"Store in RAM whatever needs to have fast access" ,what do you mean by this we can't know what needs fast acces because every point of the db needs fast access ,or you mean the bloom filter
Yeah, we can know exactly what requires fast access. It's the one for step 1: the filter / binning memory. The least significant bits of a key (all the suffixes for example, or the actual values to return) do not require fast storage, since the slow phases won't be hit often. Storing entire keys in RAM is just wasting memory that will never be hit. The issue here is that when not enough RAM exists, you need to find the right balance for what to read from disk. Optimize such that everything that's in RAM is actually useful.
Honestly, I said more than enough on the subject already. The discussion isn't even on topic, to be frank.