def METHOD_NAME(session, drive_letter):
    """
    First format tmpfs disk to wipe out cache,
    then run fio test, and return the result's bw value
    param session: a session loggined to guest os
    drive_letter: the drive to run the fio job on
    return: the bw value of the running result(bw=xxxB/s)
    """
    bw_search_reg = params["bw_search_reg"]
    test.log.info("Format tmpfs data disk")
    utils_disk.create_filesystem_windows(session, drive_letter, "ntfs")
    test.log.info("Start fio test")
    fio = generate_instance(params, vm, 'fio')
    o = fio.run(params["fio_options"] % drive_letter)
    return int(re.search(bw_search_reg, o, re.M).group(1))