You can monitor the reference-system to find out more about the runtime behavior of your application. The reference-system ensures that each object has only one in memory representation.
First you need to add the monitoring support to the db4o configuration.
configuration.Common.Add(new ReferenceSystemMonitoringSupport());
configuration.Common.Add(New ReferenceSystemMonitoringSupport())
number of object references: Tells you how many references are currently hold in the reference-system. By default db4o uses weak references to objects. If this count is very high, you might hold unnecessary references to persisted objects in your application.