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Hello,
for our project we need to cluster Magento and we had to use memcached for sessions and magento cache. I tried to overload this Model (responsible for cache backend), but I was not be able to overload or it is currently not possible by Magento to do this). So I decided to patch Magento. 
WARNING: This is the short way (patch Magento); not recommended if you want to use future upgrades!!!
Here is my step by step tutorial:
1. Install Memcached Server
2. Install PHP PECL memcache Extension like this:
# pecl install memcache
3. Go to file app/code/core/Mage/Model/App.php and copy this file to “local” folder structure
[EDIT-start]
from:
app/code/core/Mage/Model/App.php
to:
from:
app/code/local/Mage/Model/App.php
[EDIT-stop]
Replace method ”getCache()” with my patched one (shown below).
/**
* Retrieve cache object
*
* @return Zend_Cache_Core
*/
public function getCache()
{
if (!$this->_cache)
{
$backend = strtolower((string)Mage::getConfig()->getNode('global/cache/backend'));
// #################################################################
// Memcached Backend Cache PATCH
//
// Author : Bjoern Ellebrecht
// Date : 2009-09-27 (15:00)
// Notice : This mehtod should be able to overload and NOT patched!
//
// PATCH - START
if (extension_loaded('memcache') && $backend == 'memcached')
{
// Classname (end of Filename)
$backend = 'Memcached';
foreach( (array)Mage::getConfig()->getNode('global/cache/servers') as $memcached_server )
{
$memcached_servers[] = (array)$memcached_server;
}
$backendAttributes = array
(
'servers' => $memcached_servers,
'compression' => false,
'cache_dir' => null,
'hashed_directory_level' => null,
'hashed_directory_umask' => null,
'file_name_prefix' => (string)Mage::getConfig()->getNode('global/cache/prefix')
);
}
// PATCH - END
// #################################################################
else if (extension_loaded('apc') && ini_get('apc.enabled') && $backend=='apc') {
$backend = 'Apc';
$backendAttributes = array(
'cache_prefix' => (string)Mage::getConfig()->getNode('global/cache/prefix')
);
} else {
$backend = 'File';
$backendAttributes = array(
'cache_dir'=>Mage::getBaseDir('cache'),
'hashed_directory_level'=>1,
'hashed_directory_umask'=>0777,
'file_name_prefix'=>'mage',
);
}
$this->_cache = Zend_Cache::factory('Core', $backend,
array(
'caching'=>true,
'lifetime'=>7200,
'automatic_cleaning_factor'=>0,
),
$backendAttributes
);
}
return $this->_cache;
}
4. So if you have patched this method, you need to set the memcached server parameter in your app/etc/local.xml.
<config>
<global>
<!-- ...here comes your database and session save stuff -->
<cache>
<backend>memcached</backend>
<servers>
<server1>
<host>127.0.0.1</host>
<port>11211</port>
<persistent>0</persistent>
</server1>
<!-- here you can add as much servers as you like; -->
<!-- just increment number (X) in <serverX> and </serverX> -->
<!--
<server2>
<host>127.0.0.1</host>
<port>11211</port>
<persistent>0</persistent>
</server2>
-->
</servers>
</cache>
</global>
<!-- ...here comes your admin routes stuff -->
</config>
5. Bevore you can use memcache, you have to do:
- stop Apache Server
- clear magento cache path [ var/cache/ ] (if you are in the magento root directory and if you have rights to do this) rm -rf var/cache/*
- start Memcached (see homepage for configuration parameter)
# memcached -d -u {as_linux_user} -m 128 127.0.0.1 -p 11211
- start Apacher Server
So, now you (Magento-System) should be able to store all Magento cache inside Memcached-Server and boost up your system. 
Greetings
Björn
P.S.: I have attached 2 files, the patched method and the additional infomation in local.xml.
File Attachments
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