解決遠端連線mysql很慢的方法(mysql_connect 開啟串連慢)

在進行 ping和route後發現網路通訊都是正常的，而且在mysql機器上進行本地串連發現是很快的，所以網路問題基本上被排除了。以前也遇到過一次這樣的問題，可後來就不知怎麼突然好了，這次又遭遇這樣的問題，所以想看看是不是mysql的配置問題。在查詢mysql相關文檔和網路搜尋後，發現了一個配置似乎可以解決這樣的問題，就是在mysql的設定檔中增加如下配置參數：

[mysqld]
skip-name-resolve

在linux下設定檔是/etc/my.cnf，在windows下設定檔是mysql安裝目錄下的my.ini檔案。注意該配置是加在 [mysqld]下面，在更改配置並儲存後，然後重啟mysql並遠端連線測試，一切恢複如初。該參數的官方解釋資訊如下：

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.

根據文檔說明，如果你的mysql主機查詢DNS很慢或是有很多用戶端主機時會導致串連很慢，由於我們的開發機器是不能夠串連外網的，所以DNS解析是不可能完成的，從而也就明白了為什麼串連那麼慢了。同時，請注意在增加該配置參數後，mysql的授權表中的host欄位就不能夠使用網域名稱而只能夠使用 ip地址了，因為這是禁止了網域名稱解析的結果。