My understanding of defaultStartupFCV (please correct if wrong) is that this parameter was introduced in 7.0 in order to direct the server as to the level of FCV to use when creating new data files, perhaps on a newly provisioned machine.

In Atlas, we use a declarative configuration model between the control plane and data plane which involves specify this parameter at all times and we do not track which machines are expected to have data files, and which are not.

This causes an unfortunate situation when upgrading feature compatibility version with the setFCV command. setFCV itself does not require a restart, but when setting that in our declarative configuration, we also set defaultStartupFCV which does require a restart.

defaultStartupFCV itself doesn't make any sense at runtime as I understand it, any choices made off its value are made using the startup property.

This ticket requests making defaultStartupFCV a startup-and-runtime parameter, but where the runtime parameter no-ops.