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.
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.