public void setBufferedIO(boolean bufferedIO) {
        this.bufferedIO = bufferedIO;
        if (bufferedIO) {
            immediateFlush = false;
        }
    }
The <b>BufferedIO</b> option takes a boolean value. It is set to
<code>false</code> by default. If true, then <code>File</code> will be
opened and the resulting {@link java.io.Writer} wrapped around a
{@link BufferedWriter}.

BufferedIO will significatnly increase performance on heavily loaded
systems.
metamorphosis-client-extension/src/main/java/com/taobao/metamorphosis/client/extension/log4j/FileStreamAppender.java