I have a number of Dell 2950 servers with two power supplies. In normal usage how are the PSU's utilised?
Do they both draw '50%', only switching to 100% if one of them dies? Does one supply provide most of the power whilst the second is more or less redundant until the first fails?
The reason for the question is thus: I have a couple of APC UPS units in the bottom of the rack and currently both power supplies for each server are plugged into a UPS unit. Am I better off plugging one of the power supplies into the UPS and the other onto filtered mains power? My thinking is that if the power goes down and the UPS picks up, will I get a longer grace period if only one of the PSU's is drawing power?
If both PSU's draw '50%' then it would make no difference obviously, but if there is a power overhead for having two PSU's then it becomes relevant.