This feature makes it possible to use virtio balloons for memory optimization.
Every virtual machine in clusters with a compatibility level of 3.2 and higher includes a balloon device unless that device is specifically removed. This balloon device requires guest drivers and a guest agent to control the balloon size.
Ballooning optimization is a cluster-level policy attribute, which is disabled by default. To have a balloon running, the virtual machine must have a balloon device with the relevant drivers, and the cluster it belongs to must enable ballooning optimization. Each host in the cluster receives a balloon policy update when the status of that host changes to 'Up'. A manual option is also available for emergencies that makes it possible to force an update for a specific host.
Once this is set, MoM will start ballooning where and when possible to allow memory over-commitment, with a limitation of the guaranteed memory size that every virtual machine has.
Note that in some scenarios ballooning may collide with KSM. In such cases, MoM will attempt to adjust the balloon size to minimize collisions. Additionally, in some scenarios ballooning may cause sub-optimal performance for a virtual machine. Administrators are advised to use ballooning optimization with caution.