有些人觉得，解决too many connections问题，灰非简单，down了mysql,修改my.cnf调大max_connections，好吧，你想法是没错的，这的确可以解决问题，但试问对于线上在跑的MySQL，你能随便down吗？嘻嘻，如果不行，只能用另外的方法了
一旦出现了too many connections错误，DBA或者运维人员已经连接不上MySQL去动态修改max_connections了，下面做实验来演示一下：
为了方便演示，我把max_connections调小到4：
mysql> set global max_connections=4; Query OK, 0 rows affected (0.00 sec) mysql> show variables like ‘max_connec%‘; +--------------------+-------+ | Variable_name | Value | +--------------------+-------+ | max_connect_errors | 10 | | max_connections | 4 | +--------------------+-------+ 2 rows in set (0.00 sec) mysql>
然后打开几个seesion连接mysql，当连接超出4个时，报以下错误：
[root ~]$ mysql -uroot -p123456 -S /data/mysql-5.5/mysql.sock ERROR 1040 (HY000): Too many connections
提示也非常明显，就是我们配置的连接数太小，现在已经用完了，这时我们只能通过hack的方法，用过gdb直接修改mysqld内存中max_connections的值，具体做法如下：
可能很人的系统环境上没有安装gdb，安装下即可：
[root ~]$ gdb -p $(cat /data/mysql-5.5/localhost.localdomain.pid) -ex "set max_connections=500" -batch -bash: gdb: command not found [root ~]$ yum install gdb -y
然后执行gdb -p $(cat /data/mysql-5.5/localhost.localdomain.pid) -ex "set max_connections=500" -batch; cat后面跟的是你当前mysql服务的pid文件，根据自己的存放指定即可，后面是修改mysqld内存中max_connections为多少
[root ~]$ gdb -p $(cat /data/mysql-5.5/localhost.localdomain.pid) -ex "set max_connections=500" -batch [New LWP 17980] [New LWP 17948] [New LWP 17910] [New LWP 17878] [New LWP 17803] [Thread debugging using libthread_db enabled] 0x00007f49f1c82333 in poll () from /lib64/libc.so.6
这时已经可以连接mysql了，查看它的max_connections已经不再是之前的4了，而是500了
mysql -uroot -p123456 -S /data/mysql-5.5/mysql.sock Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 6 Server version: 5.5.40-log MySQL Community Server (GPL) Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement. mysql> show global variables like ‘max_conn%‘; +--------------------+-------+ | Variable_name | Value | +--------------------+-------+ | max_connect_errors | 10 | | max_connections | 500 | +--------------------+-------+ 2 rows in set (0.04 sec) mysql>
在Percona5.5的thread_pool里面提供了2个参数extra_port和extra_max_connections预留额外的连接，预防连接满了以后我们无法进入数据库进行相应的管理。
[root msb_5_5_40]$ ./use Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 1 Server version: 5.5.40-36.1 Percona Server (GPL), Release 36.1, Revision 707 Copyright (c) 2009-2014 Percona LLC and/or its affiliates Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement. mysql ((none)) > show variables like ‘%extra%‘; +-----------------------+-------+ | Variable_name | Value | +-----------------------+-------+ | extra_max_connections | 1 | | extra_port | 10010 | +-----------------------+-------+ 2 rows in set (0.00 sec)
当出现超出最大链接的错误时，可以指定用户名 密码 端口登录，端口为extra_port设置的，不是mysql服务自身的端口，相信大家都觉得这个功能很贴心吧，哈哈^.^
总结：
通常有两个参数控制控制最大连接数：
max_connections：该实例允许最大的连接数
max_user_connections：该实例允许每个用户的最大连接数
每个人要根本自己业务量，设置合适的值，不要盲目有设置过大，但也不设置过小，因为MySQL在连接数上升的情况下性能下降非常厉害，如果需要大量连接，这时可以引入thread_pool，所以我们需要保持一个原则：系统创建的用户（给应用使用用户）数* max_user_connections < max_connections。这样就不会发生文章开始说的问题。
参考资料：
http://www.mysqlperformanceblog.com/2010/03/23/too-many-connections-no-problem/
http://www.percona.com/doc/percona-server/5.5/performance/threadpool.html
http://www.cnblogs.com/gomysql/p/3834797.html