[jandrewrogers]

Some databases rely on the OS file system cache, this is true. However, it is not controversial that doing so reduces performance by integer factors. Bypassing the OS file system cache is idiomatic in database kernel designs where absolute performance is a key objective.

[tomnipotent]

Using O_DIRECT and other features still makes you reliant on the OS cache, even if all you're doing is keeping it empty and out of your business. I've only run into a few that manually manage block devices directly.