为啥我 Nginx 下启用 fastcgi_cache 不生成缓存？

先说说我在 nginx.conf 里 http 里的配置，如下：

fastcgi_cache_path /var/domain_cache levels=1:2 keys_zOne=domain:100m inactive=1d max_size=5G;
fastcgi_temp_path /var/cache_temp;
fastcgi_cache_key "$scheme$request_method$host$request_uri"; #定义哪些情况下用过期缓存
fastcgi_cache_use_stale error timeout invalid_header http_500 http_503 updating;
fastcgi_ignore_headers Cache-Control Expires Set-Cookie Vary;
然后是我的域名 myhost.conf 文件内的 Fastcgi_cache 有关配置，如下：

set $skip_cache 0;

if ($request_method = POST) {
set $skip_cache 1;
}

if ($query_string != "") {
set $skip_cache 1;
}

if ($request_uri ~* "/wp-admin/|/xmlrpc.php|wp-.*.php|/feed/|index.php|sitemap(_index)?.xml") {
set $skip_cache 1;
if ($http_cookie ~* "comment_author|wordpress_[a-f0-9]+|wp-postpass|wordpress_no_cache|wordpress_logged_in") {
set $skip_cache 1;
}


location ~ [^/]\.php(/|$)
{
try_files $uri =404;
fastcgi_pass unix:/tmp/php-cgi.sock;
fastcgi_index index.php;
include fastcgi.conf;
fastcgi_cache_bypass $skip_cache;
fastcgi_no_cache $skip_cache;
add_header X-Cache-Status $upstream_cache_status;
add_header X-Cache "$upstream_cache_status From $host";
fastcgi_cache domain;
fastcgi_cache_valid 200 301 302 1d;
}

location ~ /purge(/.*) {
allow 127.0.0.1;
allow "111.112.113.114";
deny all;
fastcgi_cache_purge domain "$scheme$request_method$host$1";
}

location ~* ^.+\.(ogg|ogv|svg|svgz|eot|otf|woff|mp4|ttf|rss|atom|jpg|jpeg|gif|png|ico|zip|tgz|gz|rar|bz2|doc|xls|exe|ppt|tar|mid|midi|wav|bmp|rtf)$ {
access_log off; log_not_found off; expires max;
}

location = /robots.txt { access_log off; log_not_found off; }
location ~ /\. { deny all; access_log off; log_not_found off; }
WordPress 里安装了 nginx-helper 插件，通过 tailf 命令查看这个插件生成的.log 文件一直是如下显示：

https://segmentfault.com/img/bVSke7?w=284&h=231