Mysql server出现“Mysql server has gone away”的错误的解决方式
一、最常见的原因分析及解决方法 wait_timeout 和 interactive_timeout
在官方的文档中给出了出现“Mysql server has gone away”的错误的分析原因：https://dev.mysql.com/doc/refman/5.7/en/gone-away.html
出现该问题的主要原因是：Mysql server服务器超时，并且关闭了与客户端的连接导致的。在这种情况下，你一般会得到下面的两种错误中的一种（具体取决于你的操作系统）：
|————–错误码———— | ———————————–错误描述——————————-|
CR_SERVER_GONE_ERROR:The client couldn’t send a question to the server.
CR_SERVER_LOST:The client didn’t get an error when writing to the server, but it didn’t get a full answer (or any answer) to the question.
默认情况下，如果在8小时没有对mysql进行查询请求的话，服务器就会自动断开连接。可以通过修改全局变量 wait_timeout和interactive_timeout两个变量的值来进行修改。
$ mysql -u root -p
$ mysql>show variables like '%timeout';
$ mysql>set global wait_timeout = 2880000;
$ mysql>set global interactive_timeout = 2880000;
对这两个变量（wait_timeout 和 interactive_timeout）的解释和比较见：http://cenalulu.github.io/mysql/mysql-timeout/
interactive_timeout：交互式客户端连接的超时时间。
wait_timeout：是非交互式的超时时间。
client_interactive：运行mysql server 在交互式超时时间interactive_timeout（而不是wait_timeout）时间之后断开连接。
简单的说 interactive就是交互式的终端，例如在shell里面直接执行mysql，出现形如mysql>的提示符后就是交互式的连接。而mysql -e ‘select 1’ 这样的直接返回结果的方式就是非交互式的连接。
继承关系
1.通过socket连接 timeout会从哪个global timeout继承 A：由下例可见，通过socket登录，timeout 继承于global.interactive_timeout;
mysql> set global interactive_timeout = 11111;
Query OK, 0 rows affected (0.00 sec)
mysql> set global wait_timeout = 22222;
Query OK, 0 rows affected (0.00 sec)
mysql> show global variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| innodb_lock_wait_timeout | 50 |
| innodb_rollback_on_timeout | OFF |
| interactive_timeout | 11111 |
| lock_wait_timeout | 31536000 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| wait_timeout | 22222 |
+----------------------------+----------+
10 rows in set (0.00 sec)
mysql -uroot -ppassword -S /usr/local/mysql3310/mysql.sock
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.5.16-log MySQL Community Server (GPL)
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show session variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| innodb_lock_wait_timeout | 50 |
| innodb_rollback_on_timeout | OFF |
| interactive_timeout | 11111 |
| lock_wait_timeout | 31536000 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| wait_timeout | 11111 |
+----------------------------+----------+
10 rows in set (0.00 sec)
通过TCP/IP client 连接， timeout会从哪个global timeout继承 A：由下例可见，通过TCP/IP client 连接后的wait_timeout 仍然继承于 global.interactive_timeout
mysql -uroot -ppassword -h 127.0.0.1 --port 3310
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.5.16-log MySQL Community Server (GPL)
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show session variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| innodb_lock_wait_timeout | 50 |
| innodb_rollback_on_timeout | OFF |
| interactive_timeout | 11111 |
| lock_wait_timeout | 31536000 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| wait_timeout | 11111 |
+----------------------------+----------+
10 rows in set (0.00 sec)
起效关系
timeout值，对于处于运行状态SQL语句是否起效（即是否等价于执行超时）？ A：由下例可见SQL正在执行状态的等待时间不计入timeout时间。即SQL运行再久也不会因为timeout的配置而中断
mysql> set session wait_timeout=10;
Query OK, 0 rows affected (0.00 sec)
mysql> set session interactive_timeout=10;
Query OK, 0 rows affected (0.00 sec)
mysql> select 1,sleep(20) from dual;
+---+-----------+
| 1 | sleep(20) |
+---+-----------+
| 1 | 0 |
+---+-----------+
1 row in set (20.00 sec)
mysql>
mysql> show session variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| innodb_lock_wait_timeout | 50 |
| innodb_rollback_on_timeout | OFF |
| interactive_timeout | 10 |
| lock_wait_timeout | 31536000 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| wait_timeout | 10 |
+----------------------------+----------+
同一个session中，wait_timeout 和 interacitve_timeout是否都会生效。 A：只有wait_timeout 会真正起到超时限制的作用
mysql> set session interactive_timeout=10;
Query OK, 0 rows affected (0.00 sec)
mysql> set session wait_timeout=20;
Query OK, 0 rows affected (0.00 sec)
mysql> show full processlist;
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| 1 | system user | | NULL | Connect | 103749 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL | 0 | 0 | 1 |
| 2 | system user | | NULL | Connect | 103750 | Connecting to master | NULL | 0 | 0 | 1 |
| 3 | root | localhost | NULL | Query | 0 | NULL | show full processlist | 0 | 0 | 11 |
| 10 | root | localhost:58946 | NULL | Sleep | 20 | | NULL | 0 | 0 | 11 |
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
