[jandrewrogers]

PostgreSQL has two main challenges with direct I/O. The basic one is that it adversely impacts portability, as mentioned, and is complicated in implementation because file system behavior under direct I/O is not always consistent.

The bigger challenge is that PostgreSQL is not architected like a database engine designed to use direct I/O effectively. Adding even the most rudimentary support will be a massive code change and implementation effort, and the end result won't be comparable to what you would expect from a modern database kernel designed to use direct I/O. This raises questions about return on investment.