本文将介绍Memcached的安装与使用

What is Memcached?
Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.
Memcached is an in-memory key-value store for small chunks of arbitrary data (strings, objects) from results of database calls, API calls, or page rendering.
Memcached is simple yet powerful. Its simple design promotes quick deployment, ease of development, and solves many problems facing large data caches. Its API is available for most popular languages.

Why Use Memcached?
Benefits of using memcached include:
? Because all information is stored in RAM, the access speed is faster than loading the information
each time from disk.
? Because the “value” portion of the key-value pair does not have any data type restrictions, you can
cache data such as complex structures, documents, images, or a mixture of such things.
? If you use the in-memory cache to hold transient information, or as a read-only cache for information
also stored in a database, the failure of any memcached server is not critical. For persistent data, you
can fall back to an alternative lookup method using database queries, and reload the data into RAM
on a different server.
The typical usage environment is to modify your application so that information is read from the cache
provided by memcached. If the information is not in memcached, then the data is loaded from the
MySQL database and written into the cache so that future requests for the same object benefit from the

cached data.

Case:
Fotolog, as they themselves point out, is probably the largest site nobody has ever heard of, pulling in more page views than even Flickr. Fotolog has 51 instances of memcached on 21 servers with 175G in use and 254G available. As a large successful photo-blogging site they have very demanding performance and scaling requirements. To meet those requirements they've developed a sophisticated approach to using memcached that others can learn from and emulate.

Memcached的下载：
http://www.memcached.org/files/memcached-1.4.21.tar.gz

在RedHat中，系统自带Memcached，可以使用yum进行安装：
yum install memcached

也可以下载包进行安装，这里就详细说明了。

基本使用：

[root@ogg1 bin]# memcached -h
memcached 1.4.4
-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: INADDR_ANY, all addresses)
-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: 64 MB)
-M            return error on memory exhausted (rather than removing items)
-c <num>      max simultaneous connections (default: 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)
-vvv          extremely verbose (also print internal state transitions)
-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)
-L            Try to use large memory pages (if available). Increasing
              the memory page size could reduce the number of TLB misses
              and improve the performance. In order to get large pages
              from the OS, memcached will allocate the total item-cache
              in one large chunk.
-D <char>     Use <char> as the delimiter between key prefixes and IDs.
              This is used for per-prefix stats reporting. The default is
              ":" (colon). If this option is specified, stats collection
              is turned on automatically; if not, then it may be turned on
              by sending the "stats detail on" command to the server.
-t <num>      number of threads to use (default: 4)
-R            Maximum number of requests per event, limits the number of
              requests process for a given connection to prevent
              starvation (default: 20)
-C            Disable use of CAS
-b            Set the backlog queue limit (default: 1024)
-B            Binding protocol - one of ascii, binary, or auto (default)
-I            Override the size of each slab page. Adjusts max item size
              (default: 1mb, min: 1k, max: 128m)

初始化Memcached：

memcached -u root -d -m 512 -p 11211 -l 192.168.56.12

[root@ogg1 bin]# ps -ef |grep mem
root      4382     1  0 02:01 ?        00:00:00 memcached -u root -d -m 512 -p 11211 -l 192.168.56.12

查看当前Memcached的状态：

[root@ogg1 bin]# telnet 192.168.56.12 11211  
Trying 192.168.56.12...
Connected to 192.168.56.12.
Escape character is '^]'.
stats
STAT pid 4382
STAT uptime 7288
STAT time 1418893354
STAT version 1.4.4
STAT pointer_size 64
STAT rusage_user 0.353946
STAT rusage_system 0.379942
STAT curr_connections 5
STAT total_connections 8
STAT connection_structures 6
STAT cmd_get 0
STAT cmd_set 0
STAT cmd_flush 0
STAT get_hits 0
STAT get_misses 0
STAT delete_misses 0
STAT delete_hits 0
STAT incr_misses 0
STAT incr_hits 0
STAT decr_misses 0
STAT decr_hits 0
STAT cas_misses 0
STAT cas_hits 0
STAT cas_badval 0
STAT auth_cmds 0
STAT auth_errors 0
STAT bytes_read 144
STAT bytes_written 1732
STAT limit_maxbytes 536870912
STAT accepting_conns 1
STAT listen_disabled_num 0
STAT threads 4
STAT conn_yields 0
STAT bytes 0
STAT curr_items 0
STAT total_items 0
STAT evictions 0
END

对应参数解释：
pid  memcache服务器的进程ID
uptime  服务器已经运行的秒数
time  服务器当前的unix时间戳
version  memcache版本
pointer_size  当前操作系统的指针大小（32位系统一般是32bit）
rusage_user  进程的累计用户时间
rusage_system  进程的累计系统时间
curr_items  服务器当前存储的items数量
total_items  从服务器启动以后存储的items总数量
bytes  当前服务器存储items占用的字节数
curr_connections  当前打开着的连接数
total_connections  从服务器启动以后曾经打开过的连接数
connection_structures  服务器分配的连接构造数
cmd_get  get命令（获取）总请求次数
cmd_set  set命令（保存）总请求次数
get_hits  总命中次数
get_misses  总未命中次数
evictions  为获取空闲内存而删除的items数（分配给memcache的空间用满后需要删除旧的items来得到空间分配给新的items）
bytes_read  总读取字节数（请求字节数）
bytes_written  总发送字节数（结果字节数）
limit_maxbytes  分配给memcache的内存大小（字节）
threads  当前线程数