You need to understand the clear distinction between these two products to understand how to use them.
- APC is both an OPCode Cache and Fast Backend
- Memcache is just a Fast Backend
Using APC as an OPCode Cache
Simply install the module on your server
pecl install apc
And enable it in your
echo "extension=apc.so" >> /usr/lib/local/php.ini (RedHat/Centos) echo "extension=apc.so" >> /etc/php5/conf.d/20apc.ini (Debian)
You then enable and fine-tune the runtime configuration to suit, eg.
Then restart PHP/Apache
/etc/init.d/httpd restart (RedHat/Centos) /etc/init.d/apache2 restart (Debian)
After that, there is nothing else to do. Confirm APC is enabled with a quick
phpinfo() - but otherwise, at this point, the OPCode cache portion of APC is active.
Nothing needs to be configured on Magento's side.
Using APC as a Fast Backend
You need to add the following to your
<global> ... <cache> <backend>apc</backend> <prefix>mystore_</prefix> </cache> ... </global>
Then flush your existing store caches. To verify it is working, load a page in the front-end and the
./var/cache directory should remain empty.
Using Memcache as a Fast Backend
You'll need to install Memcache as a PHP extension, and install the respective Memcache Daemon (Memcached) on your server.
pecl install memcache
And enable it in your php.ini
echo "extension=memcache.so" >> /usr/lib/local/php.ini (RedHat/Centos) echo "extension=memcache.so" >> /etc/php5/conf.d/20memcache.ini (Debian) /etc/init.d/httpd restart (RedHat/Centos) /etc/init.d/apache2 restart (Debian)
Then install Memcached on the server. For RH/Centos, adjust the URL to suit your release version and CPU architecture.
rpm -Uhv http://apt.sw.be/redhat/el6/en/x86_64/rpmforge/RPMS/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm yum --enablerepo=rpmforge install memcached apt-get install memcached (Debian)
Then modify Magento to use Memcache as a fast backend, change the socket path to a TCP/IP connection to suit.
<cache> <slow_backend>database</slow_backend> <fast_backend>memcached</fast_backend> <fast_backend_options> <servers> <server> <host>unix:///tmp/memcached.sock</host> <port>0</port> <persistent>0</persistent> </server> </servers> </fast_backend_options> <backend>memcached</backend> <memcached> <servers> <server> <host>unix:///tmp/memcached.sock</host> <port>0</port> <persistent>0</persistent> </server> </servers> </cache>
The caveats of Memcache and tagging - what is it storing
Memcache only supports a single level of key-value relationships, so it cannot store the Magento cache tags (that are used to flush cache data independently). As a result, you either need to specify a
slow_backend to maintain the cache content tag relationship, or don't define one at all.
If you define a
slow_backend, you run the risk of the cache tags growing so large that performance is negated; there is also the inherent problem that you cannot scale across multiple servers if each server is maintaining their own cache tags.
So when using Memcache, the better approach (with the caveat you cannot flush caches independently), is to not bother using the
In which case, we suggest removing
<slow_backend>database</slow_backend> and replacing it with:
<slow_backend>Memcached</slow_backend> <slow_backend_options> <servers> <server> <host>unix:///tmp/memcached.sock</host> <port>0</port> <persistent>0</persistent> </server> </servers> </slow_backend_options>
This will break/disable the 2nd level of caching (and prevent tag storage), but still allow the performance of Memcache.
Which to use
If its a single server deployment - there's no harm just using APC for everything.
If its a distributed set-up - then you'll need to use Memcache as the fast backend (so that all machines can access the common store).
What would we use?
Well, neither actually.
We use MageStack - which is an ultra high performance Magento Operating System that provides native operational code caching beyond that of APC (with a ~8% performance improvement), and utilises an advanced cache back-end that does support cache tagging.