alt Hacker NewsEverything "wants to" run solely in RAM, but we don't have infinite RAM, so a "production grade" database should also be able to fetch data from disk unless this is an explicit tradeoff. MariaDB and PostgreSQL do not require all indices to be stored in RAM. Obviously they can be accessed more quickly if they are in RAM but they are designed under the assumption they will often be stored on disk. It sounds like MongoDB is not, and given the reputation of MongoDB, this is as likely to be incompetence as it is to be a willing tradeoff.
Every serious database that is designed to handle moderate to high traffic, will expect you to have RAM to fit all data and indices. Relational DBs do a solid job if that's not the case, but that also sabotages the efficiency you could get from them. It will work for some time. If it's enough for your, that's fine.
I am not experienced with MongoDB, I don't know if previous comment reports were the users fault or MongoDB's. But one thing is clear to me, complaining it uses too much RAM and not knowing the reasons for it, is a user problem. A common mistake is to setup a DB and expect it just magically does works. DBs are complicated beasts, you have to know how to deal with them.