Caching in Magento Enterprise Edition

Once the cache is enabled and one of the above-mentioned shared memory backends is used, Magento automatically uses a TwoLevel cache backend (see http://ping.fm/cb1yD for more information). In which case, the specified cache backend is used as a fast cache. The file system cache backend is used as a slow cache for single web node environments, and the MySQL database is used as a slow cache for multiple web node environments.

This is fully customizable and can be easily set in app/etc/local.xml, as shown in the Appendix F. When using APC, the Source Code Compiler must be disabled.
Make sure that if you are using APC or memcached as backends, you configure each of them with enough memory to include all cached data; otherwise, a backend may purge the required cache hierarchy structure.