工作原理
repcached实现了memcached复制的功能，它是一个单master单slave的方案，但master/slave都是可读写的，而且可以相互同步，如果master坏掉slave侦测到连接断了，它会自动listen而成为master，这时坏掉的master只能启用为slave，它们之间互换角色，才能保持复制功能，换句话说，master没有抢占功能；而如果slave坏掉，master也会侦测到连接断，它就会重新listen等待新的slave加入。
应用场景
用memcached做session共享或其它服务时会存在memcached的单点故障，如果memcached宕机，那么整个系统用户无法登陆(session)。
基于这种情况，采用repcached做memcached的主从冗余。
Repcached下载地址
http://sourceforge.net/projects/repcached/files/repcached/
Repcached安装方式
Repcached有两种安装方式：
1.补丁版本安装
先安装相应版本的memcached，然后对应版本的Repcached补丁。
2.整合版本安装
直接安装整合版本的memcached
方式一：补丁版本安装
1. 安装Memcache，相关安装方法可以参见博文：
http://ultrasql.blog.51cto.com/9591438/1632179
2. 下载对应的repcached版本补丁安装文件：
假设安装的memcahced版本为1.2.8，下载针对该版本最新的补丁:
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wget http:
//softlayer-dal
.dl.sourceforge.net
/project/repcached/repcached/2
.2.1-1.2.8
/repcached-2
.2.1-1.2.8.patch.gz
gzip
-
cd
..
/repcached-2
.2.1-1.2.8.patch.gz | patch -p1
.
/configure
--
enable
-replication
make
&&
make
install
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方式二：整合版本安装
1. 安装libevent：
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cd
/tmp
wget http:
//downloads
.sourceforge.net
/levent/libevent-2
.0.22-stable.
tar
.gz
tar
zxvf libevent-2.0.22-stable.
tar
.gz
cd
libevent-2.0.22-stable
.
/configure
--prefix=
/usr/local/lib
make
&&
make
install
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2. 将libevent的库文件添加到动态库中：
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vi
/etc/ld
.so.conf
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在最后添加如下行：
/usr/local/lib //此处为要添加的libevent库目录
重新加载动态lib库
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ldconfig
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注意：如果无此步骤，在启动memcached时，会提示看不到libevent的库文件。
3. 测试libevent是否安装成功：
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ls
-al
/usr/lib
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grep
libevent-
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4. 创建启动帐号：
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groupadd memcached
useradd
-g memcached memcached
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5. 创建PID进程目录并修改所有者：
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mkdir
/var/run/memcached
chown
-R memcached.memcached
/var/run/memcached
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6. 安装整合memcached-repcached包：
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cd
/tmp
wget http:
//softlayer-dal
.dl.sourceforge.net
/project/repcached/repcached/2
.2.1-1.2.8
/memcached-1
.2.8-repcached-2.2.1.
tar
.gz
cp
memcached-1.2.8-repcached-2.2.1.
tar
.gz
/usr/local
cd
/usr/local
tar
zxvf memcached-1.2.8-repcached-2.2.1.
tar
.gz
mv
memcached-1.2.8-repcached-2.2.1 memcached
cd
memcached
.
/configure
--prefix=
/usr/local/memcached
--with-libevent=
/usr/local/lib
--
enable
-replication --
enable
-64bit
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注意：默认memcached单个进程只支持到2G内存，需要更大内存支持的话，需要打开64位支持，编译的时候加参数：
--enable-64bit
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make
&&
make
install
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提示编译出错：
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make all-recursive
make[1]: Entering directory `/usr/local/memcached'
Making all in doc
make[2]: Entering directory `/usr/local/memcached/doc'
make[2]: Nothing to be done for `all'.
make[2]: Leaving directory `/usr/local/memcached/doc'
make[2]: Entering directory `/usr/local/memcached'
gcc -DHAVE_CONFIG_H -I. -DNDEBUG -m64 -g -O2 -MT memcached-memcached.o -MD
MP -MF .deps/memcached-memcached.Tpo -c -o memcached-memcached.o `test -f
memcached.c' || echo './'`memcached.c
memcached.c: In function ‘add_iov’:
memcached.c:697: error: ‘IOV_MAX’ undeclared (first use in this function)
memcached.c:697: error: (Each undeclared identifier is reported only once
memcached.c:697: error: for each function it appears in.)
make[2]: *** [memcached-memcached.o] Error 1
make[2]: Leaving directory `/usr/local/memcached'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/usr/local/memcached'
make: *** [all] Error 2
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解决方案：
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vi
memcached.c
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将下面的几行
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/* FreeBSD 4.x doesn't have IOV_MAX exposed. */
#ifndef IOV_MAX
#if defined(__FreeBSD__) || defined(__APPLE__)
# define IOV_MAX 1024
#endif
#endif
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修改为
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/* FreeBSD 4.x doesn't have IOV_MAX exposed. */
#ifndef IOV_MAX
# define IOV_MAX 1024
#endif
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重新编译和安装：
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make
&&
make
install
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查看帮助：
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.
