MySQL server has gone away报错原因分析及解决办法

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.log 
130101 22:22:30 InnoDB: Initializing buffer pool, size = 256.0M 
130101 22:22:30 InnoDB: Completed initialization of buffer pool 
130101 22:22:30 InnoDB: highest supported file format is Barracuda. 
130101 22:22:30 InnoDB: 1.1.8 started; log sequence number 63444325509 
130101 22:22:30 [Note] Server hostname (bind-address): '127.0.0.1'; port: 3306 
130101 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 events 
130101 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 seconds 

mysql> SELECT NOW(); 
ERROR 2006 (HY000): MySQL server has gone away 
No connection. Trying to reconnect... 
Connection id: 132361 
Current 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 | 
+--------------------+----------+ 

参考： http://www.lai18.com/content/319554.html