BerkeleyDB may be a good solution for you, in conjunction with Storable for serialization. Assuming that your entities don't have to point to each other (you don't say whether there is advanced intra-object connection or not), this has several advantages:

• Can run using a remote RPC server if you wish
• Allows for arbitrarily large objects with diverse structure to be saved and retrieved
• Simple interface, without requiring RDBMS overhead, limitations, or complexity
• Good performance, locking, etc.