QoS provides lower-limit control for individual volumes to secure a certain level of performance (specified as the lower-limit value) for services that request I/Os. Set the lower-limit value for all volumes, where the value for each volume must be determined depending on the amount of I/Os.
When there are few I/O requests from hosts on individual volumes (#0 to #3) as in the following figure, the lower limit control function does not work because it is not necessary.
In this condition, if I/O requests from hosts on a specific volume (for example, take the specific volume as volume #0 in the figure) increase and require the throughput no less than the lower limit value set for volume #0, the throughput is in the state of not satisfying the lower limit (as shown by A in the following figure). Therefore, this function controls assignment of the order of I/O processing so that I/O processing on volume #0 takes priority and the throughput of volume #0 satisfies the lower limit (as shown by B in the following figure). Furthermore, if there is enough room for the entire I/O processing (for example, there are few I/O requests from hosts on volumes #1 to #3, and the throughput of volumes #1 to #3 does not need to satisfy each lower limit (as shown by C in the following figure)), this function allows I/O processing on volume #0 even if the throughput becomes larger than the lower limit (as shown by D in the following figure).
In this condition, if I/O requests from hosts on volumes #1 to #3 also increase, this function controls assignment of the order of I/O processing so that I/O processing on volumes #1 to #3 takes priority and the throughput of volumes #1 to #3 satisfies each lower limit (as shown by E in the following figure). Because the throughput of volume #0 already exceeds the lower limit, assignment of the order of I/O processing is adjusted so that I/O processing on volumes #1 to #3 takes priority over I/O processing on volume #0. Then, the throughput of volume #0 is decreased to around the lower limit value (as shown by F in the following figure).
This approximates the throughput of individual volumes to the lower-limit value.
The lower-limit control can be applied to the I/O rate, transfer rate, or both. If the lower-limit control is applied to both the I/O rate and the transfer rate, the I/O rate and the transfer rate are monitored on individual volumes. Then, when an I/O occurs, the volumes for which either of the I/O rate or the transfer rate does not reach the lower-limit value take priority for I/O processing.
- I/O rate (IOPS): 10 to 2,147,483,647
- Transfer rate (MB/s): 1 to 2,097,151
- Specify the lower-limit values for all volumes for which the LUN path is set, or you can specify the lower limit for all volumes for which a namespace on the NVM subsystem with an NVM subsystem port added for NVMe is set. If there are volumes for which the lower-limit values are not set, the volumes take priority over the other volumes and you cannot obtain expected results.
- If you specify the lower-limit value that exceeds the performance capacity of the storage system, you cannot obtain expected results. Specify an appropriate value considering the performance capacity of the storage system.
- The QoS function controls the lower limit only for the I/Os between hosts and the target volumes. When a volume is an external volume or a pair volume for remote copy operations, the lower limit for I/Os between the storage systems is not controlled.
- The lower-limit control for a volume and the priority control for a QoS group cannot be set at the same time. For details, see Combinations of the QoS settings for a volume and a QoS group.