在MySQL数据库中，在进行数据迁移和从库只读状态设置时，都会涉及到只读状态和Master-slave的设置和关系。
经过实际测试，对于MySQL单实例数据库和master库，如果需要设置为只读状态，需要进行如下操作和设置：
一般单实例情况下将MySQL设置为只读状态的命令：
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# mysql -uroot -p
mysql> show
global
variables
like
"%read_only%"
;
mysql>flush tables
with
read
lock;
mysql>
set
global
read_only=1;
mysql> show
global
variables
like
"%read_only%"
;
|
将MySQL从只读设置为读写状态的命令：
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mysql> unlock tables;
mysql>
set
global
read_only=0;
|
对于需要保证master-slave主从同步的salve库，如果要设置为只读状态，需要执行的命令为：
mysql> set global read_only=1; 打开只读开关
将salve库从只读状态变为读写状态，需要执行的命令是：
mysql> set global read_only=0; 打开读写开关
对于数据库读写状态，主要靠 “read_only”全局参数来设定；
默认情况下，数据库是用于读写操作的，所以read_only参数也是0或faluse状态，这时候不论是本地用户还是远程访问数据库的用户，都可以进行读写操作；
如需设置为只读状态，将该read_only参数设置为1或TRUE状态，但设置 read_only=1 状态有两个需要注意的地方：
1.read_only=1只读模式，不会影响slave同步复制的功能，所以在MySQL slave库中设定了read_only=1后，通过 show slave status\G 命令查看salve状态，可以看到salve仍然会读取master上的日志，并且在slave库中应用日志，保证主从数据库同步一致；
2.read_only=1只读模式，可以限定普通用户进行数据修改的操作，但不会限定具有super权限的用户的数据修改操作；在MySQL中设置read_only=1后，普通的应用用户进行insert、update、delete等会产生数据变化的DML操作时，都会报出数据库处于只读模式不能发生数据变化的错误，但具有super权限的用户，例如在本地或远程通过root用户登录到数据库，还是可以进行数据变化的DML操作；
为了确保所有用户，包括具有super权限的用户也不能进行读写操作，就需要执行给所有的表加读锁的命令 “flush tables with read lock;”，这样使用具有super权限的用户登录数据库，想要发生数据变化的操作时，也会提示表被锁定不能修改的报错。
这样通过 设置“read_only=1”和“flush tables with read lock;”两条命令，就可以确保数据库处于只读模式，不会发生任何数据改变，在MySQL进行数据库迁移时，限定master主库不能有任何数据变化，就可以通过这种方式来设定。
但同时由于加表锁的命令对数据库表限定非常严格，如果再slave从库上执行这个命令后，slave库可以从master读取binlog日志，但不能够应用日志，slave库不能发生数据改变，当然也不能够实现主从同步了，这时如果使用 “unlock tables;”解除全局的表读锁，slave就会应用从master读取到的binlog日志，继续保证主从库数据库一致同步。
为了保证主从同步可以一直进行，在slave库上要保证具有super权限的root等用户只能在本地登录，不会发生数据变化，其他远程连接的应用用户只按需分配为select,insert,update,delete等权限，保证没有super权限，则只需要将salve设定“read_only=1”模式，即可保证主从同步，又可以实现从库只读。
相对的，设定“read_only=1”只读模式开启的解锁命令为设定“read_only=0”;设定全局锁“flush tables with read lock;”，对应的解锁模式命令为：“unlock tables;”.
