Comment by adrian_b

You can use io_uring very well with sequential files, where the kernel will do read-ahead for you, possibly helped by you with fadvise.

O_DIRECT is to be used with files that will be accessed in a random order (random meaning that the order is not predictable by the kernel) and with buffer sizes per access great enough that you want to avoid their copying between kernel and application (e.g. at least a few kByte per access).

Whenever O_DIRECT is used, the programmer takes responsibility to implement an adequate form of read-ahead, based on the access pattern that is predicted for the application, and which cannot be guessed by the kernel.

If the programmer did not implement read-ahead, like it was the case before doing the update described in TFA, that was an incompletely written program. One should not enable O_DIRECT without a complete implementation for its requirements, as that can lead only to lower performance than standard I/O.