Riak in-memory store per-key overhead

Question

As a follow-up to THIS question I asked earlier, are there any estimates of the per-key overhead in Riak when using the in-memory backend? Google hasn't helped much thus far, most searches leading me back to Bitcask estimates.

I'm basically trying to understand if the in-memory backend will be a better fit for storing session-ids per user, (each user being a separate key and the session-id being the ONLY value in this key) versus Bitcask. The per-key overhead is one of the variables I'm evaluating in this process.

Answer 1

The memory backend does require the whole object to be stored in memory, which apart from the data, bucket and key includes the metadata that Riak adds to an object, e.g. vector clock, even if no user metadata or indexes are defined. I do not have an exact estimate of how large this metadata is, but believe that a few hundred bytes as outlined in the links provided in your previous question as well as this post on the Riak mailing list is fairly accurate.