mysql主从架构搭建
主机环境
关闭防火墙跟selinux
两台版本、安装方式一致的数据库
配置搭建环境脚本《mysql项目需要的主机基础环境设置》
安装
rpm -ivh libev4-4.15-7.1.x86_64.rpm libev-devel-4.15-21.1.x86_64.rpm percona-xtrabackup-24-2.4.4-1.el6.x86_64.rpm
安装依赖包
yum install perl-DBD-mysql
主从架构+mha软件包
都关闭dns
vim /etc/ssh/sshd_config
#useDNS yes 改为 useDNS no修改主从的配置文件
主库配置：
binlog_format=row
server-id=3306100
binlog_rows_query_log_events=on
log-bin=server
从库配置：
read_only=1
server-id=3306101
binlog_format=row
binlog_rows_query_log_events=on
log-bin=slave
对主库进行复制
innobackupex --user=root --password=123 /backup/得到
2018-07-17_14-42-41这个文件然后应用这个文件
innobackupex --apply-log 2018-07-17_14-42-41/然后关闭从库数据库，清空数据库
rm –rf * //因为这是实验，所以用rm -rf比较方便，但不推荐用这种方式，可以考虑把文件拷贝到别的地方！！！！很危险的操作！！！在主库中通过scp传输 将刚才得到的文件传送到从库的/backup目录下/
scp -r 2018-07-17_14-05-15/ root@172.16.10.4:/backup/回到从库进入到/backup/目录确认
然后执行恢复操作
innobackupex --move-back /backup/2018-07-13_14-05-15/授权
chown -R mysql:mysql /var/lib/mysql查看
var/lib/mysql/xtrabackup_info确保
binlog_pos = filename 'server.000002', position '154'参数有值
这样就知道主从关系了
在主库
创建从库复制的用户
grant replication slave on *.* to 'repl'@'%' identified by 'repl';进入从库数据库设置主从关系（在从库上设置）
mysql>
CHANGE MASTER TO MASTER_HOST='172.16.10.6', //主库的ip
MASTER_USER='repl',MASTER_PASSWORD='repl',
MASTER_PORT=3306,
MASTER_LOG_FILE='server.000002',MASTER_LOG_POS=154, //来自上面查看 /var/lib/mysql/xtrabackup_info
MASTER_CONNECT_RETRY=10; //重连次数启动从库
mysql> start slave;启动从库前后 线程对比
mysql> show processlist;
+----+------+-----------+------+---------+------+----------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+----------+------------------+
| 3 | root | localhost | NULL | Query | 0 | starting | show processlist |
+----+------+-----------+------+---------+------+----------+------------------+
1 row in set (0.00 sec)
mysql> start slave
-> ;
Query OK, 0 rows affected (0.01 sec)mysql> show processlist;
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| 3 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 4 | system user | | NULL | Connect | 39 | Connecting to master | NULL | //连接io线程
| 5 | system user | | NULL | Connect | 39 | Slave has read all relay log; waiting for more updates | NULL |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+ //SQL线程，日志应用线程
3 rows in set (0.00 sec)
mysql>
mysql> show slave status \G
Slave_IO_State: Waiting for master to send event
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
主库show processlist;
mysql> show processlist;
+----+------+-------------------+-------+-------------+------+---------------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-------------------+-------+-------------+------+---------------------------------------------------------------+------------------+
| 4 | root | localhost | mysql | Query | 0 | starting | show processlist |
| 5 | repl | 172.16.10.4:47768 | NULL | Binlog Dump | 465 | Master has sent all binlog to slave; waiting for more updates | NULL |
+----+------+-------------------+-------+-------------+------+---------------------------------------------------------------+------------------+
2 rows in set (0.00 sec)多了一个Binlog Dump线程
主库 新建一个测试库跟测试表
mysql> create database zhucongfuzhiceshi1 character set =utf8;
Query OK, 1 row affected (0.02 sec)
mysql> use zhucongfuzhiceshi1;
Database changed
mysql> create table ceshi as select * from mysql.user;
Query OK, 4 rows affected (0.08 sec)
Records: 4 Duplicates: 0 Warnings: 0到从库看一下是否有复制过来
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| zhucongfuzhiceshi1 |
+--------------------+
5 rows in set (0.00 sec)mysql> use zhucongfuzhiceshi1;
Database changed
mysql> show tables;
