简述
比较常见的一个报错日志：
ERROR 2006 (HY000): MySQL server has gone away
原因
原因一：MySQL 服务宕了
排查方法：
1.1 查看MySQL运行时长
mysql> show global status like 'uptime';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Uptime | 138 |
+---------------+-------+
1.2 查看MySQL日志
[root@ZTEST-218 ~]# tail /data/logs/mysql/mysqld.log
2017-09-21 11:12:47 7f13935fe700 InnoDB: Loading buffer pool(s) from .//ib_buffer_pool
2017-09-21 11:12:48 9744 [Note] Server hostname (bind-address): '*'; port: 63306
2017-09-21 11:12:48 9744 [Note] IPv6 is available.
2017-09-21 11:12:48 9744 [Note] - '::' resolves to '::';
2017-09-21 11:12:48 9744 [Note] Server socket created on IP: '::'.
2017-09-21 11:12:48 9744 [Warning] 'proxies_priv' entry '@ root@ztest-217' ignored in --skip-name-resolve mode.
2017-09-21 11:12:48 9744 [Note] Event Scheduler: Loaded 0 events
2017-09-21 11:12:48 9744 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.6.34-log' socket: '/var/lib/mysql/mysql.sock' port: 63306 MySQL Community Server (GPL)
2017-09-21 11:12:50 7f13935fe700 InnoDB: Buffer pool(s) load completed at 170921 11:12:50
如果Uptime的值比较大，说明MySQL运行时间长，最近一段时间内没有重启服务之类的。
如果MySQL错误日志中没有相关日志记录，也说明MySQL最近未重启。
原因二：连接超时
排查方法：
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活动，达到了MySQL 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MySQL server has gone away。
mysql> show global variables like 'wait_timeout';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| wait_timeout | 3600 |
+---------------+-------+
1 row in set (0.00 sec)
mysql> select now();
+---------------------+
| now() |
+---------------------+
| 2017-09-21 11:27:31 |
+---------------------+
1 row in set (0.00 sec)
mysql> SET SESSION wait_timeout=5; #将wait_timeout修改为5s
Query OK, 0 rows affected (0.00 sec)
mysql> select now(); #等待一段时间后，再次执行SQL
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 7
Current database: *** NONE ***
+---------------------+
| now() |
+---------------------+
| 2017-09-21 11:27:53 |
+---------------------+
1 row in set (0.00 sec)
当连接sleep时间超过wait_timeout时间后，再次发起活动查询时，就会报这个错。
原因三：进程在Server端被主动kill
排查方法：
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> show global status like 'com_kill';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Com_kill | 1 |
+---------------+-------+
1 row in set (0.00 sec)
原因四：Your SQL statement was too large
排查方法：
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 | 16777216 |
+--------------------+----------+
1 row in set (0.00 sec)
mysql> set global max_allowed_packet=1024*1024*32; #修改参数
Query OK, 0 rows affected (0.00 sec)
mysql> show global variables like 'max_allowed_packet';
+--------------------+----------+
| Variable_name | Value |
+--------------------+----------+
| max_allowed_packet | 33554432 |
+--------------------+----------+
1 row in set (0.00 sec)
reference
http://cenalulu.github.io/mysql/mysql-has-gone-away/
