Hello, thanks for your interest in Knot Resolver :-)

cache('['count_entries']'): 4953
cache('['usage_percent']'): 16.953125

These values do persist for me, but I don't really know FreeBSD in particular and don't have it at hand.

Have you checked that noone clears the files and you don't have cache.clear() in some config, etc?  Say, /var/run is usually considered ephemeral and not persisted across reboots.

#) become cache.stats() reset after a reboot?

Beware that most of those stats are tied the single current process (PID), counting the number of an operation was performed.  But those two above are indeed shared and persisted, as they're properties of the data.mdb file contents.

--Vladimir