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 -pTCP port number to listen on (default: 11211) -U UDP port number to listen on (default: 11211, 0 is off) -s UNIX socket path to listen on (disables network support) -a access mask for UNIX socket, in octal (default: 0700) -l interface to listen on (default: INADDR_ANY, all addresses) -d run as a daemon -r maximize core file limit -u assume identity of (only when run as root) -m max memory to use for items in megabytes (default: 64 MB) -M return error on memory exhausted (rather than removing items) -c 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 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 save PID in , only used with -d option -f chunk size growth factor (default: 1.25) -n minimum space allocated for key+value