QoS is a means to ensure specific performance levels for applications and workloads. There are two ways to accomplish this:
1. Ability to set limitations to avoid unwanted behavior from non-critical VMs.
2. Ability to set guarantees to ensure service levels for mission-critical VMs.

Windows 2019 Failover Clustering includes Storage QoS for use in scenarios where S2D is used in conjunction with Hyper-V. Storage QoS supports both methods (maximums as well as minimums) and mostly focusses on IOPs. A Storage QoS policy can be tied to an individual virtual disk.

Two kind of QoS policies can be used:
1. Aggregated policies apply maximums and minimum for the combined set of VHD/VHDX files and virtual machines where they apply.
2. Dedicated policies apply the minimum and maximum values for each VHD/VHDx, separately.

A single policy can be tied to one or more virtual disks.

Storage QoS can only be used when all servers (storage clients and storage servers) are running Windows Server 2019.