mysql 主从，双主同步

1、创建用户并设置远程访问授权

1). A上添加： //ip地址为B的ip地址，用于B访问

　　# grant replication slave,replication client,reload,super on *.* to 'sync_user'@'192.168.2.220' identified by '123456' with grant option;  

2). B上添加：//ip地址为A的ip地址，用于A访问 

　　# grant replication slave,replication client,reload,super on *.* to 'sync_user'@'192.168.2.67' identified by '123456' with grant option;

3). 执行命令更新数据库使用户生效。

　　# flush privileges; 

2、确保mysql对外部ip开放

1). 确认一下3306是否对外开放，MySQL默认状态下是不开放对外访问功能的。查看的办法如下：

　　~# netstat -an | grep 3306
　　tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN

　　从上面可以看出，mysql的3306端口只是监听本地的连接，这样就阻碍了外部IP对该数据库的访问，修改的办法其实很简单，进入到mysql的配置文件所在目录（/etc/mysql/my.cnf(默认)，有的是在/etc/my.cnf）下，找到文件中的如下内容：

　　# Instead of skip-networking the default is now to listen only on  
　　# localhost which is more compatible and is not less secure.  

　　bind-address = 127.0.0.1

　　将bind-address注释掉，或者改成你想要使用的客户端主机IP。
　　这样mysql的远程访问端口就算开启了

　　~# netstat -an | grep 3306
　　tcp        0      0 0.0.0.0:3306            0.0.0.0:*               LISTEN

2). 关闭防火墙
　　# service iptables stop

 3. 创建同步测试表

create database testsync;
use testsync;

create table t_user (
    id int primary key auto_increment,
    username varchar(20),
    age int
);

4. 配置A B的mysql（查看my.cnf位置 mysql --help | grep my.cnf）

# vi /etc/my.cnf

 在[mysqld]下添加如下字段：

#两台机器的server-id不能相同
server-id       = 1
log-bin=mysql-bin
binlog_format=mixed
replicate-do-db=testsync
replicate-ignore-db=mysql,information_schema,performance_schema

5. 重启并进入mysql

0). 查看主机状态
　　# show master status;

+-------------------+----------+--------------------+------------------+-------------------+
| File              | Position | Binlog_Do_DB       | Binlog_Ignore_DB | Executed_Gtid_Set |
+-------------------+----------+--------------------+------------------+-------------------+
| mysqld-bin.000005 |    68113 | happy,ipaddressmgr |                  |                   |
+-------------------+----------+--------------------+------------------+-------------------+
1 row in set (0.00 sec)

1). 设置主机信息
　　# change master to master_host='192.168.2.220',master_user='sync_user',master_password='123456',master_log_file='mysql-bin.000005',master_log_pos=68113; 

2). 启动slave 
　　# start slave;

3). 查看是否启动成功
　　# show processlist;

+----+-------------+-------------------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+
| Id | User        | Host                    | db   | Command     | Time | State                                                                       | Info             |
+----+-------------+-------------------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+
|  1 | system user |                         | NULL | Connect     | 1264 | Waiting for master to send event                                            | NULL             |
|  2 | system user |                         | NULL | Connect     | 1262 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL             |
| 38 | sync_user   | pc.local:36102 | NULL | Binlog Dump | 1258 | Master has sent all binlog to slave; waiting for binlog to be updated       | NULL             |
| 40 | root        | localhost               | NULL | Query       |    0 | NULL                                                                        | show processlist |
+----+-------------+-------------------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+

4). 查看slave上的master状态
　　#show master status;

5). 查看slave状态
　　# show slave statusG;

6. 以上便实现主从备份，如果想要双主，只需要在另一台mysql上，执行步骤4, 5即可。

错误处理：

101027 16:50:58 [Note] Slave I/O thread: connected to master 'xxx@xxxxxx:3306',replication started in log 'xxx-bin.000001' at position 1440046 
101027 16:50:58 [ERROR] Error reading packet from server: Could not find first log file name in binary log index file ( server_errno=1236) 
101027 16:50:58 [ERROR] Slave I/O: Got fatal error 1236 from master when reading data from binary log: 'Could not find first log file name in binary log index file', Error_code: 1236 
101027 16:50:58 [Note] Slave I/O thread exiting, read up to log 'xxx-bin.000001', position 1440046 

Slave_IO_Running: No 
Slave_SQL_Running: Yes 

原因：master_log_file或者master_log_pos配置错误 或者 权限错误

按照正常流程解决，应该是：
stop slave; 
CHANGE MASTER TO MASTER_HOST='xxx',MASTER_USER='xx',MASTER_PASSWORD='xx',MASTER_PORT=3306,MASTER_LOG_FILE='xxx-bin.000001',MASTER_LOG_POS=1440046;
start slave;
 
问题依然存在，
 
后来经过以下步骤解决，不过有点困惑：
1、重启主库
2、stop slave;
3、给从库重新授权
4、执行以下命令 reset slave; start slave;