Hi Dave,
Thanks for such detailed response. The error in my case was not the garbage collector, rather the logic how the sessions work (sess_regenerate_destroy = TRUE didn't behave as documented).
It seems the problem was that during various tests we didn't change the name for cookie and somehow old one could affect.
Cleaning sessions tables and also changing the sess_cookie_name in the settings did the trick and the sessions started to work properly (removing the previous record when updating).
Comparing file storage and db, file might be better performance (although not significantly), one of the consideration factors is convenience. When someone programmatically grabs the site there are plenty sessions produced, it is pretty easy to delete all at once via DB, rather than work with flat files.
In our experience, the session table grew to several millions and then performance degraded. This is why it was a concern to keep it low. But now seems there is no large problem with it and we move forward with deployment.
Thank you for help and advice!
