QoS provides priority control for individual volumes and groups of volumes. The QoS priority control function controls the priority order of I/O processing that the storage system performs for services that issue I/O requests.
Priority has three levels as described in the following table. When the storage system receives I/O requests for volumes or QoS groups for which priority control is set, I/O processing is controlled so that the I/O processing for volumes or QoS groups with higher priority is performed and the I/O processing for volumes or QoS groups with lower priority is delayed. The target response time is defined for each priority level. The storage system controls I/Os so that the target response time is satisfied.
| Priority level | Target response time |
|---|---|
| 3 (High) | 1 ms |
| 2 (Medium) | 5 ms |
| 1 (Low) | 10 ms |
CAUTION:
Specify the priority for all volumes for which the LUN path is set or a namespace on the NVM subsystem with an NVM subsystem port added for NVMe is set, or a QoS group containing those volumes. If there are volumes or QoS groups for which the priority is not set, the I/O processing on the volume or QoS group takes precedence, and the expected effect is not achieved.
Note:
- The QoS function controls priority only for the I/Os between hosts and the target volume. When a QoS-controlled volume is used as a pair volume for remote copy operations, the priority for the remote I/Os between the storage systems is not controlled.
- The priority control defines the priority of response and does not secure the target response time. The target response time might not be satisfied if a heavy load is placed on the storage system.
- The priority control does not suppress the response time of a low-priority volume to its target response time even if the target response time of a high-priority volume may not be attained.
- The target response time has a fixed value and cannot be changed.