The OPTIONS+="db_persist" may be quite important for, primarily, handling the tranfer of udev database from initramfs to root fs. This options is not yet documented in udev(7) man page. Please, add a note about this option, possibly explaining properly what is its primary use or any other information that is good to know when using this option (e.g. a relation to udevadm info --cleanup-db used mostly at boot after switching from initramfs to root fs).