How to control VM traffic on KVM

Question

I have multiple VMs running on a KVM hypervisor. One of the VMs is consuming most of the available bandwidth, making it difficult for other VMs to use. How can I control the bandwidth usage for each VM. I would like to guarantee bandwidth for each individual VM. I would like to know if this is usually done on a hypervisor level, if so how? or on a virtual switch level provided that I have one.

Solution

The libvirt domain specification includes this functionality already. You can specify separate settings for incoming and outgoing traffic. The example given in the libvirt documentation is:

<devices>
  <interface type='network'>
    <source network='default'/>
    <target dev='vnet0'/>
    <bandwidth>
      <inbound average='1000' peak='5000' floor='200' burst='1024'/>
      <outbound average='128' peak='256' burst='256'/>
    </bandwidth>
  </interface>
<devices>

You can read more upstream at http://libvirt.org/formatdomain.html#elementQoS