+------------------------------+
| Tables_in_zhucongfuzhiceshi1 |
+------------------------------+
| ceshi |
+------------------------------+
1 row in set (0.00 sec)
select * from ceshi1;这就达到了一个主从同步的状态
再看一下
（查看主从应用的日志点跑到哪里了）
mysql> show slave status \G;
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 172.16.10.6
Master_User: repl
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: server.000003
Read_Master_Log_Pos: 4112
Relay_Log_File: jiqi4-relay-bin.000003
Relay_Log_Pos: 4319
Relay_Master_Log_File: server.000003 //当前应用到主库的文件位置
现在已经读到server.000003这个日志的4112点了主从调整为并行复制
并行复制 要修改从库配置文件
并行参数：
slave_preserve_commit_order=1
log_slave_updates=1
slave_parallel_type=logical_clock
slave_parallel_workers=4 开启4个线程
master_info_repository=TABLE 调优的值 原来是=file
relay_log_info_repository=TABLE 调优的值 原来是=file
relay_log_recovery=ON
mysql> show processlist;
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| 1 | system user | | NULL | Connect | 154 | Waiting for master to send event | NULL |
| 2 | system user | | NULL | Connect | 154 | Slave has read all relay log; waiting for more updates | NULL |
| 3 | system user | | NULL | Connect | 154 | Waiting for an event from Coordinator | NULL |
| 4 | system user | | NULL | Connect | 154 | Waiting for an event from Coordinator | NULL |
| 5 | system user | | NULL | Connect | 154 | Waiting for an event from Coordinator | NULL |
| 6 | system user | | NULL | Connect | 154 | Waiting for an event from Coordinator | NULL |
| 9 | root | localhost | NULL | Query | 0 | starting | show processlist |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
7 rows in set (0.00 sec)
搭建成一主两从架构，并建立互信
先把主从并行复制的两行在配置文件中注释掉
#master_info_repository=TABLE
#relay_log_info_repository=TABLE
准备一台新的机器，与主从复制的两台机器mysql版本一致且环境相同(确保binlog版本相同)
配置新的机器与主库能够主从复制
在主库中show processlist中看到两个dump线程说明一注两从的环境已经搭建好了
三台机器
修改hosts文件
vim /etc/hosts把三台主机都给备注上，都要备注
127.0.0.1 localhost master localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
172.16.10.6 master
172.16.10.4 beizhu
172.16.10.7 jiankong建立ssh互信 一台一台机器的来
rsa密钥
建立ssh互信 一台一台机器的来
rsa密钥
#ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
e1:cd:28:b0:b4:c3:4b:4b:4c:f6:4c:39:18:79:50:c6 root@master
The key's randomart image is:
+--[ RSA 2048]----+
| o=o |
| .+E. |
| *.+ . |
| * B o = |
| O + S o |
| o + . |
| o |
| |
| |
+-----------------+
将公钥追加到认证文件中
cat /root/.ssh/id_rsa.pub >> authorized_keys
修改权限
chmod 700 /root/.ssh
chmod 600 /root/.ssh/authorized_keys
将公钥追传到另外两个机器上
#ssh-copy-id -i /root/.ssh/id_rsa.pub root@beizhu
Warning: Permanently added 'beizhu,172.16.10.4' (RSA) to the list of known hosts.
root@beizhu's password:
Now try logging into the machine, with "ssh 'root@beizhu'", and check in:
.ssh/authorized_keys
to make sure we haven't added extra keys that you weren't expecting.
#ssh-copy-id -i /root/.ssh/id_rsa.pub root@jiankong
Warning: Permanently added 'jiankong,172.16.10.7' (RSA) to the list of known hosts.
root@jiankong's password:
Now try logging into the machine, with "ssh 'root@jiankong'", and check in:
.ssh/authorized_keys
to make sure we haven't added extra keys that you weren't expecting.
测试现在是不是免密了
[root@master ~]# ssh root@beizhu
Warning: Permanently added 'beizhu,172.16.10.4' (RSA) to the list of known hosts.
Last login: Mon Jul 16 14:00:18 2018 from 172.16.65.123
[root@beizhu ~]# exit
logout
Connection to beizhu closed.
[root@master ~]# ssh root@jiankong
Warning: Permanently added 'jiankong,172.16.10.7' (RSA) to the list of known hosts.
Last login: Mon Jul 16 13:34:30 2018 from 172.16.65.123
[root@jiankong ~]#
没有提示输入密码 就可以直接登陆这样master主机的免密登录另外两台登录做完了，然后做另外两台的
.