当然设定了read_only=1后，所有的select查询操作都是可以正常进行的。
实验一：设置了read_only=1后，远程业务用户进行数据库修改会提示ERROR 1290错误：
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copy
(test01@172.32.1.200) [data03]> show tables;
+
------------------+
| Tables_in_data03 |
+
------------------+
| t01 |
| t02 |
|
user
|
+
------------------+
3
rows
in
set
(0.00 sec)
(test01@172.32.1.200) [data03]>
(test01@172.32.1.200) [data03]>
(test01@172.32.1.200) [data03]> show
global
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)
(test01@172.32.1.200) [data03]>
(test01@172.32.1.200) [data03]>
(test01@172.32.1.200) [data03]>
delete
from
t01
where
id1=3;
<span style=
"color:#ff0000;"
>ERROR 1290 (HY000): The MySQL server
is
running
with
the
--read-only option so it cannot execute this statement</span>
(test01@172.32.1.200) [data03]>
update
t01
set
id1=id1+30
where
id1=3;
ERROR 1290 (HY000): The MySQL server
is
running
with
the
--read-only option so it cannot execute this statement
(test01@172.32.1.200) [data03]>
insert
into
t01(id1,a1,b1)
values
(9,9,9);
ERROR 1290 (HY000): The MySQL server
is
running
with
the
--read-only option so it cannot execute this statement
(test01@172.32.1.200) [data03]>
(test01@172.32.1.200) [data03]>
select
*
from
t01;
+
-----+------+------+
| id1 | a1 | b1 |
+
-----+------+------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 4 | 4 | 4 |
| 5 | 5 | 5 |
| 6 | 6 | 6 |
+
-----+------+------+
5
rows
in
set
(0.00 sec)
(test01@172.32.1.200) [data03]>
|
实验二：设定了全局读写后，具有super权限的用户进行数据修改后，也会提示错误ERROR 1223：
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view
plain copy
mysql> use data03;
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_data03 |
+
------------------+
| t01 |
| t02 |
|
user
|
+
------------------+
3
rows
in
set
(0.00 sec)
mysql>
select
*
from
t01;
+
-----+------+------+
| id1 | a1 | b1 |
+
-----+------+------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 4 | 4 | 4 |
| 5 | 5 | 5 |
| 6 | 6 | 6 |
+
-----+------+------+
5
rows
in
set
(0.00 sec)
mysql>
mysql> show
global
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)
mysql>
mysql>
mysql>
insert
into
t01(id1,a1,b1)
values
(8,8,8);
Query OK, 1 row affected (0.00 sec)
mysql>
update
t01
set
a1=a1+10
where
id1=2;
Query OK, 1 row affected (0.00 sec)
Rows
matched: 1 Changed: 1 Warnings: 0
mysql>
delete
from
t01
where
id1=4;
Query OK, 1 row affected (0.00 sec)
mysql>
select
*
from
t01;
+
-----+------+------+
| id1 | a1 | b1 |
+
-----+------+------+
| 1 | 1 | 1 |
| 2 | 12 | 2 |
| 5 | 5 | 5 |
| 6 | 6 | 6 |
| 8 | 8 | 8 |
+
-----+------+------+
5
rows
in
set
(0.00 sec)
mysql>
mysql> flush tables
with
read
lock;
Query OK, 0
rows
affected (0.00 sec)
mysql>
mysql>
insert
into
t01(id1,a1,b1)
values
(9,9,9);
<span style=
"color:#ff0000;"
>ERROR 1223 (HY000): Can
't execute the query because you have a conflicting read lock</span>
mysql>
mysql> update t01 set a1=a1+10 where id1=5;
ERROR 1223 (HY000): Can'
t
execute
the query because you have a conflicting
read
lock
mysql>
mysql>
delete
from
t01
where
id1=5;
ERROR 1223 (HY000): Can't
execute
the query because you have a conflicting
read
lock
mysql>
mysql>
|
实验三：MySQL从库设定read_only=1后主从同步正常，设定表读锁后，不能同步，解除读锁后，主从同步恢复。
