Hi Greg.
I resolved my problem with a good, old-fashioned reboot. The root cause in my case will forever be a mystery. I looked for evidence of a significant event, and found nothing.
If your replica is indeed creating files at a crazy rate, I’d expect to see elevated RAM too.
I’m not expert enough to advise you on your case, but I can say what I’d be inclined to do:
If I was confident that those million files were erroneous (i.e. expendable), I’d stop that replica, trash the data directory and restart it. If it did’t fix itself after that - and I had no other clues - I’d probably replace the host with a clean server.
As a side note, I’d also look to be upgrading from 3.6 as soon as practical. I know it’s a pain playing catch up without immediate benefit to your application, but the further behind you get the worse it becomes; especially as Mongo requires version upgrades to be done one minor version at a time.