.
.然后都完成了
在主库上建一个 监控用户
在master上创建监控用户，这样2个从上自动有该用户
mysql> grant all on *.* to 'mha_monitor'@'%' identified by 'mha_monitor';
Query OK, 0 rows affected, 1 warning (0.00 sec)
在另外两台机器上都能以这个用户登录
[root@beizhu mysql]# mysql -umha_monitor -pmha_monitor
mysql: [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 10
Server version: 5.7.21-log MySQL Community Server (GPL)
Copyright (c) 2000, 2018, 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.
配置MHA
在mha-manager即监控主机上执行：
1、创建mha的工作目录
mkdir -p /etc/masterha在~家目录下建个目录把软件包考进去
mha4mysql-manager-0.55-0.el6.noarch.rpm perl-Mail-Sender-0.8.16-3.el6.noarch.rpm perl-MIME-Types-1.28-2.el6.noarch.rpm
mha4mysql-node-0.54-0.el6.noarch.rpm perl-Mail-Sendmail-0.79-12.el6.noarch.rpm perl-Parallel-ForkManager-0.7.9-1.el6.noarch.rpm
perl-Config-Tiny-2.12-7.1.el6.noarch.rpm perl-MailTools-2.04-4.el6.noarch.rpm perl-Params-Validate-0.92-3.el6.x86_64.rpm
perl-Email-Date-Format-1.002-5.el6.noarch.rpm perl-MIME-Lite-3.027-2.el6.noarch.rpm perl-TimeDate-1.16-13.el6.noarch.rpm
perl-Log-Dispatch-2.27-1.el6.noarch.rpm perl-MIME-Lite-HTML-1.23-2.el6.noarch.rpm
yum 安装这些文件
yum localinstall ./*.rpm改一下配置文件，这是自己编写的一个配置文件
vim /etc/masterha/app1.cnf[root@jiankong ~]# cat /etc/masterha/app1.cnf
[server default]
manager_workdir=/var/log/masterha/app1
manager_log=/var/log/masterha/app1/manager.log
master_binlog_dir=/var/lib/mysql #主库上binlog的位置
master_ip_failover_script=/usr/bin/master_ip_failover
master_ip_online_change_script=/usr/bin/master_ip_online_change
user=mha_monitor
password=mha_monitor
ping_interval=1
remote_workdir=/tmp
repl_user=repl
repl_password=repl
ssh_user=root
[server1]
hostname=172.16.10.6
candidate_master=1
port=3306
[server2]
hostname=172.16.10.4
candidate_master=1
check_repl_delay=0
port=3306
[server3]
hostname=172.16.10.7
port=3306在2个从节点（备主跟监控）设置relay_log_purge=0，不自动清除relay_log，
mysql> set global relay_log_purge=0;
Query OK, 0 rows affected (0.00 sec)主库关闭,从库开启
并设置到配置文件中,最好是这三台机器主从都给配置上
可以利用自带的purge_relay_log脚本实现定时任务自动清理relay log，此处不赘述
将mha的故障切换脚本和在线切换脚本放在/usr/bin下去：
master_ip_failover
master_ip_online_change
mha的故障切换脚本和在线切换脚本
给这两个脚本可执行权限
[root@jiankong bin]# chmod +x master_ip_failover
[root@jiankong bin]# chmod +x master_ip_online_change 检查ssh配置：
masterha_check_ssh --conf=/etc/masterha/app1.cnf最后显示
Mon Jul 16 19:45:18 2018 - [info] All SSH connection tests passed successfully.在主库上配置vip
vip的作用是当有应用连接数据库的时候，连接的是一个虚拟ip 作用是 当后端有故障切换的时候，对于应用来说切换是透明的，不需要修改任何东西 （啥意思 ，，？？）
在主库上配置vip：ifconfig eth0:1 192.168.120.120/24[root@master bin]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 00:0c:29:3b:d7:0f brd ff:ff:ff:ff:ff:ff
inet 172.16.10.6/16 brd 172.16.255.255 scope global eth1
inet6 fe80::20c:29ff:fe3b:d70f/64 scope link
valid_lft forever preferred_lft forever[root@master bin]# ifconfig eth1:1 172.16.10.9/16
[root@master bin]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 00:0c:29:3b:d7:0f brd ff:ff:ff:ff:ff:ff
inet 172.16.10.6/16 brd 172.16.255.255 scope global eth1
inet 172.16.10.9/16 brd 172.16.255.255 scope global secondary eth1:1
inet6 fe80::20c:29ff:fe3b:d70f/64 scope link
valid_lft forever preferred_lft forever
(如何删除虚拟ip：ip addr del 192.168.120.120/24 dev eth0)[root@jiankong ~]# ifconfig eth1:1 172.16.10.8/16
