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After setting up caching in Joomla 2.5, cache files will be saved to disk, and over time you’ll need to delete those cache files. When clearing cache in Joomla 2.5 you have two options:
Let’s say that you have the cache time set to 30 minutes, meaning if a page has a cache file younger than 30 minutes, that cache file will be used when displaying the page. Let’s also say you have 500 cache files saved to disk.
How to Clear Cache or Purge Expired Cache
If you choose to clear the cache, it will delete ALL cache files. This is a quick process because all cache files are deleted, Joomla 2.5 does not need to “think”, it simply deletes cache files.
Purge Expired Cache:
If you purge expired cache, Joomla 2.5 will only delete cache files older than the specified time, which in our example is 30 minutes. This process is more resource intensive on your Joomla server because Joomla has to “think” and figure out which cache files are too old before it deletes them.