The point is Redis is beginning to move away from being just an in-memory database. See my other comment for a more detailed answer as to the use case.

Persistent caches are an important commodity for many websites. For some services trying to handle standard traffic patterns with an empty cache is suicide and can cause cascading failures across the board. Without a strong persistence solution Redis can't be used here. antirez mentioned he considered it a strange move for Reddit to use to Cassandra and not Redis as a persistent cache[1] and I think the issues with Redis persistence may well have caused that.

Additionally I still think there's reasonable ground for a database that's primarily in-memory but drops least used data off to disk. The vast majority of the web follows a Zipfian/long tail distribution so although your "working dataset" can be far larger than RAM your actual "active working dataset" can fit in there. Why trade away the advantages of an in-memory data structure driven database when almost all your queries can be satisfied in this manner?

[1] http://www.reddit.com/r/programming/comments/bcqhi/reddits_n...