20.16.9.14. Quality of service

This section of the domain XML provides setting quality of service. Incoming and outgoing traffic can be shaped independently. The bandwidth element can have at most one inbound and at most one outbound child elements. Leaving any of these children element out results in no QoS being applied on that traffic direction. Therefore, when you want to shape only domain's incoming traffic, use inbound only, and vice versa.

Each of these elements has one mandatory attribute average (or floor as described below). average specifies average bit rate on the interface being shaped. Then there are two optional attributes: peak, which specifies maximum rate at which interface can send data, and burst, which specifies the amount of bytes that can be burst at peak speed. Accepted values for attributes are integer numbers.

The units for average and peak attributes are kilobytes per second, whereas burst is only set in kilobytes. In addition, inbound traffic can optionally have a floor attribute. This guarantees minimal throughput for shaped interfaces. Using the floor requires that all traffic goes through one point where QoS decisions can take place. As such it may only be used in cases where the interface type='network'/ with a forward type of route, nat, or no forward at all). It should be noted that within a virtual network, all connected interfaces are required to have at least the inbound QoS set (average at least) but the floor attribute does not require specifying average. However, peak and burst attributes still require average. At the present time, ingress qdiscs may not have any classes, and therefore floor may only be applied only on inbound and not outbound traffic.

To specify the QoS configuration settings, use a management tool to make the following changes to the domain XML:

  ...
  <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>
  ...