Hows Data center cooling work

I use VISI for my data center, and they've given me a tour of their data center in St. Paul. They have 250 tons of cooling capacity provided by CRAC units. The units are backed by generator power in case of failure. They also have what they call "cool aisles" and "hot aisles". The backs of the servers are positioned so that when you walk down one aisle, it is hot (backs of servers on either side of you are toward you), and you walk down the next aisle and it is cool (fronts of servers on either side of you are toward you). You can read more at http://www.visi.com. I would recommend them if you're looking for a data center.