magento Сайт не работает должным образом после включения APC

**Recommended APC configuration for Magento**

By default APC configuration is not adjusted for Magento. In some cases you will see fatal errors in server logs, e.g. "PHP Fatal error: Cannot redeclare class". To fix such errors you need to change default APC configuration in PHP.INI file or APC.INI (depending on your configuration).

Here are recommended APC settings for Magento Community and Enterprise:

apc.enabled=1
apc.shm_size=512M
apc.num_files_hint=10000
apc.user_entries_hint=10000
apc.max_file_size=5M
apc.stat=0
apc.optimization=0
apc.shm_segments=1
apc.enable_cli=1
apc.cache_by_default=1
apc.include_once_override=1

In case of using APC bytecode cache, the modification time check option (apc.stat) must be disabled.

**Activate APC support in Magento configuration**

In order to activate APC support in Magento core, you should modify /app/etc/local.xml configuration file.
Add these lines between GLOBAL tags:

<global>
...
<cache>
<backend>apc</backend>
<prefix>MYSTORE_</prefix>
</cache>
...
</global>
The "Prefix" value is a short descriptor that will allow you to use APC caching for several Magento stores on the same server, it should be unique for each Magento store you have.

If you want to enabled 2 level cache for your Magento, you can use slow_backend tag in local.xml.

Magento has two levels cache: a fast level and a slow level cache. Magento will use the fast level if possible, and if there is no memory available in APC, it will revert to the slow level cache. It will eliminate any down time connected with cache memory issues.

You can define "database" or "file" values for slow_backend:<global>
...
<cache>
<backend>apc</backend>
<slow_backend>database</slow_backend>
<prefix>MYSTORE_</prefix>
</cache>
...
</global>
Save local.xml and test Magento storefront. If you will notice any errors, make sure that local.xml was modified correctly.

Больше подробностей: http://turnkeye.com/blog/magento-performance-apc-cache/