4 rows in set (0.00 sec)
mysql> show full processlist;
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| 1 | system user | | NULL | Connect | 103749 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL | 0 | 0 | 1 |
| 2 | system user | | NULL | Connect | 103750 | Connecting to master | NULL | 0 | 0 | 1 |
| 3 | root | localhost | NULL | Query | 0 | NULL | show full processlist | 0 | 0 | 11 |
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
3 rows in set (0.00 sec)
global timeout和session timeout是否都会作为超时判断依据？ A：只有session级别 timeout 会起作用。即一个session开始后，无论如何修改global级别的timeout都不会影响该session
mysql> set session interactive_timeout = 10;
Query OK, 0 rows affected (0.00 sec)
mysql> set session wait_timeout = 10;
Query OK, 0 rows affected (0.00 sec)
mysql> show session variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| interactive_timeout | 10 |
| wait_timeout | 10 |
+----------------------------+----------+
10 rows in set (0.00 sec)
mysql> show global variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| interactive_timeout | 20 |
| wait_timeout | 20 |
+----------------------------+----------+
10 rows in set (0.00 sec)
mysql> show full processlist;
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| 3 | root | localhost | NULL | Query | 0 | NULL | show full processlist | 0 | 0 | 11 |
| 17 | root | localhost:60585 | NULL | Sleep | 10 | | NULL | 10 | 10 | 11 |
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
2 rows in set (0.00 sec)
mysql> show full processlist;
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| 3 | root | localhost | NULL | Query | 0 | NULL | show full processlist | 0 | 0 | 11 |
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
1 rows in set (0.00 sec)
测试2
mysql> show session variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| interactive_timeout | 20 |
| wait_timeout | 20 |
+----------------------------+----------+
10 rows in set (0.00 sec)
mysql> show global variables like '%timeout%';
+----------------------------+----------+
| Variable_name | Value |
+----------------------------+----------+
| interactive_timeout | 10 |
| wait_timeout | 10 |
+----------------------------+----------+
10 rows in set (0.00 sec)
mysql> show full processlist;
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| 3 | root | localhost | NULL | Query | 0 | NULL | show full processlist | 0 | 0 | 11 |
| 19 | root | localhost:50276 | NULL | Sleep | 19 | | NULL | 10 | 10 | 11 |
+----+-------------+-----------------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
2 rows in set (0.00 sec)
mysql> show full processlist;
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
| 3 | root | localhost | NULL | Query | 0 | NULL | show full processlist | 0 | 0 | 11 |
+----+-------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+-----------------------+-----------+---------------+-----------+
1 rows in set (0.00 sec)
二、还有一种就是 你发送给服务器的查询是错误的 或者太大了
这种情况下，需要修改 变量max_allowed_packet的大小
$ mysql> set global max_allowed_packet = 64M;
三、其他问题见官方文档
Reference:解决方式参考
修改过程中遇到的其他问题
没有通过mysql command line来修改变量的时候 是通过在/etc/mysql/my.cnf中添加变量来修改的，
max_allowed_packet = 64M
wait_timeout = 2880000
interactive_timeout = 2880000
这种方式。但是修改完之后重启mysql service的时候 就出问题了，报错：“Job for mysql.service failed because the control process exited with error code , see systemctl status mysql.service and journalctl -xe for details”（这里是因为语法出错了，要在变量设置前 加上[mysqld]标志）
于是注释掉了 上面的变量修改，但是还是不起作用
后来通过Stackoverflow查到了，能够解决问题的答案：
1.Check the permission of mysql data dir using below command
# ls -ld /var/lib/mysql/
2.Check the permission of databases inside mysql data dir using below command
# ls -lh /var/lib/mysql/
3.Check the listening network tcp ports using below command
# netstat -ntlp
4.Check the mysql log files for any error using below command.
# cat /var/log/mysql/mysqld.log
5.Try to start mysql using below command
# mysqld_safe --defaults-file=/etc/my.cf
我的问题在执行完最后一条之后 就解决了，但是安装的PHPMyadmin不能连接上了 ，所以又通过下面的命令：
$ sudo service mysql restart
重启了下 就好了。
附加内容：在使用Mysql server的时候出现 “Mysql server has gone away”的时候 ，大概有两个方面的原因：
一是 查询太大 太长
二是 连接超时自动断开（这个是最常见的错误）
这两个修改方式都是 设置[mysqld]中的变量:
[mysqld]
max_allowed_packet = 64M
wait_timeout = 2880000
interactive_timeout = 2880000
当时在 /etc/mysql/my.cnf中修改的时候 没有加上 [mysqld]这个标志 导致语法错误 而不能启动mysql
还有网上别人遇到的错误是 修改了变量之后没有生效。
这个方面的原因大概就是：
my.cnf中 还有引用了两个地方的设置：
!includedir /etc/mysql/conf.d/
!includedir /etc/mysql/mysql.conf.d/
这两个文件里面的mysql.conf配置也会被引进来 ，导致my.cnf中的配置被这两个文件中的配置覆盖。
所以尽量到这两个文件中，去修改需要修改的变量。