nginx做代理的服务器的时候是默认等后端服务器完成响应后再一次把内容返回给用户。这个关键的配置项目是proxy_buffering.proxy_buffering 默认是on
[proxy_buffering](http://wiki.nginx.org/HttpProxyModule#proxy_buffers)
This directive activate response buffering of the proxied server.If buffering is activated, then nginx reads the answer from the proxied server as fast as possible, saving it in the buffer as configured by directives proxy_buffer_size andproxy_buffers. If the response does not fit into memory, then parts of it will be written to disk.