-
mysql>
-
mysql> show databases;
-
+--------------------+
-
| Database |
-
+--------------------+
-
| information_schema |
-
| bitvc |
-
| data03 |
-
| ga |
-
| jiradb |
-
| meibi |
-
| meibi02 |
-
| mysql |
-
| performance_schema |
-
| sbtest |
-
+--------------------+
-
10 rows in set (0.00 sec)
-
-
mysql>
-
mysql>
-
mysql>
-
mysql> show global 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)
-
-
mysql>
-
mysql> show slave status\G
-
*************************** 1. row ***************************
-
Slave_IO_State: Waiting for master to send event
-
Master_Host: 172.32.1.200
-
Master_User: repl
-
Master_Port: 3307
-
Connect_Retry: 60
-
Master_Log_File: mysql-bin.000009
-
Read_Master_Log_Pos: 5853
-
Relay_Log_File: huobiDBtest-relay-bin.000002
-
Relay_Log_Pos: 6016
-
Relay_Master_Log_File: mysql-bin.000009
-
Slave_IO_Running: Yes
-
Slave_SQL_Running: Yes
-
Replicate_Do_DB:
-
Replicate_Ignore_DB:
-
Replicate_Do_Table:
-
Replicate_Ignore_Table:
-
Replicate_Wild_Do_Table:
-
Replicate_Wild_Ignore_Table:
-
-
Last_Errno: 0
-
Last_Error:
-
Skip_Counter: 0
-
Exec_Master_Log_Pos: 5853
-
Relay_Log_Space: 6195
-
Until_Condition: None
-
Until_Log_File:
-
Until_Log_Pos: 0
-
Master_SSL_Allowed: No
-
Master_SSL_CA_File:
-
Master_SSL_CA_Path:
-
Master_SSL_Cert:
-
Master_SSL_Cipher:
-
Master_SSL_Key:
-
Seconds_Behind_Master: 0
-
Master_SSL_Verify_Server_Cert: No
-
Last_IO_Errno: 0
-
Last_IO_Error:
-
Last_SQL_Errno: 0
-
Last_SQL_Error:
-
Replicate_Ignore_Server_Ids:
-
Master_Server_Id: 2003307
-
Master_UUID: 6f68eea7-76e9-11e4-8f99-00221904cd5d
-
Master_Info_File: /data/mysqldata/3308/data/master.info
-
SQL_Delay: 0
-
SQL_Remaining_Delay: NULL
-
Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it
-
Master_Retry_Count: 86400
-
Master_Bind:
-
Last_IO_Error_Timestamp:
-
Last_SQL_Error_Timestamp:
-
Master_SSL_Crl:
-
Master_SSL_Crlpath:
-
Retrieved_Gtid_Set:
-
Executed_Gtid_Set:
-
Auto_Position: 0
-
1 row in set (0.00 sec)
-
-
mysql>
-
mysql> flush tables with read lock;
-
Query OK, 0 rows affected (0.00 sec)
-
-
mysql> show slave status\G
-
*************************** 1. row ***************************
-
Slave_IO_State: Waiting for master to send event
-
Master_Host: 172.32.1.200
-
Master_User: repl
-
Master_Port: 3307
-
Connect_Retry: 60
-
Master_Log_File: mysql-bin.000009
-
Read_Master_Log_Pos: 6531
-
Relay_Log_File: huobiDBtest-relay-bin.000002
-
Relay_Log_Pos: 6016
-
Relay_Master_Log_File: mysql-bin.000009
-
Slave_IO_Running: Yes
-
Slave_SQL_Running: Yes
-
Replicate_Do_DB:
-
Replicate_Ignore_DB:
-
Replicate_Do_Table:
-
Replicate_Ignore_Table:
-
Replicate_Wild_Do_Table:
-
Replicate_Wild_Ignore_Table:
-
Last_Errno: 0
-
Last_Error:
-
Skip_Counter: 0
-
-
Exec_Master_Log_Pos: 5853
-
Relay_Log_Space: 6873
-
Until_Condition: None
-
Until_Log_File:
-
Until_Log_Pos: 0
-
Master_SSL_Allowed: No
-
Master_SSL_CA_File:
-
Master_SSL_CA_Path:
-
Master_SSL_Cert:
-
Master_SSL_Cipher:
-
Master_SSL_Key:
-
Seconds_Behind_Master: 120
-
Master_SSL_Verify_Server_Cert: No
-
Last_IO_Errno: 0
-
Last_IO_Error:
-
Last_SQL_Errno: 0
-
Last_SQL_Error:
-
Replicate_Ignore_Server_Ids:
-
Master_Server_Id: 2003307
-
