最近在Linux服務器上安裝MySql5后，本地使用客戶端連MySql速度超慢，本地程序連接也超慢。
解決方法：在配置文件my.cnf的[mysqld]下加入skip-name-resolve。

原因是默認安裝的MySql開啟了DNS的反向解析。如果禁用的話就不能在MySQL的授權表中使用主機名了而只能用ip格式。

附：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 lookup 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.

或者host中添加

192.168.1.21 N-21

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