It's funny how the meaning of "bare metal" changes over time. To me it means taking the OS out of the loop (either the application being the OS, or at least circumventing the normal OS facilities). So I was fully expecting this to be about databases with their own filesystems and/or buffer caches.

Or, for that matter, a custom OS whose only task is to run an (in-kernel, perhaps?) database server.

Came here to say the same. Also raw disk slices vs file systems for databases.