在manager上编辑2个切换脚本内容，修改vip地址和vip设备名
检查read_only参数，主上一定是off，从上一定时on（该参数可动态全局修改）[root@jiankong bin]# vim master_ip_failover
my $vip = '172.16.10.9/16'; # Virtual IP 主库上设置的虚拟ip
my $key = "1";
my $ssh_start_vip = "/sbin/ifconfig eth0:$key $vip"; beizhu的网口
my $ssh_stop_vip = "/sbin/ifconfig eth1:$key down"; 主库的网口
my $exit_code = 0;
[root@jiankong bin]# vim master_ip_online_change
vip=`echo '172.16.10.9/16'` # Virtual IP
key=`echo '1'`
stop_vip=`echo "ssh root@$orig_master_host /sbin/ifconfig eth1:$key down"`
start_vip=`echo "ssh root@$new_master_host /sbin/ifconfig eth0:$key $vip"`检查复制环境：
masterha_check_repl --conf=/etc/masterha/app1.cnf
MySQL Replication Health is OK.
检查manager节点的状态：
masterha_check_status --conf=/etc/masterha/app1.cnf
开启manager监控：
nohup masterha_manager --conf=/etc/masterha/app1.cnf --remove_dead_master --ignore_last_failover < /dev/null > /var/log/masterha/app1/manager.log 2>&1 &
[root@jiankong bin]# nohup masterha_manager --conf=/etc/masterha/app1.cnf --remove_dead_master --ignore_last_failover < /dev/null > /var/log/masterha/app1/manager.log 2>&1 &
[1] 4704
[root@jiankong bin]# masterha_check_status --conf=/etc/masterha/app1.cnf
app1 (pid:4704) is running(0:PING_OK), master:172.16.10.6
故障切换测试
直接在新的主库上进行failover，杀掉mysqld进程
这时候会自动完成故障切换，vip的漂移，但是manager进程会自动关闭 就是原来的 主库又重新变成了主库
mysql> show processlist;
+----+------+----------------+------+-------------+------+---------------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+----------------+------+-------------+------+---------------------------------------------------------------+------------------+
| 31 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 53 | repl | jiankong:43568 | NULL | Binlog Dump | 6 | Master has sent all binlog to slave; waiting for more updates | NULL |
+----+------+----------------+------+-------------+------+---------------------------------------------------------------+------------------+
2 rows in set (0.00 sec)
master主机又变成了主库
beizhu主机从mha环境中剥离出来
通过修复重新加入
旧主上默认不会自动指向新主，需要进行恢复操作：
通常情况下，旧主在修复完整后，可能想把旧主作为新主的slave，这时可以借助当时自动切换时刻的MHA日志来完成对旧主的修复。
手工在线测试
检查master_ip_online_change脚本中所有虚拟ip是否修改完成。1、MHA里有2个角色，一个是manager节点，一个是node节点，要实现这个MHA，最好是不少于3台机器，一主2从，1台是主，一台当备用master，另一台从机当监控机，一旦主库宕机，备主开始充当主库提供服务，如果旧主上线也不会在成为master了，除非修复完整后强制做主库。2、一旦发生切换，manager进程将会退出，无法进行再次测试，需要将故障数据库加入到MHA环境中。
在线切换步骤：
1、手工停掉mha监控
masterha_stop --conf=/etc/masterha/app1.cnf
2、执行在线切换
masterha_master_switch --conf=/etc/masterha/app1.cnf --master_state=alive --new_master_host=172.16.10.4 --new_master_port=3306 --orig_master_is_new_slave --running_updates_limit=10000 改成备主的ip 让备主 切换成主
其中参数的意思：
–orig_master_is_new_slave 切换时加上此参数是将原 master 变为 slave 节点，如果不加此参数，原来的 master 将不启动。
–running_updates_limit=10000,故障切换时,候选 master 如果有延迟的话，mha 切换不能成功，加上此参数表示延迟在此时间范围内都可切换（单位为 s），但是切换的时间长短是由 recover 时 relay 日志的大小决定。
在备主和监控机上查看是否完成切换：show processlist;
yes，yes
From:
172.16.10.6 (current master)
+--172.16.10.4
+--172.16.10.7
To:
172.16.10.4 (new master)
+--172.16.10.7
+--172.16.10.6
Tue Jul 17 09:08:01 2018 - [info] Switching master to 172.16.10.4(172.16.10.4:3306) completed successfully.这个时候看一下新的主库beizhu的状态
mysql> show processlist;
