If I may offer some comments...

At a previous job I implemented a message board type system using DB_File (although the rest of the site used Sybase). This was for a large site (several million users), but the number of messages per forum was expected to be small (i.e. several million forums, but each one quite small).

This worked really well, until the system was bought out by another company that wanted to migrate their message boards to our code base. They had 40000 boards, some of which had several thousand messages, and requirements that differed enough from the original code to cause some serious performance problems due to the need to look through the entire DB file to get the information that was needed.

Ouch.

So I ended up re-implementing this code to use a relational database back-end (which was actually quite easy for this particular task).

So - what I mean to say is that using a database engine is a good idea - it makes things easier to extend, and it allows you to make performance decisions at the database level.

I'd also like to add that you can get Sybase 11.0.3.3 completely free for linux. It's not the latest version of Sybase, but it is stable, and offers stored procs, etc.

Just my opinion, of course :-)

Michael