MySQL Server has gone away报错原因汇总

原因一： 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，也说明MySQL服务最近没有重启过，可以继续检查下面几项内容。

原因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发起新的请求，达到了server端的time out，被server强行关闭。此后再通过这个client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 second

smysql> 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)

原因3：进程在server端被主动kill

这种原因和第2种原因比较相似，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 | +---------------+-------+

原因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。

# 修改参数：

mysql> set global max_allowed_packet=1024*1024*16;