Ensuring data reaches disk

If the storage device has a write cache but no independent power supply you have the problem that you will loose data on power loss because O_DIRECT will only guarantee that the write reaches the device, not that it reaches persistent storage inside that device.
For that you need to issue some special commands - which e.g. fsync() knows how to do.
Besides an O_DIRECT write doesn't guarantee that metadata updates have reached stable storage.