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在平时和开发的交流 以及 在论坛回答问题的或称中会发现这个问题被问及的频率非常高。程序中报错： MySQL server has gone away 是什么意思？ 如何避免？因此，感觉有必要总结一下发生这个问题的原因。今天正好看到一篇外文blog总结的比较好，就翻译过来了原文：http://ronaldbradford.com/blog/sqlstatehy000-general-error-2006-mysql-server-has-gone-away-2013-01-02/
原因1. MySQL 服务宕了
判断是否属于这个原因的方法很简单，执行以下命令，查看mysql的运行时长
$ mysql -uroot -p -e "show global status like 'uptime';"+---------------+-------+| Variable_name | Value |+---------------+-------+| Uptime | 68928 |+---------------+-------+1 row in set (0.04 sec)
或者查看MySQL的报错日志，看看有没有重启的信息
$ tail /var/log/mysql/error.log130101 22:22:30 InnoDB: Initializing buffer pool, size = 256.0M130101 22:22:30 InnoDB: Completed initialization of buffer pool130101 22:22:30 InnoDB: highest supported file format is Barracuda.130101 22:22:30 InnoDB: 1.1.8 started; log sequence number 63444325509130101 22:22:30 [Note] Server hostname (bind-address): '127.0.0.1'; port: 3306130101 22:22:30 [Note] - '127.0.0.1' resolves to '127.0.0.1';130101 22:22:30 [Note] Server socket created on IP: '127.0.0.1'.130101 22:22:30 [Note] Event Scheduler: Loaded 0 events130101 22:22:30 [Note] /usr/sbin/mysqld: ready for connections.Version: '5.5.28-cll' socket: '/var/lib/mysql/mysql.sock' port: 3306 MySQL Community Server (GPL)
如果uptime数值很大，表明mysql服务运行了很久了。说明最近服务没有重启过。
如果日志没有相关信息，也表名mysql服务最近没有重启过，可以继续检查下面几项内容。
2. 连接超时
如果程序使用的是长连接，则这种情况的可能性会比较大。
即，某个长连接很久没有新的请求发起，达到了server端的timeout，被server强行关闭。
此后再通过这个connection发起查询的时候，就会报错server has gone away
$ mysql -uroot -p -e "show global variables like '%timeout';"+----------------------------+----------+| Variable_name | Value |+----------------------------+----------+| connect_timeout | 30 || delayed_insert_timeout | 300 || innodb_lock_wait_timeout | 50 || innodb_rollback_on_timeout | OFF || interactive_timeout | 28800 || lock_wait_timeout | 31536000 || net_read_timeout | 30 || net_write_timeout | 60 || slave_net_timeout | 3600 || wait_timeout | 28800 |+----------------------------+----------+
mysql> SET SESSION wait_timeout=5;## Wait 10 secondsmysql> SELECT NOW();ERROR 2006 (HY000): MySQL server has gone awayNo connection. Trying to reconnect...Connection id: 132361Current database: *** NONE ***+---------------------+| NOW() |+---------------------+| 2013-01-02 11:31:15 |+---------------------+1 row in set (0.00 sec)
3. 进程在server端被主动kill
这种情况和情况2相似，只是发起者是DBA或者其他job。发现有长时间的慢查询执行kill xxx导致。
$ mysql -uroot -p -e "show global status like 'com_kill'"+---------------+-------+| Variable_name | Value |+---------------+-------+| Com_kill | 0 |+---------------+-------+
4. Your SQL statement was too large.
当查询的结果集超过 max_allowed_packet 也会出现这样的报错。定位方法是打出相关报错的语句。
用select * into outfile 的方式导出到文件，查看文件大小是否超过 max_allowed_packet ，如果超过则需要调整参数，或者优化语句。
mysql> show global variables like 'max_allowed_packet';+--------------------+---------+| Variable_name | Value |+--------------------+---------+| max_allowed_packet | 1048576 |+--------------------+---------+1 row in set (0.00 sec)
修改参数：
mysql> set global max_allowed_packet=1024*1024*16;mysql> show global variables like 'max_allowed_packet';+--------------------+----------+| Variable_name | Value |+--------------------+----------+| max_allowed_packet | 16777216 |+--------------------+----------+1 row in set (0.00 sec)
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