing intensive applications, access to more advanced CPU capabilities (CPU “flags”) can make an enormous difference in the efficiency of a calculation. Consequently, the generic CPU profile provided by libvirt and KVM is inadequate for these applications.

Most modern processors have the advanced CPU capabilities that numerical applications need. The only problem is how to get libvirt and KVM to expose these capabilities to the hosted virtual machines?

Unfortunately, libvirt does not provide a mechanism for passing arbitrary options to the underlying hypervisor. However, the call to the hypervisor can be wrapped to provide the additional options.

Create a wrapper script for the call to qemu that will add the -cpu option:

/usr/libexec/qemu-kvm -cpu host $@

This additional option will allow the host CPU specification to be passed directly to virtual machines, allowing all of the advanced capabilities of the host processor to be used.


The wrapper script must be installed on all of the nodes of the StratusLab cloud infrastructure. This script is named /var/lib/one/bin/ below; change this value as necessary.

Once the wrapper script is installed, then the OpenNebula configuration must be changed to force this script to be used to start virtual machines. Modify the configuration files /etc/one/vmm_exec/vmm_exec_kvm*.conf setting the value of the EMULATOR parameters:

EMULATOR = /var/lib/one/bin/

Change the value to match the wrapper script that you have created. Once this is done, restart OpenNebula. Execute:

$ service oned restart

on the frontend node of your StratusLab cloud infrastructure.

Once this is done, any new virtual machines will use the wrapper script and will have the full CPU capabilities of the host processor. You can verify the CPU capabilities from the virtual machine with the command:

$ cat /proc/cpuinfo

The relevant capabilities are in the “flags” part of the response.

How to fix the keyboard mapping for virt-manager?

Occasionally when debugging problems with virtual machines it is useful to use virt-manager via X11 to bring up the machine console. The default configuration unfortunately makes the keyboard mapping completely useless. To fix this, you must use the -k option when starting virtual machines with QEMU.

To do this, follow the same procedure as for exposing the CPU flags above and add the -k en-us option to the qemu command line. This will cause the keyboard mapping to always be a US English. You may use other language values, although the US English is the safest default.