介绍：
memcached本身没有内置分布式功能，无法实现使用多台Memcache服务器来存储不同的数据，最大程度的使用相同的资源；无法同步数据，容易造成单点故障。
在 Memcached中可以保存的item数据量是没有限制的，只要内存足够 。
Memcached单进程最大使用内存为2G，要使用更多内存，可以分多个端口开启多个Memcached进程
最大30天的数据过期时间,设置为永久的也会在这个时间过期，常量REALTIME_MAXDELTA
60*60*24*30控制
最大键长为250字节，大于该长度无法存储，常量KEY_MAX_LENGTH 250控制
单个item最大数据是1MB，超过1MB数据不予存储，常量POWER_BLOCK 1048576进行控制，
它是默认的slab大小
最大同时连接数是200，通过 conn_init()中的freetotal进行控制，最大软连接数是1024，通过
settings.maxconns=1024 进行控制
跟空间占用相关的参数：settings.factor=1.25, settings.chunk_size=48, 影响slab的数据占用和步进方式
memcached是一种无阻塞的socket通信方式服务，基于libevent库，由于无阻塞通信，对内存读写速度非常之快。
memcached分服务器端和客户端，可以配置多个服务器端和客户端，应用于分布式的服务非常广泛。
memcached作为小规模的数据分布式平台是十分有效果的。
memcached是键值一一对应，key默认最大不能超过128个字 节，value默认大小是1M，也就是一个slabs，如果要存2M的值（连续的），不能用两个slabs，因为两个slabs不是连 续的，无法在内存中 存储，故需要修改slabs的大小，多个key和value进行存储时，即使这个slabs没有利用完，那么也不会存放别的数据。
magent能够消除单点故障问题，但是不能实现负载均衡。（mcrouter流量分发）
1:cache安装脚本：
install_cache(){
cd $INSTALL_HOME
mkdir -p /opt
if [ -f libevent-1.4.14b-stable.tar.gz ] ; then
tar zxvf libevent-1.4.14b-stable.tar.gz >> $INSTALL_HOME/install.log
else
lftp -c "pget -n 10 $FTP_ADDR/softwares/memcache/libevent-1.4.14b-stable.tar.gz"
tar zxvf libevent-1.4.14b-stable.tar.gz >> $INSTALL_HOME/install.log
fi
cd libevent-1.4.14b-stable
./configure --prefix=/usr >> $INSTALL_HOME/install.log
make >> $INSTALL_HOME/install.log
make install >> $INSTALL_HOME/install.log
cd $INSTALL_HOME
rm -rf libevent-1.4.14b-stable*
echo 'install libevent succesed!'
cd $INSTALL_HOME
echo 'install the memcached server...'
cd $INSTALL_HOME
if [ -f memcached-1.4.5.tar.gz ] ; then
tar zxvf memcached-1.4.5.tar.gz >> $INSTALL_HOME/install.log
else
lftp -c "pget -n 10 $FTP_ADDR/softwares/memcache/memcached-1.4.5.tar.gz"
tar zxvf memcached-1.4.5.tar.gz >> $INSTALL_HOME/install.log
fi
cd memcached-1.4.5
./configure --prefix=/opt/memcached --with-libevent=/usr >> $INSTALL_HOME/install.log
make >> $INSTALL_HOME/install.log
make install >> $INSTALL_HOME/install.log
cd ..
