how to archive old messages

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Hi, I'm going to install YABB with mySQL but I have a problem:  
- I have a limited space on my host  
- I want to keep old messages  
 
When I run a Snitzforum, I had an ASP script that: pruned topics older than a certain date, put them in another dbase, transformed them in a text file, deleted the temporary dbase and compacted the original one.  
 
In this way I could keep the dbase small and the messages in smaller text file.  
 
Is there something like this in phpBB? Am I having the right approach to this? (I know that with Unix-mySQL things may be different)  
 
Thank you for your help,  
Ciao, N.