MySQL bug [#45892](https://bugs.mysql.com/bug.php?id=45892) contains additional information:
> Some testing by Domas has shown that some filesystems (XFS) do not sync metadata without the fsync. If the metadata would change, then you need to still use fsync (or O_SYNC for file open).
>
> For example, if a file grows while O_DIRECT is enabled it will still write to the new part of the file, however since the metadata doesn
>
> Solution:
>
> Continue to use fsync when important metadata changes or use O_SYNC in addition to O_DIRECT.
O_DIRECT (Since Linux 2.4.10)
Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applica‐
tions do their own caching. File I/O is done directly to/from user-space buffers. The O_DIRECT flag on its own makes an effort to transfer data synchronously, but does
not give the guarantees of the O_SYNC flag that data and necessary metadata are transferred. To guarantee synchronous I/O, O_SYNC must be used in addition to O_DIRECT.
See NOTES below for further discussion.
A semantically similar (but deprecated) interface for block devices is described in raw(8).
