Storage Spaces Tiering and QoS Integration
From what I understand so far, SOFS Storage QoS operates independently from Storage Spaces automated tiering.
Storage Spaces automated tiering optimization process should take into consideration VHDXs QoS policies in order to meet the IOPS objective. If tiering only looks at the number of time a certain piece of data has been accessed between the tier optimization executions, the data could possibly be not placed on the optimal tier to meet the performance target.
If the SSD tier could be balanced fairly between VHDXs based on QoS policy targets, that would most likely ensure more consistent IO performance.