ERROR 2006 (HY000): MySQL server has gone away

1.版本

1)操作系统

 cat /etc/issue
Red Hat Enterprise Linux Server release 5.5 (Tikanga)
Kernel \r on an \m

 cat /proc/version
Linux version 2.6.32-504.el6.x86_64 ([email protected]) (gcc version 4.4.7 20120313 (Red Hat 4.4.7-11) (GCC) ) #1 SMP Wed Oct 15 04:27:16 UTC 2014

2)mysql数据库版本

mysql --version
mysql  Ver 14.14 Distrib 5.6.27, for Linux (x86_64) using  EditLine wrapper

2.问题描述

  使用source导入某个库的导出文件时报如下错误：

<span style="color:#ff0000;">ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    125987
Current database: xwiki
...........
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    125988
Current database: xwiki</span><span style="color:#333333;">
...........
ERROR 1231 (42000): Variable 'time_zone' can't be set to the value of 'NULL'
ERROR 1231 (42000): Variable 'sql_mode' can't be set to the value of 'NULL'
ERROR 1231 (42000): Variable 'foreign_key_checks' can't be set to the value of 'NULL'
ERROR 1231 (42000): Variable 'unique_checks' can't be set to the value of 'NULL'
ERROR 1231 (42000): Variable 'character_set_client' can't be set to the value of 'NULL'
Query OK, 0 rows affected (0.00 sec)</span>

##这里我们需要关注标红部分的报错，下面的ERROR 1231都是由标红错误引起的。

3.解决方案

  后来我通过修改max_allowed_packet参数，解决了该问题。

1)查看修改之前max_allowed_packet参数

mysql> show variables like 'max_allowed%';
+--------------------+---------+
| Variable_name      | Value   |
+--------------------+---------+
| max_allowed_packet | 4194304 |
+--------------------+---------+

2)修改 max_allowed_packet参数

mysql> set global max_allowed_packet=16777218;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+----------------------------------------------------------+
| Level   | Code | Message                                                  |
+---------+------+----------------------------------------------------------+
| Warning | 1292 | Truncated incorrect max_allowed_packet value: '16777218' |
+---------+------+----------------------------------------------------------+
1 row in set (0.00 sec)

##注意此处告警是因为max_allowed_packet值必须为1024的整数倍，上例中max_allowed_packet值会被设置为小于16777218的1024最大的整数倍(即16777216)
The value should be a multiple of 1024; nonmultiples are rounded down to the nearest multiple

3)重新连接数据库并执行source

导入成功

4.官方文档中关于max_allowed_packet说明

1)

17.4.1.21 Replication and max_allowed_packet

1)

max_allowed_packet sets an upper limit on the size of any single message between the MySQL server and clients, including replication slaves. If you are replicating large column values (such as might be found in TEXT or BLOB columns) and max_allowed_packet is too small on the master, the master fails with an error, and the slave shuts down the I/O thread. If max_allowed_packet is too small on the slave, this also causes the slave to stop the I/O thread.

Row-based replication currently sends all columns and column values for updated rows from the master to the slave, including values of columns that were not actually changed by the update. This means that, when you are replicating large column values using row-based replication, you must take care to setmax_allowed_packet large enough to accommodate the largest row in any table to be replicated, even if you are replicating updates only, or you are inserting only relatively small values.

2)

•  max_allowed_packet

  Command-Line Format	--max_allowed_packet=#
  System Variable	Name	max_allowed_packet
  	Variable Scope	Global
  	Dynamic Variable	Yes
  Permitted Values (<= 5.6.5)	Type	integer
  	Default	1048576
  	Min Value	1024
  	Max Value	1073741824
  Permitted Values (>= 5.6.6)	Type	integer
  	Default	4194304
  	Min Value	1024
  	Max Value	1073741824

  The maximum size of one packet or any generated/intermediate string, or any parameter sent by the mysql_stmt_send_long_data() C API function. The default is 4MB as of MySQL 5.6.6, 1MB before that.

  The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch large (possibly incorrect) packets.

  You must increase this value if you are using large BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit formax_allowed_packet is 1GB. The value should be a multiple of 1024; nonmultiples are rounded down to the nearest multiple.

  When you change the message buffer size by changing the value of the max_allowed_packet variable, you should also change the buffer size on the client side if your client program permits it. The default max_allowed_packet value built in to the client library is 1GB, but individual client programs might override this. For example, mysql and mysqldump have defaults of 16MB and 24MB, respectively. They also enable you to change the client-side value by setting max_allowed_packet on the command line or in an option file.

  The session value of this variable is read only.