+----+------+----------------+------+-------------+------+---------------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+----------------+------+-------------+------+---------------------------------------------------------------+------------------+
| 2 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 10 | repl | jiankong:52817 | NULL | Binlog Dump | 44 | Master has sent all binlog to slave; waiting for more updates | NULL |
| 11 | repl | master:37647 | NULL | Binlog Dump | 44 | Master has sent all binlog to slave; waiting for more updates | NULL |
+----+------+----------------+------+-------------+------+---------------------------------------------------------------+------------------+
3 rows in set (0.00 sec)跟新的从库master的状态
mysql> show processlist;
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| 31 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 41 | system user | | NULL | Connect | 138 | Waiting for master to send event | NULL |
| 42 | system user | | NULL | Connect | 138 | Slave has read all relay log; waiting for more updates | NULL |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
3 rows in set (0.00 sec)这样主库上虚拟ip没了 虚拟ip到了beizhu上
在jiankong上启动监控
nohup masterha_manager --conf=/etc/masterha/app1.cnf --remove_dead_master --ignore_last_failover < /dev/null > /var/log/masterha/app1/manager.log 2>&1 &
masterha_check_status --conf=/etc/masterha/app1.cnf修复
在manager上查看当时的change master 信息：
cat /var/log/masterha/app1/manager.log |grep -i “All other slaves should start”
Tue Jul 17 11:22:08 2018 - [info] All other slaves should start replication from here. Statement should be: CHANGE MASTER TO MASTER_HOST=’172.16.10.6’, MASTER_PORT=3306, MASTER_LOG_FILE=’server.000001’, MASTER_LOG_POS=154, MASTER_USER=’repl’, MASTER_PASSWORD=’xxx’;
可利用这个信息做beizhu（原主）的修复
在jiankong上检查监控状态（默认故障切换后关闭）
检查复制看到只有一个从
在beizhu上
mysql> CHANGE MASTER TO MASTER_HOST='172.16.10.6', MASTER_PORT=3306, MASTER_LOG_FILE='server.000001', MASTER_LOG_POS=154, MASTER_USER='repl', MASTER_PASSWORD='repl';
Query OK, 0 rows affected, 2 warnings (0.03 sec)
mysql> start slave;
Query OK, 0 rows affected (0.00 sec)mysql> show processlist;
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| 2 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 3 | system user | | NULL | Connect | 8 | Waiting for master to send event | NULL |
| 4 | system user | | NULL | Connect | 8 | Slave has read all relay log; waiting for more updates | NULL |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
3 rows in set (0.00 sec)再次检查复制看到还是只有一主一从
这是因为切换后故障的主机已经从配置文件中删除了
查看配置文件发现[server2]这一栏没有了，需要手动添加
或者是通过这个命令
masterha_conf_host –command=add –conf=/etc/masterha/app1.cnf –hostname=172.16.10.4 –block=server1 –params=”no_master=1;ignore_fail=1”
添加完成后检查复制环境，已经恢复到三台主机
MHA切换原理分析
MHA架构有MHA manager和MHA node组成，关系图如下：
MHA manager介绍：
实际上，一个manager可以用力多个master-slave集群，通过运行工具可以实现master 主机的故障切换、手动故障切换、连接检查等等功能。
MHA node
部署在所有运行MySQL的服务器上，无论是master还是slave。主要作用有三个。
1).保存二进制日志
如果能够访问故障master，会拷贝master的二进制日志
2).应用差异中继日志
从拥有最新数据的slave上生成差异中继日志，然后应用差异日志。
3).清除中继日志
在不停止SQL线程的情况下删除中继日志
切换过程及原理：
1). 从宕机崩溃的Master保存二进制日志事件(binlogevent)
2). 识别含有最新更新的Slave
3). 应用差异的中继日志(relaylog)到其他Slave
4). 应用从Master保存的二进制日志事件
5). 提升一个Slave为新的Master
6). 使其他的Slave连接新的Master进行复制