解决远程连接mysql很慢的方法(mysql_connect 打开连接慢)

问题描述：

远程访问mysql特别慢，慢到经常连接超时，但是直接去服务查看系统状况发现mysql状况正常，通过命令行连接mysql、执行sql语句都不慢。

查理办法：

在mysql配置文件中添加以下配置项

[mysqld]  

skip-name-resolve

下面是有关该配置项的解释

How MySQL uses DNS

When a new thread connects to mysqld, mysqld will spawn a new thread to handle the request. This thread will first check if the hostname is in the hostname cache. If not the thread will call gethostbyaddr_r() and gethostbyname_r() to resolve the hostname.

If the operating system doesn't support the above thread-safe calls, the thread will lock a mutex and call gethostbyaddr() and gethostbyname() instead. Note that in this case no other thread can resolve other hostnames that is not in the hostname cache until the first thread is ready.

You can disable DNS host lookup by starting mysqld with –skip-name-resolve. In this case you can however only use IP names in the MySQL privilege tables.

If you have a very slow DNS and many hosts, you can get more performance by either disabling DNS lookop with –skip-name-resolve or by increasing the HOST_CACHE_SIZE define (default: 128) and recompile mysqld.

You can disable the hostname cache with –skip-host-cache. You can clear the hostname cache with FLUSH HOSTS or mysqladmin flush-hosts.

If you don't want to allow connections over TCP/IP, you can do this by starting mysqld with –skip-networking.

大概意思是当一个新的请求到达mysql服务时，mysql服务会派生一个子进程来处理这个请求的全部事宜。这个子进程首先会检查发来请求的主机名是否在缓存中，如果不存在则会进程DNS解析来查找主机名。

可以通过在mysqld标签下skip-name-resolve禁用DNS解析。

注意：在mysql配置文件中禁止DNS解析后，mysql库中user表中的host字段不能使用域名。