关于pos请求的疑问？？

func postFile(filename string, targetURL string) error {
    bodyBuf := &bytes.Buffer{}
    bodyWriter := multipart.NewWriter(bodyBuf)//这一步不是很懂？？
    //关键的一步操作
    _, err := bodyWriter.CreateFormFile("file", filename)
    len := len(bodyBuf.Bytes())  
    buf1 := &bytes.Buffer{}
    buf1.Write(bodyBuf.Bytes()[0:len])//这是什么在干什么？？？
    bodyWriter.Close()
    buf2 := &bytes.Buffer{}
    buf2.Write(bodyBuf.Bytes()[len:]) //这样不会是空的吗？？

    //打开文件句柄操作
    fh, err := os.Open(filename)
    if err != nil {
        flog.LOG_ERROR("can not opening file")
        return err
    }
    defer fh.Close()
    contentType := bodyWriter.FormDataContentType()
    mr := io.MultiReader(buf1, fh, buf2) //将其串联
    //mr := io.MultiReader(buf1, fh, buf2)
    resp, err := http.Post(targetURL, contentType, mr)
    if err != nil {
        return err
    }
    defer resp.Body.Close()
    if resp.StatusCode != 200 {
        return nil
    }
    return nil
}

func HttpPost(targetUrl string, filename string) error {
    bodyBuf := &bytes.Buffer{}
    bodyWriter := multipart.NewWriter(bodyBuf)
    //关键的一步操作
    fileWriter, err := bodyWriter.CreateFormFile("file", filename) //创建表头
    if err != nil {
        fmt.Println("error writing to buffer")
        return err
    }
    //fmt.Println("url:", targetUrl)
    //打开文件句柄操作
    fh, err := os.Open(filename)
    if err != nil {
        fmt.Println("error opening file")
        return err
    }
    _, err = io.Copy(fileWriter, fh)
    if err != nil {
        return err
    }
    contentType := bodyWriter.FormDataContentType()//得到类型
    bodyWriter.Close()
    //bodyBuf
    resp, err := http.Post(targetUrl, contentType, bodyBuf)
    if err != nil {
        fmt.Println(err)
        return err
    }
    defer resp.Body.Close()
    resp_body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        return err
    }
    fmt.Println(resp.Status)
    fmt.Println(string(resp_body))
    return nil
}

第一段代码比第二段怎么快这么多？？？ 第一段代码buf1、buf2是干什么的