nginx 的 upstream timed out 问题

nginx 作为负载服务，表现为网站访问很慢，有些文件或页面要等待到60s才会返回，我注意到60s就是超时时间，但是超时后返回状态是正常值200，网站可以正常打开，就是会一直等待到超时才打开，而且问题出现不定时，不定文件，静态文件也会出现这个问题。貌似很奇葩是吧，最终查看错误信息没有注意到 IPv6，导致饶了不少弯。

错误信息：

[error] #: * upstream timed out (: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond) while connecting to upstream, client: 66.249.69.219, server: xxx.com, request: "GET /shop/view.jhtml HTTP/1.1", upstream: "http://[::1]:12002/shop/view.jhtml", host: "www.xxx.com"

配置如下：

    upstream zcun_pool {
        server localhost: max_fails= fail_timeout=30s;
        server localhost: max_fails= fail_timeout=30s;
        server localhost: max_fails= fail_timeout=30s;
    }

发现可能是 IPv6 的问题后，改 localhost 为本地 IPv4 即可解决.....这错误说起来还真是低级~

    upstream zcun_pool {
        server 127.0.0.1: max_fails= fail_timeout=30s;
        server 127.0.0.1: max_fails= fail_timeout=30s;
        server 127.0.0.1: max_fails= fail_timeout=30s;
    }

网站访问回复正常。