Master_UUID: 6f68eea7-76e9-11e4-8f99-00221904cd5d
-
-
Master_Info_File: /data/mysqldata/3308/data/master.info
-
SQL_Delay: 0
-
SQL_Remaining_Delay: NULL
-
Slave_SQL_Running_State: Waiting for global read lock
-
Master_Retry_Count: 86400
-
Master_Bind:
-
Last_IO_Error_Timestamp:
-
Last_SQL_Error_Timestamp:
-
Master_SSL_Crl:
-
Master_SSL_Crlpath:
-
Retrieved_Gtid_Set:
-
Executed_Gtid_Set:
-
Auto_Position: 0
-
1 row in set (0.00 sec)
-
-
mysql>
-
mysql>
-
mysql> select * from data03.t01;
-
+-----+------+------+
-
| id1 | a1 | b1 |
-
+-----+------+------+
-
| 1 | 1 | 1 |
-
| 2 | 2 | 2 |
-
| 4 | 4 | 4 |
-
| 5 | 5 | 5 |
-
| 6 | 6 | 6 |
-
+-----+------+------+
-
5 rows in set (0.00 sec)
-
-
mysql>
-
mysql> unlock tables;
-
Query OK, 0 rows affected (0.00 sec)
-
-
mysql> show slave status\G
-
*************************** 1. row ***************************
-
Slave_IO_State: Waiting for master to send event
-
Master_Host: 172.32.1.200
-
Master_User: repl
-
Master_Port: 3307
-
Connect_Retry: 60
-
Master_Log_File: mysql-bin.000009
-
Read_Master_Log_Pos: 6531
-
Relay_Log_File: huobiDBtest-relay-bin.000002
-
Relay_Log_Pos: 6694
-
Relay_Master_Log_File: mysql-bin.000009
-
Slave_IO_Running: Yes
-
Slave_SQL_Running: Yes
-
Replicate_Do_DB:
-
Replicate_Ignore_DB:
-
Replicate_Do_Table:
-
Replicate_Ignore_Table:
-
Replicate_Wild_Do_Table:
-
Replicate_Wild_Ignore_Table:
-
Last_Errno: 0
-
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Last_Error:
Skip_Counter: 0
Exec_Master_Log_Pos: 6531
Relay_Log_Space: 6873
Until_Condition: None
Until_Log_File:
Until_Log_Pos: 0
Master_SSL_Allowed:
No
Master_SSL_CA_File:
Master_SSL_CA_Path:
Master_SSL_Cert:
Master_SSL_Cipher:
Master_SSL_Key:
Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert:
No
Last_IO_Errno: 0
Last_IO_Error:
Last_SQL_Errno: 0
Last_SQL_Error:
Replicate_Ignore_Server_Ids:
Master_Server_Id: 2003307
Master_UUID: 6f68eea7-76e9-11e4-8f99-00221904cd5d
Master_Info_File: /data/mysqldata/3308/data/master.info
SQL_Delay: 0
SQL_Remaining_Delay:
NULL
Slave_SQL_Running_State: Slave has
read
all
relay log; waiting
for
the slave I/O thread
to
update
it
Master_Retry_Count: 86400
Master_Bind:
Last_IO_Error_Timestamp:
Last_SQL_Error_Timestamp:
Master_SSL_Crl:
Master_SSL_Crlpath:
Retrieved_Gtid_Set:
Executed_Gtid_Set:
Auto_Position: 0
1 row
in
set
(0.00 sec)
mysql>
select
*
from
data03.t01;
+
-----+------+------+
| id1 | a1 | b1 |
+
-----+------+------+
| 1 | 1 | 1 |
| 2 | 12 | 2 |
| 5 | 5 | 5 |
| 6 | 6 | 6 |
| 8 | 8 | 8 |
+
-----+------+------+
5
rows
in
set
(0.00 sec)
|
##########################################
.FLUSH TABLES WITH READ LOCK
这个命令是全局读锁定，执行了命令之后所有库所有表都被锁定只读。一般都是用在数据库联机备份，这个时候数据库的写操作将被阻塞，读操作顺利进行。
解锁的语句也是unlock tables。
2.LOCK TABLES tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}
这个命令是表级别的锁定，可以定制锁定某一个表。例如： lock tables test read; 不影响其他表的写操作。
解锁语句也是unlock tables。
重点：
这两个语句在执行的时候都需要注意个特点，就是 隐式提交的语句。在退出MySQL终端的时候都会隐式的执行unlock tables。也就是如果要让表锁定生效就必须一直保持对话。
P.S. MYSQL的read lock和wirte lock
read-lock: 允许其他并发的读请求，但阻塞写请求，即可以同时读，但不允许任何写。也叫共享锁
write-lock: 不允许其他并发的读和写请求，是排他的(exclusive)。也叫独占锁
本文转自crazy_charles 51CTO博客，原文链接：http://blog.51cto.com/douya/1868243，如需转载请自行联系原作者
