When the Automated QoS Priority is set, the Automated QoS function calculates the targeted value (in milliseconds) at the response time. The targeted value is calculated based on the storage device that is equipped with a high performance disk, from the Automated QoS Priority that was set and the current I/O performance. In addition, if the target response time is set, the value (in milliseconds) that was set is the target value of the performance adjustment.
Automatic tuning for the Automated QoS function adjusts the bandwidth so the volume performance that was set as the target is reached. In order to adjust the bandwidth of the volume that was set with a target, the bandwidth for the volume that was not set with a target is also adjusted.
A bandwidth adjustment is performed based on the measured response time of the read + write volume. However, automatic tuning does not always guarantee the target value that was set.
Structure of Automatic Tuning
The QoS function for the ETERNUS Disk storage system sets the bandwidth restriction in 16 levels from 60 IOPS, 5 MB/s to unlimited. The Automated QoS function adjusts I/O performance by changing the bandwidth using this function.
Immediately following the target value that was initially set in the target volume, there may be cases when the effects are not immediately apparent.
By setting the Automated QoS Priority level and target response time for automatic tuning, performance information for each evaluation interval (1 minute) is obtained, and checks and adjustments are repeated as follows:
A common resource with the largest Busy rate (CM, Port, switch Port, and Tier pool) is found.
The bandwidth is adjusted between the volumes that share resources with the largest busy rate.
The bandwidth of the volume with a measured performance that exceeds the target performance is narrowed by one level.
The bandwidth of the volume that is controlled by the ETERNUS Disk storage system's QoS setting with a measured performance lower than the target performance is widened by one level.
For bandwidth insufficiencies that arise for the results described above, adjustments are made by narrowing the bandwidth for volumes without a set target value.
Point
When the Busy rate is low, automatic tuning is not performed. Refer to "Consideration of System in Low Load State" for details.
The following example shows the adjustments that are performed with automatic tuning depending on the Automated QoS Priority setting.
When a business that is using three volumes (Vol#1 - Vol #3) are set to high priority, medium priority and low priority, set the volumes with Automated QoS priority.
Here, Vol#1 is set to "High", Vol#2 is set to "Middle", and Vol#3 is set to "Low".
The Automated QoS function calculates the target response times for Vol#2 and Vol#3 internally based on the measured performance for Vol#1.
The Automated QoS function compares the calculated target response times and measured performance for Vol#2 and Vol#3. If the results of the measured performance exceed the target performance, the bandwidth is narrowed by one level. If the measured performance is lower than the target performance and the volume bandwidth is controlled by the ETERNUS Disk storage system QoS setting, the bandwidth is widened by one level.
Depending on the above process, the bandwidth is adjusted for the three volumes according to the Automated QoS Priority setting.
Figure 5.1 Example of Bandwidth Adjustment when Automated QoS Priority is Set
The following example shows adjustments that are performed with automatic tuning depending on the target response time setting:
Check the I/O performance of the business application with the Web Console performance graph screen. When the measured value is 50 milliseconds for the response time of Vol#1 that is used and the I/O performance is determined to be low, reduce the response time lower than 50 milliseconds (for example, 30 milliseconds).
At this time, it is not necessary to set the target response time for the other volumes (Vol#2, Vol#3).
By setting the target response time, the Automated QoS function performs adjustments by widening the bandwidth range for Vol#1 and narrowing the bandwidth for Vol#2 and Vol#3 so that the response time approaches the target value.
Depending on the above process, the bandwidth is adjusted for Vol#1 according to the target response time setting.
Figure 5.2 Example of Bandwidth Adjustment when Target Response Time is Set
Coordination with Automated Storage Tiering
When set to link with Automated Storage Tiering, and it is not possible to achieve performance objectives with the Automated QoS function alone, Automated Storage Tiering can be used to relocate data in accordance with the capacity allocation ratio. In this way, the performance target can be achieved.
Note
The Automated QoS function refers to the performance information of the storage device that belongs to the performance adjustment target FTV, and the performance information of the switch.
The Automated QoS function can be used even when the performance function for the switch is disabled, but in this case, performance is adjusted only for shared resources related to the CMs and Tier pools.
Consideration of System in Low Load State
In general, the system load state differs according to the timeframe.
With the system in a low load state, the business application I/O performance is not affected by the operation of other business applications, and the measured response time may be shorter than the target response time. In this case, when adjusting the bandwidth to be narrower with automatic tuning, I/O access to the storage device increases suddenly and puts the system under high load and may lead to longer response times. The Automated QoS function, in order to prevent this kind of decrease in I/O performance, only narrows the bandwidth when the following conditions are satisfied.
For volumes that are set with a target response time, the measured response time is shorter than the target response time.
For other volumes that are set with the target response time under shared resources with a high busy rate, volumes with a longer measured response time than the target response time exist.
For this reason, automatic tuning is not performed in the following cases.
For all volumes under the shared resources, where the measured response time is shorter than the target response time
For volumes, when there is no load from the business application
In addition, if there is no business load but there is room in the bandwidth, the remaining bandwidth is temporarily accommodated to other volumes where the bandwidth is insufficient, and bandwidth shortage is eliminated. As a result, a higher I/O performance target response time might be obtained when there is room in the bandwidth of other volumes.
