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 |
+--------------------+----------+