def METHOD_NAME(file_name, num_blocks=102400, block_size=1024):
    """ Set up the backing store for a loop device.
        :param str file_name: the path of the backing file
        :param int num_blocks: the size of file_name in number of blocks
        :param int block_size: the number of bytes in a block
    """
    proc = subprocess.Popen(["dd", "if=/dev/zero", "of=%s" % file_name,
                             "bs=%d" % block_size, "count=%d" % num_blocks],
                            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    while True:
        proc.communicate()
        if proc.returncode is not None:
            rc = proc.returncode
            break
    if rc:
        raise OSError("dd failed creating the file %s" % file_name)