The response time of a QoS group is the average response time of I/O processing for the QoS group. Setting the priority level for the QoS group enables the I/O processing control so that the response time of the QoS group meets the target response time. The I/O processing control is not enabled for the individual volumes in the QoS group. To enable the I/O processing control for the individual volumes in the QoS group to meet the target response time, make sure to set the priorities for the individual volumes.
The priority level setting for a QoS group and the lower-limit setting for a volume cannot be set at the same time. For more information, see Combinations of the QoS settings for a volume and a QoS group.
The following examples describe the differences in the priority setting between multiple volumes and a QoS group.
Example 1:
When the medium priority level (target response time of 5 ms) is set for three volumes and the low priority level (target response time of 10 ms) is set for a volume without using a QoS group, to secure the response time (5 ms) of the volume with medium priority, the I/O processing on the volume with low priority is performed at the low priority within the target response time (10 ms).
Example 2:
When the medium priority level (target response time of 5 ms) is set for the QoS group that contains three volumes with no priority levels set, if the average response time of the QoS group is 5 ms or more, the I/O processing priority control is enabled so that the response time is less than 5 ms.
If the response time of individual volumes is 1 ms, 4 ms, and 8 ms respectively, and that of the QoS group is less than 5 ms, the response time of the QoS group meets the target time (5 ms). Therefore, the I/O processing priority control for the volume with response time of 8 ms (exceeding 5 ms) is not available.