Buffer manager

Hello! We are trying to model the delays in a database. To do this we need answers on the following questions. 1. If you have a number of SQL questions, how many percent of the needed datablocks is it likely that you have to get from the diskstorage?? 2. Does the answer from question number 1 differ a lot if the ratio between buffer size and the total amount of data stored in the database is changed?? 3. How does a buffer manager work??? / Best regards Krister and Robert.