Performance Testing

Hi, I have a very critical problem here. How many users the Informix DB engine can handle concurrently? In detail, how many read locks, write locks, and login locks can be handled in the same time by Informix DB engine? In what condition, the DB engine probably die. Had anyone here done the pressure testing before for the Informix DB engine? Because I runs an information system based on the Informix DB now, I am afraid that the performance will go poor or down while the number of users is up and the access frequencies is increased. If anyone here have any experience on the problem, please give me a hand. Thanks a lot! Elisa