I think there are a lot of reasons to use this flag besides a write()+f(data)sync() sequence:

* If you're putting something in a write-ahead log then applying it to the primary storage, you want it to be fully committed to the write-ahead log before you start changing the primary storage, so if there's a crash halfway through the primary storage change you can use the log to get to a consistent state (via undo or redo).

* If you're trying to atomically replace a file via the rename-a-temporary-file-into-place trick, you can submit the whole operation to the ring at once, but you'd want to use `IOSQE_IO_LINK` to ensure the temporary file is fully written/synced before the rename happens.

btw, a clarification about my earlier comment: `O_SYNC` (no `D`) should be equivalent to calling `fsync` after every write. `O_DSYNC` should be equivalent to calling the weaker `fdatasync` after every write. The difference is the metadata stored in the inode.