What it means by cache


I have see this types of caches config in local.xml.additional file. I want to is know

what is meant by this type of caches and

why this is used and which section is affected by it on Magento

and I need details of explanation


maybe this helps:

The Two Level cache mechanisms

The « two level cache » system allows to get a fast cache backend and a slow cache backend. The main problem here is that only file and database structures allow the slow backend to be really efficient because a “home made” structuring of these content can be done.

With APC or Memcached, we can get a two way associative structure but still very simple. Magento handles large data collections, with type, category, groups, etc… A simple structure can’t really fulfill the need of Magento’s cache. In order to be able to work properly, Magento need a slow backend cache that could be structured properly and have extended value/capabilities. In the fast_backend, we then only have a key (element id) and the content of this key (the element that is cached). It’s a bit like if you have a mailing system that store a mail content with an ID and another DB that help structuring it with folders and attributes. The “raw” data is stored in the fast_backend, the intelligent mapping of these data is stored in the slow_backend.

This way, Magento can selectively delete or update a key or a category without having to flush all the cache every time an update is made.

If you impose a “non configurable” slow cache backend to Magento, it won’t be able to selectively clean the part of the cache that are impacted by a manipulation, it then renders the whole caching mechanism quite useless.

via https://www.nbs-system.co.uk/blog-2/magento-optimization-howto-en.html

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