For the topic of directIO / O_DIRECT, this is specific to databases that do their own buffering, and thus one would have double-buffering: once at the DB app layer and again at the OS VFS layer.

Not all databases do buffering: Postgresql does not, and is generally built around an OS's normal I/O buffering.

2017 sub-thread on this topic:

* https://news.ycombinator.com/item?id=15722708

That seems like terrible advice considering it has no silent data corruption protection. You should treat your storage hardware as if it's trying to corrupt your data.