/bin/memcached
-h
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memcached 1.2.8
repcached 2.2.1
-p <num> TCP port number to listen on (default: 11211)
-U <num> UDP port number to listen on (default: 11211, 0 is off)
-s <file> unix socket path to listen on (disables network support)
-a <mask> access mask for unix socket, in octal (default 0700)
-l <ip_addr> interface to listen on, default is INDRR_ANY
-d run as a daemon
-r maximize core file limit
-u <username> assume identity of <username> (only when run as root)
-m <num> max memory to use for items in megabytes, default is 64 MB
-M return error on memory exhausted (rather than removing items)
-c <num> max simultaneous connections, default is 1024
-k lock down all paged memory. Note that there is a
limit on how much memory you may lock. Trying to
allocate more than that would fail, so be sure you
set the limit correctly for the user you started
the daemon with (not for -u <username> user;
under sh this is done with 'ulimit -S -l NUM_KB').
-v verbose (print errors/warnings while in event loop)
-vv very verbose (also print client commands/reponses)
-h print this help and exit
-i print memcached and libevent license
-P <file> save PID in <file>, only used with -d option
-f <factor> chunk size growth factor, default 1.25
-n <bytes> minimum space allocated for key+value+flags, default 48
-R Maximum number of requests per event
limits the number of requests process for a given con nection
to prevent starvation. default 20
-b Set the backlog queue limit (default 1024)
-x <ip_addr> hostname or IP address of peer repcached
-X <num:num> TCP port number for replication. <listen:connect> (default: 11212)
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修改memcached目录所有者：
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cd
..
chown
-R memcached.memcached memcached
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配置主从复制
参数说明：
-x 设置从哪个IP上进行同步。
-X 指定数据同步的端口。
Memcached默认服务端口是11211，默认同步监听端口是11212。
启动Master：
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/usr/local/memcached/bin/memcached
-d -m 100 -l 192.168.11.51 -p 11211 -u memcached -c 1024 -x 192.168.11.52 -X 11212 -P
/var/run/memcached/memcached-rep
.pid
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查看监听端口：
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netstat
-tupln |
grep
memcached
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tcp 0 0 192.168.11.51:11211 0.0.0.0:* LISTEN 18634/memcached
tcp 0 0 192.168.11.51:11212 0.0.0.0:* LISTEN 18634/memcached
udp 0 0 192.168.11.51:11211 0.0.0.0:* 18634/memcached
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启动Slave：
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/usr/local/memcached/bin/memcached
-d -m 100 -l 192.168.11.52 -p 11211 -u memcached -c 1024 -x 192.168.11.51 -X 11212 -P
/var/run/memcached/memcached-rep
.pid
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说明：-x 192.168.11.51用于同步的Master的IP地址。
查看监听端口：
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netstat
-tupln |
grep
memcached
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tcp 0 0 192.168.11.52:11211 0.0.0.0:* LISTEN 24262/memcached
udp 0 0 192.168.11.52:11211 0.0.0.0:* 24262/memcached
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验证数据同步
在Master创建数据：
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[root@test01 bin]
# telnet 192.168.11.51 11211
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Trying 192.168.11.51...
Connected to 192.168.11.51.
Escape character is '^]'.
get key1
END
set key1 0 0 2
aa
STORED
quit
Connection closed by foreign host.
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在Slave获取数据：
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[root@test02 bin]
# telnet 192.168.11.52 11211
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Trying 192.168.11.52...
Connected to 192.168.11.52.
Escape character is '^]'.
get key1
END
get key1
VALUE key1 0 2
aa
END
quit
Connection closed by foreign host.
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在Slave创建数据：
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[root@test02 bin]
# telnet 192.168.11.52 11211
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Trying 192.168.11.52...
Connected to 192.168.11.52.
Escape character is '^]'.
get key2
END
set key2 0 0 3
b
STORED
get key2
VALUE key2 0 3
b
END
quit
Connection closed by foreign host.
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在Master获取数据：
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[root@test01 bin]
# telnet 192.168.11.51 11211
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Trying 192.168.11.51...
Connected to 192.168.11.51.
Escape character is '^]'.
get key2
VALUE key2 0 3
b
END
quit
Connection closed by foreign host.
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Memcached高可用
启动Master和Slave时不要加-l参数指定监听地址，否则keepalived无法监听VIP的地址。
然后配置上keepalived就可以作为高可用了。
本文转自UltraSQL51CTO博客，原文链接： http://blog.51cto.com/ultrasql/1632929，如需转载请自行联系原作者
