From Sasha:

> Last week, as I was exploring the system I learned that most *nix systems do not support O_DIRECT on RAM-based file systems, like /tmpfs. If you try to create a connection with a direct_io flag when running over a tmpfs, you'll get a not-very-descriptive error "Invalid option". I don't know if any of the future customers will actually try that (perhaps those that want to run WT without retaining persistent records), but perhaps it's worth adding half a sentence to the documentation explaining that direct_io option is not supported on RAM-based file systems. Might save people (and ourselves) some time in the future.