4.2. Virtual disk QoS settings (XenEnterprise only)

In XenEnterprise, virtual disks have an optional i/o priority Quality of Service (QoS) setting. This setting can be made to existing virtual disks with the CLI as described in this section.

The first parameter is qos_algorithm_type. This parameter needs to be set to the value ionice, which is the only type of QoS algorithm supported for virtual disks in this release.

The QoS parameters themselves are set with key/value pairs assigned to the qos_algorithm_type parameter. For virtual disks, qos_algorithm_type takes a sched key, and depending on the value, also requires a class key.

Possible values of qos_algorithm_type:sched are

sched=rt or sched=real-time sets the QoS scheduling parameter to real time priority, which requires a class parameter to set a value
sched=idle sets the QoS scheduling parameter to idle priority, which requires no class parameter to set any value
sched=<anything> sets the QoS scheduling parameter to best effort priority, which requires a class parameter to set a value

The possible values for class are

One of the following keywords: highest, high, normal, low, lowest
an integer between 0 and 7, where 0 is the highest priority and 7 is the lowest

For example, the following CLI commands set the virtual disk's VBD to use real time priority=5:

xe vbd-param-set uuid=<vbd UUID> qos_algorithm_type=ionice
xe vbd-param-set uuid=<vbd UUID> qos_algorithm_params:sched=rt
xe vbd-param-set uuid=<vbd UUID> qos_algorithm_params:class=5