ivity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also
解释：一个持续SLEEP状态的线程多久被关闭。线程每次被使用都会被唤醒为acrivity状态，执行完Query后成为interactive状态，重新开始计时。wait_timeout不同在于只作用于TCP/IP和Socket链接的线程，意义是一样的。一般设置是8小时，一般网站白天都有人访问，从夜里到早上一般都会超过8小时，所以再来访问就会这个问题。
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net_read_timeout / net_write_timeout
手册描述：
The number of seconds to wait for more data from a connection before aborting the read. Before MySQL 5.1.41, this timeout applies only to TCP/IP connections, not to connections made through Unix socket files, named pipes, or shared memory. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout.
On
Linux, the NO_ALARM build flag affects timeout behavior as indicated in the description of the net_retry_count system variable.
解释：这个参数只对TCP/IP链接有效，分别是
数据库等待接收客户端发送网络包和发送网络包给客户端的超时时间，这是在Activity状态下的线程才有效的参数
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slave_net_timeout
手册描述：
The number of seconds to wait for more data from the master before the slave considers the connection broken, aborts the read, and tries to reconnect. The first retry occurs immediately after the timeout. The interval between retries is controlled by the MASTER_CONNECT_RETRY option for the CHANGE MASTER TO statement or –master-connect-retry option, and the number of reconnection attempts is limited by the –master-retry-count option. The default is 3600 seconds (one hour).
解释：这是Slave判断主机是否挂掉的超时设置，在设定时间内依然没有获取到Master的回应就人为Master挂掉了