One of the largest forums online in 2003 was was ezboard. A billion page views per month. Back then - insane - well that was beyond insane. I started work there as a young idiot. "What db you guys using, mysql?...." long pause... "We use a file system." I dug in. Yep. A fucking file system. Files allowed: simple backups, impossible speed, obvious cache systems, partitioning.

Later on i met the roomate of the author of memcache. Few hundred lines of C. Completely changed the game how caching systems worked. Years later a super senior FB engineer explained to me over a beer - "we basically use mysql as a file store, everything else is in memcache." Good ideas are stupid simple.