Understanding events from adaptive QoS policies that have a defined block size

Adaptive QoS policy groups automatically scale a throughput ceiling or floor based on the volume size, maintaining the ratio of IOPS to TBs as the size of the volume changes. Starting with ONTAP 9.5 you can specify the block size in the QoS policy to effectively apply a MBps threshold at the same time.

Assigning an IOPS threshold in an adaptive QoS policy places a limit only on the number of operations that occur in each workload. Depending on the block size that is set on the client that generates the workloads, some IOPS include much more data and therefore place a much larger burden on the nodes that process the operations.

The MBps value for a workload is generated using the following formula:
MBps = (IOPS * Block Size) / 1000
If a workload is averaging 3,000 IOPS and the block size on the client is set to 32 KB, then the effective MBps for this workload is 96. If this same workload is averaging 3,000 IOPS and the block size on the client is set to 48 KB, then the effective MBps for this workload is 144. You can see that the node is processing 50% more data when the block size is larger.

Let's look at the following adaptive QoS policy that has a defined block size and how events are triggered based on the block size that is set on the client.

Create a policy and set the peak throughput to 2,500 IOPS/TB with a block size of 32KB. This effectively sets the MBps threshold to 80 MBps ((2500 IOPS * 32KB) / 1000) for a volume with 1 TB used capacity. Note that Unified Manager generates a Warning event when the throughput value is 10% less than the defined threshold. Events are generated under the following situations:

Used Capacity Event is generated when throughput exceeds this number of ...
IOPS MBps
1 TB 2,250 IOPS 72 MBps
2 TB 4,500 IOPS 144 MBps
5 TB 11,250 IOPS 360 MBps

If the volume is using 2TB of the available space, and the IOPS is 4,000, and the QoS block size is set to 32KB on the client, then the MBps throughput is 128 MBps ((4,000 IOPS * 32 KB) / 1000). No event is generated in this scenario because both 4,000 IOPS and 128 MBps are below the threshold for a volume that is using 2 TB of space.

If the volume is using 2TB of the available space, and the IOPS is 4,000, and the QoS block size is set to 64KB on the client, then the MBps throughput is 256 MBps ((4,000 IOPS * 64 KB) / 1000). In this case the 4,000 IOPS does not generate an event, but the MBps value of 256 MBps is above the threshold of 144 MBps and an event is generated.

For this reason, when an event is triggered based on a MBps breach for an adaptive QoS policy that includes the block size, a MBps chart is displayed in the System Diagnosis section of the Event details page. If the event is triggered based on an IOPS breach for the adaptive QoS policy, an IOPS chart is displayed in the System Diagnosis section. If a breach occurs for both IOPS and MBps you will receive two events.

For more information on adjusting QoS settings, see the ONTAP 9 Performance Monitoring Power Guide.

ONTAP 9 Performance Monitoring Power Guide