今天看到语句set global read_only=on; 以为这条语句只对设置变量之后的连接才有效果。 
因为记得set global 就是只对本设置之后再次连接到数据库上的连接才有效，对于已经存在的连接，没有效果。 
查看了一下文档： 
If you change a global system variable, the value is remembered and used for new connections until 
the server restarts. 

但是测试read_only: 

[mysql@localhost ~]$ mysql -u root 
Welcome to the MySQL monitor. Commands end with ; or \g. 
Your MySQL connection id is 1 
Server version: 5.6.23-enterprise-commercial-advanced-log MySQL Enterprise Server - Advanced Edition (Commercial) 

Copyright (c) 2000, 2015, 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> grant all on a.* to test identified by 'test'; 
Query OK, 0 rows affected (0.00 sec) 

mysql> select user,host ,super_priv from mysql.user; 
+-------+-----------------------+------------+ 
| user | host | super_priv | 
+-------+-----------------------+------------+ 
| root | localhost | Y | 
| root | localhost.localdomain | Y | 
| root | 127.0.0.1 | Y | 
| root | ::1 | Y | 
| test | % | N | 
| ttt | % | Y | 
| ttttt | % | Y | 
+-------+-----------------------+------------+ 
7 rows in set (0.00 sec) 





[mysql@localhost ~]$ mysql -u test -ptest 
Warning: Using a password on the command line interface can be insecure. 
Welcome to the MySQL monitor. Commands end with ; or \g. 
Your MySQL connection id is 4 
Server version: 5.6.23-enterprise-commercial-advanced-log MySQL Enterprise Server - Advanced Edition (Commercial) 

Copyright (c) 2000, 2015, 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> use a; 
Reading table information for completion of table and column names 
You can turn off this feature to get a quicker startup with -A 

Database changed 
mysql> show tables; 
+-------------+ 
| Tables_in_a | 
+-------------+ 
| aa | 
| aaaa在 | 
| t | 
| t2 | 
| t3 | 
| t3_new | 
| tt | 
+-------------+ 
7 rows in set (0.00 sec) 

mysql> desc t; 
+---------+---------+------+-----+---------+-------+ 
| Field | Type | Null | Key | Default | Extra | 
+---------+---------+------+-----+---------+-------+ 
| sysdate | int(11) | YES | | NULL | | 
+---------+---------+------+-----+---------+-------+ 
1 row in set (0.00 sec) 


mysql> insert into t values (1); 
Query OK, 1 row affected (0.06 sec) 



这时在另一个有super权限会话中： 
mysql> set global read_only=on; 
Query OK, 0 rows affected (0.00 sec) 


然后回到已经连接的test用户中去： 
mysql> insert into t values (1); 
ERROR 1290 (HY000): The MySQL server is running with the --read-only option so it cannot execute this statement 
mysql> show variables like '%read_only%'; 
+------------------+-------+ 
| Variable_name | Value | 
+------------------+-------+ 
| innodb_read_only | OFF | 
| read_only | ON | 
| tx_read_only | OFF | 
+------------------+-------+ 
3 rows in set (0.00 sec) 

再次查看文档：

原来read_only的范围只有 global 。没有session级别。对于别的只有global级别的变量，这种同样有效。