Session garbage collection

Hi all, I'm implementing php4 session management in a huge website we're developing. Now i want to start a session for every visitor hitting our website, but i'm wondering about whether it is a smart thing. Php4 is configured to write session files to a directory on a EXT2 partition, and i don't know the limitations for this type of file system, but when there's lots of visitors in a small timespan this might get tricky. So my question is how often do old, unused registered sessions get purged? Do i really run into trouble in this situation? Or should i use a different type of filesystem for storing session files? Thanks a bunch :-) Hans Rakers