一：redis 主从复制过程

当配置好slave 后，slave 与master 建立连接，然后发送sync 命令。无论是第一次连接还是重新连接，master 都会启动一个后台进程，将数据库快照保存到文件中，同时master 主进
程会开始收集新的写命令并缓存。后台进程完成写文件后，master 就发送文件给slave，slave将文件保存到硬盘上，再加载到内存中，接着master 就会把缓存的命令转发给slave，后续
master 将收到的写命令发送给slave。如果master 同时收到多个slave 发来的同步连接命令，master 只会启动一个进程来写数据库镜像，然后发送给所有的slave。

二：如何配置
配置slave 服务器很简单，只需要在slave 的配置文件中加入如下配置
slaveof 192.168.1.1 6379 #指定master 的ip 和端口
下面我们做一个实验来演示如何搭建一个主从环境:
# slaveof
slaveof localhost 6379
我们在一台机器上启动主库(端口6379)，从库(端口6378)
启动后主库控制台日志如下:
[root@localhost redis-2.2.12]# src/redis-server redis.conf
[7064] 09 Aug 20:13:12 * Server started, Redis version 2.2.12
[7064] 09 Aug 20:13:12 # WARNING overcommit_memory is set to 0! Background save may fail under
low memory condition. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and
then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.
[7064] 09 Aug 20:13:12 * The server is now ready to accept connections on port 6379
[7064] 09 Aug 20:13:13 - 0 clients connected (0 slaves), 539512 bytes in use
[7064] 09 Aug 20:13:18 - 0 clients connected (0 slaves), 539512 bytes in use
[7064] 09 Aug 20:13:20 - Accepted 127.0.0.1:37789
[7064] 09 Aug 20:13:20 * Slave ask for synchronization
[7064] 09 Aug 20:13:20 * Starting BGSAVE for SYNC
[7064] 09 Aug 20:13:20 * Background saving started by pid 7067
[7067] 09 Aug 20:13:20 * DB saved on disk
[7064] 09 Aug 20:13:20 * Background saving terminated with success
[7064] 09 Aug 20:13:20 * Synchronization with slave succeeded
[7064] 09 Aug 20:13:23 - 0 clients connected (1 slaves), 547380 bytes in use
启动后从库控制台日志如下:
[root@localhost redis-2.2.12]# src/redis-server redis.slave
[7066] 09 Aug 20:13:20 * Server started, Redis version 2.2.12
[7066] 09 Aug 20:13:20 # WARNING overcommit_memory is set to 0! Background save may fail under
low memory condition. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and
then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.
[7066] 09 Aug 20:13:20 * The server is now ready to accept connections on port 6378
[7066] 09 Aug 20:13:20 - 0 clients connected (0 slaves), 539548 bytes in use
[7066] 09 Aug 20:13:20 * Connecting to MASTER...

三：redis 主从复制特点:
(1)、master 可以拥有多个slave
(2)、多个slave 可以连接同一个master 外，还可以连接到其他slave
(3)、主从复制不会阻塞master，在同步数据时，master 可以继续处理client 请求
(4)、提高系统的伸缩性

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Redis 主从配置

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