io.Copy导致golang内存不足

bodyBuf := &bytes.Buffer{}
bodyWriter := multipart.NewWriter(bodyBuf)

//key step
fileWriter,err := bodyWriter.CreateFormFile(paramName,fileName)
if err != nil {
    return nil,err
}

file,err := os.Open(fileName) //the file size is about 700Mb
if err != nil {
    return nil,err
}
defer file.Close()

//iocopy
copyLen,err := io.Copy(fileWriter,file) // this cause out of memory
if err != nil {
    fmt.Println("io.copy(): ",err)

    return nil,err
}

错误消息如下：

runtime: memory allocated by OS (0x752cf000) not in usable range [0x18700000,0x98700000)
runtime: out of memory: cannot allocate 1080229888-byte block (1081212928 in use)
fatal error: out of memory

我为buf分配了足够的内存,导致bodyWriter.CreateFormFile()内存不足

buf := make([]byte,766509056)
bodyBuf := bytes.NewBuffer(buf)
bodyWriter := multipart.NewWriter(bodyBuf)

fileWriter,fileName) // out of memory
if err != nil {
    return nil,err
}