MySQL根据配置文件会限制Server接受的数据包大小。有时候大的插入和更新会受 max_allowed_packet 参数限制，导致大数据写入或者更新失败。
查看目前配置：
mysql> show VARIABLES like 'max_allowed_packet'; +--------------------+------------+ | Variable_name | Value | +--------------------+------------+ | max_allowed_packet | 1073741824 | +--------------------+------------+ 1 row in set max_allowed_packet的单位为字节:
-- 转化为Mb,就是1024Mb mysql> select 1073741824/1024/1024; +----------------------+ | 1073741824/1024/1024 | +----------------------+ | 1024.00000000 | +----------------------+ 1 row in set 修改方法1-配置文件修改
可以编辑my.cnf,在[mysqld]段或者mysql的server配置段进行修改。
max_allowed_packet = 20M
修改my.cnf,配置要重载才能生效
修改方法2-命令修改
参数生效范围为global,不是session. 如果服务器重启设置会失效
set global max_allowed_packet = 2*1024*1024*10 注意
查询时使用的是show variables的话，发现设置好像并没有生效，这是因为show variables等同于show session variables，查询的是会话变量，只有使用show global variables，查询的才是全局变量。
示例:
- 查询 max_allowed_packet = 1073741824
mysql> show VARIABLES like 'max_allowed_packet'; +--------------------+------------+ | Variable_name | Value | +--------------------+------------+ | max_allowed_packet | 1073741824 | +--------------------+------------+ 1 row in set - 设置 max_allowed_packet = 1048576
mysql> set global max_allowed_packet = 1*1024*1024; Query OK, 0 rows affected - 查询结果没变
mysql> show VARIABLES like 'max_allowed_packet'; +--------------------+------------+ | Variable_name | Value | +--------------------+------------+ | max_allowed_packet | 1073741824 | +--------------------+------------+ 1 row in set - 全局查看
mysql> show global VARIABLES like 'max_allowed_packet'; +--------------------+---------+ | Variable_name | Value | +--------------------+---------+ | max_allowed_packet | 1048576 | +--------------------+---------+ 1 row in set max_allowed_packet 值设置过小将导致单个记录超过限制后写入数据库失败，且后续记录写入也将失败，为了数据完整性，需要考虑到事务因素。
作者：战神悟空
链接：https://www.jianshu.com/p/629685b97030
來源：简书