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Memcached should be used only for caching small and static data, such as HTML code fragments. Reason is Memcached will consume comparatively less memory resources for metadata. Strings, which are the only data type that are supported by Memcached, are ideal for storing data that's only being read because strings require no further processing.
Redis in all other cases. Reason is Redis' ability to fine-tune cache contents and durability -- and greater efficiency overall.
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