mysql连接慢的问题——dns惹的祸

原文地址:http://www.jb51.net/article/27616.htm

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症状描述###

• 本机连接mysql速度很快
• 远程ping mysql主机速度正常
• 远程连接mysql速度需要等待30秒左右（问题所在）
• 远程连接后访问mysql中的表正常

原因是mysql主机不能访问广域网，连接DNS时连接不上，因此连接的时候会有延时。

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解决办法###

在my.cnf中添加如下配置
[mysqld]
skip-name-resolve

重启mysql服务

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说明###

根据文档说明，如果你的mysql主机查询DNS很慢或是有很多客户端主机时会导致连接很慢，由于我们的开发机器是不能够连接外网的，所以DNS解析是不可能完成的，从而也就明白了为什么连接那么慢了。同时，请注意在增加该配置参数后，mysql的授权表中的host字段就不能够使用域名而只能够使用 ip地址了，因为这是禁止了域名解析的结果。

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以下是官方信息###

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.