有次同事提出開(kāi)發(fā)使用的MySQL數(shù)據(jù)庫(kù)連接很慢，因?yàn)槲覀兊腗ySQL開(kāi)發(fā)數(shù)據(jù)庫(kù)是單獨(dú)一臺(tái)機(jī)器部署的，所以認(rèn)為可能是網(wǎng)絡(luò)連接問(wèn)題導(dǎo)致的。
在進(jìn)行 ping和route后發(fā)現(xiàn)網(wǎng)絡(luò)通信都是正常的，而且在MySQL機(jī)器上進(jìn)行本地連接發(fā)現(xiàn)是很快的，所以網(wǎng)絡(luò)問(wèn)題基本上被排除了。以前也遇到過(guò)一次這樣的問(wèn)題，可后來(lái)就不知怎么突然好了，這次又遭遇這樣的問(wèn)題，所以想看看是不是MySQL的配置問(wèn)題。在查詢MySQL相關(guān)文檔和網(wǎng)絡(luò)搜索后，發(fā)現(xiàn)了一個(gè)配置似乎可以解決這樣的問(wèn)題，就是在MySQL的配置文件中增加如下配置參數(shù)：
在Linux下配置文件是/etc/my.cnf，在windows下配置文件是MySQL安裝目錄下的my.ini文件。注意該配置是加在 [mysqld]下面，在更改配置并保存后，然后重啟mysql并遠(yuǎn)程連接測(cè)試，一切恢復(fù)如初。該參數(shù)的官方解釋信息如下：
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(jù)文檔說(shuō)明，如果你的mysql主機(jī)查詢DNS很慢或是有很多客戶端主機(jī)時(shí)會(huì)導(dǎo)致連接很慢，由于我們的開(kāi)發(fā)機(jī)器是不能夠連接外網(wǎng)的，所以DNS解析是不可能完成的，從而也就明白了為什么連接那么慢了。同時(shí)， 請(qǐng)注意在增加該配置參數(shù)后，mysql的授權(quán)表中的host字段就不能夠使用域名而只能夠使用 ip地址了 ，因?yàn)檫@是禁止了域名解析的結(jié)果。
