Hey I am also working on a none-commercial MMOG project and my teamates and I are responsible for the data representation and storage part of it. I searched your site for documentations on your implementation in that field and have not found much I want. So if you guys could provide any information or just throw links at me it would be much appreciated =D
The persistence system in Cyphesis uses an object-relational model using PostgreSQL. The performance is dreadful, and the entire subsystem is not good enough to work. It is disabled by default, and probably no longer works.
Account and rules data is still stored in simple SQL tables, and this seems to work just fine.