I just disabled Varnish Server on this VPS. The varnish server version is v3.0.5.
The performance on my blog is improved, but not that much. And also sometimes it return 503 error.
There are a lot of other Varnish user said same 503 issues.
So, I stop using Varnish server for the time being.
And at the same time, I noticed that Varnish 4.0 released. I tried to install it on my Centos testing machine, Installation is fine. but I can not make it work. When I try to start varnish, it failed.
There are not many documents about Varnish 4.0. I will wait and see if I can use it later.
Varnish official site.