I can't work out if the bit labelled "hash buckets" is in RAM, while data volume is SSD or if it is all SSD. I am guessing this is the case so it is quite similar to eg Twitter's fatcache.
It's actually all SSD by default, but you can pin the buckets into memory if you'd like. We generally do this in the memcached-replacement use case and specifically for CDN. However, we do have another use case not emphasized in the article that has a mix largely consisting of smaller values which we store in the bucket cache as direct records on SSD.
In general, if it fits in memory, it's faster to keep it there. If you don't have space for whatever reason, you leave it on the SSD.
