What causes a Persistent Settings Stream to be created initially?

The creation of the first domain or any PSS settings change (via SNMP, management API, or the storage UIs) will create the PSS if it does not already exist.

Also, any user-initiated shutdown or reboot will create a PSS.  One of the shutdown steps is to make sure we have the latest PSS, which will create one if there is not one.  And then we update the PSS with volumes we expect to return before shutting off.