Propagate power consumption metrics from hypervisor to virtual machines (Qemu/KVM)


A major pain point in measuring power consumption is doing so inside a virtual machine. A virtual machine usually doesn't have access to power metrics.

Scaphandre aims at solving that by enabling a communication between a scaphandre instance on the hypervisor/bare metal machine and another one running on the virtual machine. The scaphandre agent on the hypervisor will compute the metrics meaningful for that virtual machine and the one on the VM access those metrics to allow its user/administrator to use the data as if they had access to power metrics in the first place (as if they were on a bare metal machine).

This allows to break opacity in a virtualization context, if you have access to the hypervisor, or in a public cloud context if the provider uses scaphandre on its hypervisors.

How to

This is working on Qemu/KVM hypervisors only.

The idea is to run the agent on the hypervisor, with the qemu exporter:

scaphandre qemu

More examples for a production ready setup will be added soon (systemd service, docker container, ...). If you think the documentation needs a refresh now, please contribute :)

For each virtual machine you want to give access to its metrics, create a tmpfs mountpoint:

 mount -t tmpfs tmpfs_DOMAIN_NAME /var/lib/libvirt/scaphandre/DOMAIN_NAME -o size=5m

In the definition of the virtual machine (here we are using libvirt), ensure you have a filesystem configuration to give access to the mountpoint:

virsh edit DOMAIN_NAME

Then add this filesystem configuration block inside the <devices></devices> block:

<filesystem type='mount' accessmode='passthrough'>
    <driver type='virtiofs'/>
    <source dir='/var/lib/libvirt/scaphandre/DOMAIN_NAME'/>
    <target dir='scaphandre'/>
    <readonly />

Save and (re)start the virtual machine.

If you get this error: "error: unsupported configuration: 'virtiofs' requires shared memory", you might add this configuration section to the <domain> section.

  <source type='memfd'/>
  <access mode='shared'/>

Then connect to the virtual machine and mount the filesystem:

 mount -t 9p -o trans=virtio scaphandre /var/scaphandre

You can now run scaphandre to export the metrics with the exporter of your choice (here prometheus):

 scaphandre --vm prometheus

Please refer to the qemu exporter reference for more details.

Note: This how to is only suitable for a "manual" use case. For all automated systems like openstack or proxmox, some more work needs to be done to make the integration of those steps easier.