MySQL Server has gone away报错原因汇总分析（转自：http://cenalulu.github.io/mysql/mysql-has-gone-away/）

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
 :: InnoDB: Initializing buffer pool, size = 256.0M
 :: InnoDB: Completed initialization of buffer pool
 :: InnoDB: highest supported file format is Barracuda.
 :: InnoDB: 1.1. started; log sequence number
 :: [Note] Server hostname (bind-address): '127.0.0.1'; port:
 :: [Note]   - '127.0.0.1' resolves to '127.0.0.1';
 :: [Note] Server socket created on IP: '127.0.0.1'.
 :: [Note] Event Scheduler: Loaded  events
 :: [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.5.28-cll'  socket: '/var/lib/mysql/mysql.sock'  port:   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            |        |
| delayed_insert_timeout     |       |
| innodb_lock_wait_timeout   |        |
| innodb_rollback_on_timeout | OFF      |
| interactive_timeout        |     |
| lock_wait_timeout          |  |
| net_read_timeout           |        |
| net_write_timeout          |        |
| slave_net_timeout          |      |
| wait_timeout               |     |
+----------------------------+----------+
mysql> SET SESSION wait_timeout=;

# Wait  seconds

mysql> SELECT NOW();
ERROR  (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:
Current database: *** NONE ***

+---------------------+
| NOW()               |
+---------------------+
| -- :: |
+---------------------+
 row in set (0.00 sec)

原因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      |      |
+---------------+-------+

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。

mysql> show global variables like 'max_allowed_packet';
+--------------------+---------+
| Variable_name      | Value   |
+--------------------+---------+
| max_allowed_packet |  |
+--------------------+---------+
 row in set (0.00 sec)

# 修改参数：

mysql> set global max_allowed_packet=**;
mysql> show global variables like 'max_allowed_packet';
+--------------------+----------+
| Variable_name      | Value    |
+--------------------+----------+
| max_allowed_packet |  |
+--------------------+----------+
 row in set (0.00 sec)