Question

Is it possible to persist only certain keys to disk using Redis? Is the best solution for this as of right now to run separate Redis servers where one server can have throw away caches and the other one has more important data that we need to flush to disk periodically (such as counters to visits on a web page)

Answer 1

You can set expirations on a subset of your keys. They will be persisted to disk, but only until they expire. This may be sufficient for your use case.

You can then use the redis maxmemory and maxmemory-policy configuration options to cap memory usage and tell redis what to do when it hits the max memory. If you use the volatile-lru or volatile-ttl options Redis will discard only those keys that have an expiration when it runs out of memory, throwing out either the Least Recently Used or the one with the nearest expiration (Time To Live), respectively.

However, as stated, these values are still put to disk until expiration. If you really need to avoid this then your assumption is correct and another server looks to be the only option.