1:（限定普通用户），数据库只读不能写
**不影响主从同步
mysql> set global read_only=1; #1是只读，0是读写
mysql> show global variables like "%read_only%";
+------------------+-------+
| Variable_name | Value |
+------------------+-------+
| innodb_read_only | OFF |
| read_only | ON |
| tx_read_only | OFF |
+------------------+-------+
恢复：
mysql> set global read_only=0; #1是只读，0是读写
2:（限定所有用户），只能读不能写
**影响主从同步，解锁后恢复
mysql> set global read_only=1; #1是只读，0是读写
mysql> flush tables with read lock;
恢复：
mysql> set global read_only=0; #1是只读，0是读写
mysql> unlock tables;