+A flush request will not be sent unless NBD_FLAG_SEND_FLUSH is set,
+and indicates the backing file should be fdatasync()'d to disk.
+
+The top 16 bits of the request are flags. NBD_CMD_FLAG_FUA implies
+a force unit access, and can currently only be usefully combined
+with NBD_CMD_WRITE. This is implemented using sync_file_range
+if present, else by fdatasync() of that file (note not all files
+in a multifile environment). NBD_CMD_FLAG_FUA will not be set
+unless NBD_FLAG_SEND_FUA is set.
+