When I first started at the Bandwith Barn, the traffic accounting that such an environment required just wasn't available off-the-shelf or in the open source world.  I've often been asked for the hacking combination of scripts and pmacct that maintain the Bandwidth Barn traffic system - which includes "buying" more monthly traffic, setting traffic limits per month per person, up-to-date graphs of usage per protocol and per client available to each company in the Barn, and months of historical data in case of queries or complaints about the billing.

Looks like ulogd, some iptables rules, and a few simple cronned SQL scripts make this a lot easier these days, thanks to this post about ulogd for bandwidth accounting by Stefano.

If you didn't live through Operating Systems of the 70s, 80s, and 90s (or caught the lecture at university), then you might want to read this ArsTechnica article on the history of the more mainstream filesystems so you can fake your way authoritatively when the topic comes up.

(It doesn't cover less mainstream or specific-purpose filesystems or cover more interesting work done on some of the filesystems it does mention - particularly on the BSD side of things.  You'll have to do your own research on those.  Try FreeBSD 7.0, maybe?)