Database -- What are log_url + log_visitor ? Can I truncate them?

Question

I've to move a 9.8GB database from a live server to local so I can fix stuff without going offline. But, I just noticed that there are

log_url_info = ~23,039,809 = 2.1 GiB
log_url = ~22,131,158 = 2.9 GiB
log_visitor = ~17,249,673 = 2.1 GiB
log_visitor_info = ~17,720,546 = 2.4 Gib

so I have few question about this tables;

1) What info are stored in those tables? What Am I going to lose?

2) Can I truncate them? If yes, can I do it live? Won't it block the system or take too much time?!

Answer 1

Those tables remember the pages visited by your customers. They also have a connection with the compared products. If you truncate them some customers might lose the list of compared products. You can truncate them, but it could block the access to the database. I recommend putting the website offline while you do it.
Whenever you decide to truncate them back-up first just to make sure.

• Fabian Blechschmidt: Yes Marius is right, and btw. everyone I know truncates the tables before updates :)