The last parameter @datasync of fsync() has following indication:
* if datasync=0, we'd always flush data and metadata
* if datasync=1, we'd always flush data while does not flush modifed
metadata unless that metadata is needed in order to allow a
subsequent data retrieval to be correctly handled. For example, a
change to the file size would require a metadata flush.
Lustre client can not tell the difference easily, and would issue
MDS_SYNC and OST_SYNC in all cases.