Redis is an open source, Berkeley Software Distribution (BSD) licensed, advanced key-value store that can optionally be used in Magento for back end and session storage. In fact, you can replace memcached with Redis.
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What happens if Redis runs out of memory? ... If this limit is reached Redis will start to reply with an error to write commands (but will continue to accept read-only commands), or you can configure it to evict keys when the max memory limit is reached in the case where you are using Redis for caching.
To improve the Redis example for your necessities, you get best so it is adequately huge to store your whole Magento reserve.