Javascript NodeJS：使用可读事件和排水事件

我正在尝试使用节点的可读流读取一个大文件（~800MB）。 我已经订阅了“readable”事件，在获取我正在写入响应的数据时，然后等待drain事件再次读取数据。但排水事件仅被触发3-4次。如果有什么问题，请告诉我

// Assume res to be any writable stream.

var readStream = fs.createReadStream('C:/movie.mp4');

readStream
    .on('error', function (err) {
    })
    .on('readable', function () {
        console.log('READABLE event fired');
        writeStreamData(readStream, res);
    })
    .on('end', function () {
        console.log('END event fired');
        res.end();
    });

function writeStreamData(readStream, res) {
    var chunk, isFlushed;
    console.log('in writeStreamData');
    if (null !== (chunk = readStream.read(1024 * 1000))) {
        console.log('writing data');
        isFlushed = res.write(chunk);
        if (!isFlushed) {
            console.log('waiting for drain event');
            res.once('drain', function () {
                console.log('*** DRAIN event triggered');
                writeStreamData(readStream, res);
            });
        } else {
            writeStreamData(readStream, res);
        }
    }
}