title: What does it mean for Redis to be an "in memory" database?
body: <p><a href="http://en.wikipedia.org/wiki/Redis" rel="noreferrer">Wikipedia says</a> that Redis is an in-memory database, but it also says that it can persist "data to the disk at least every 2 seconds".  I feel like these two things are mutually exclusive.  How can it be considered in-memory yet (it can) store data on disk?  I assumed the definition of in-memory meant that <strong>it does not</strong> store to disk.  </p>
  
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  <p>This is a similar question: <a href="https://stackoverflow.com/questions/25151866/redis-concept-in-memory-or-db">Redis concept: In memory or DB?</a>  The difference is that he's asking about the persistence implementation.  My question is about the concept of in-memory vs persistence.  </p>