rm -rf memcached-1.4.5*
ln -s /usr/lib/libevent-1.4.so.2 /usr/lib64/libevent-1.4.so.2
echo '========********install memcached successed!******* ======'
echo "/opt/memcached/bin/memcached -d -m 1024 -u root -p 11211" >>/etc/rc.local
}
2:安装magent代理
下载 magent-0.5.tar.gz
mkdir magent
tar -zxvf magent-0.5.tar.gz -C kmagent
[root@shell magent]# /sbin/ldconfig
[root@shell magent]# sed -i "s#LIBS = -levent#LIBS = -levent -lm#g" Makefile
[root@shell magent]# make
gcc -Wall -O2 -g -c -o magent.o magent.c
magent.c: 在函数‘writev_list’中:
magent.c:623: 错误：‘SSIZE_MAX’未声明(在此函数内第一次使用)
magent.c:623: 错误：(即使在一个函数内多次出现，每个未声明的标识符在其
magent.c:623: 错误：所在的函数内也只报告一次。)
make: *** [magent.o] 错误 1
参考：http://www.php-note.com/article/detail/820
在开头加入：vim magent.c
#ifndef SSIZE_MAX
#define SSIZE_MAX 32767
#endif
重新执行：
[root@shell magent]# make
gcc -Wall -O2 -g -c -o magent.o magent.c
gcc -Wall -O2 -g -c -o ketama.o ketama.c
gcc -Wall -O2 -g -o magent magent.o ketama.o -levent -lm
cp magent /usr/bin/magent
执行下magent试试
[root@shell ~]# magent
please provide -s "ip:port" argument
memcached agent v0.4 Build-Date: Nov 26 2015 09:58:35
Usage:
-h this message
-u uid
-g gid
-p port, default is 11211. (0 to disable tcp support)
-s ip:port, set memcached server ip and port
-b ip:port, set backup memcached server ip and port
-l ip, local bind ip address, default is 0.0.0.0
-n number, set max connections, default is 4096
-D don't go to background
-k use ketama key allocation algorithm
-f file, unix socket path to listen on. default is off
-i number, set max keep alive connections for one memcached server, default is 20
-v verbose
3：查看memcached缓存（服务器为：192.168.16.239，192.168.16.238，192.168.16.241）
[root@shell ~]# ps aux|grep cache
root 18751 0.0 0.0 335052 896 ? Ssl 10:05 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11211
root 18758 0.0 0.0 335052 892 ? Ssl 10:05 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11212
root 18765 0.0 0.0 335052 892 ? Ssl 10:05 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11213
[root@oracle10g src]# ps aux|grep cache
root 5411 0.0 0.0 335052 896 ? Ssl 10:15 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.238 -p 11211
root 5419 0.0 0.0 335052 896 ? Ssl 10:15 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.238 -p 11212
root 5426 0.0 0.0 335052 896 ? Ssl 10:15 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.238 -p 11213
1）：先才用单台服务器进行测试
magent -u root -n 51200 -l 192.168.16.239 -p 12000 -s 192.168.16.239:11211 -s 192.168.16.239:11212 -b 192.168.16.239:11213
[root@shell ~]# telnet 192.168.16.239 12000
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
set key1 16 0 4
zhao
STORED
set key2 16 0 5
liang
STORED
[root@shell opt]# telnet 192.168.16.239 11211
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
get key1
END
get key2
VALUE key2 16 5
liang
END
[root@shell opt]# telnet 192.168.16.239 11212
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
get key1
VALUE key1 16 4
zhao
END
get key2
END
[root@shell opt]# telnet 192.168.16.239 11213
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
get key1
VALUE key1 16 4
zhao
END
get key2
VALUE key2 16 5
liang
END
2)不用备份服务端，停掉一个server
[root@shell opt]# magent -u root -n 51200 -l 192.168.16.239 -p 12000 -s 192.168.16.239:11211 -s 192.168.16.239:11212
[root@shell opt]# /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11211
[root@shell opt]# /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11212
[root@shell ~]# telnet 192.168.16.239 12000
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
set key1 16 0 4
zhao
STORED
set key2 16 0 5
liang
STORED
set key3 16 0 5
liang
STORED
[root@shell opt]# telnet 192.168.16.239 11211
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
get key1
END
get key2
VALUE key2 16 5
liang
END
quit
Connection closed by foreign host.
[root@shell opt]# telnet 192.168.16.239 11212
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
get key1
VALUE key1 16 4
zhao
END
get key2
END
[root@shell ~]# ps aux|grep cache
root 30357 0.0 0.1 336076 1968 ? Ssl 14:54 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11211
root 30364 0.0 0.1 336076 1972 ? Ssl 14:54 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11212
[root@shell ~]# kill -9 30357
[root@shell ~]# ps aux|grep cache
root 30364 0.0 0.1 336076 1972 ? Ssl 14:54 0:00 /opt/memcached/bin/memcached -d -m 64 -u root -l 192.168.16.239 -p 11212
[root@shell opt]# telnet 192.168.16.239 11212
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
get key1
VALUE key1 16 4
zhao
END
get key2
END
get key1
VALUE key1 16 4
zhao
END
get key3
VALUE key3 16 5
liang
END
quit
Connection closed by foreign host.
[root@shell opt]# telnet 192.168.16.239 12000
Trying 192.168.16.239...
Connected to 192.168.16.239.
Escape character is '^]'.
get key3
VALUE key3 16 5
liang
END
脚本：
#!/bin/bash
for ((i=1; i<=100; i++))
do
printf "set key$i 0 0 5 12345 " | nc 192.168.16.239 11211
done
printf "get key$i " | nc 192.168.16.239 11211