Up until now I've been cooling the thermally (and physically) isolated server room from the rest of my house and using a large mini-split to keep it cool. However the mini-split has reached its limits; The room now has 9-10 kW of gear in it.
My next move is probably a second mini-split to augment the first. This seems the best solution since in the next few months I'll be decommissioning my SAN. That'll reduce the thermal load a good bit. Plus long term I can move one of them on to the UPS and set it a few degrees higher than the other. That'll be a win-win for redundancy and power efficiency.
Side note: Opening a door/window isn't really a great option for a few reasons. One, it's only cool enough to do that like 1/3 of the year here. Two, it lets a TON of dust/dirt/particulates in. Seeing as I have over 1PiB of tape now, that's a serious issue. Although as a short term measure I might put one of my air filter units in a window and suck in some of the cold air for the time being. I think of the wen filter units I have should get enough out to be good.
The company I normally use for HVAC work, SNELL, only does residential work and no longer does mini-splits. So I'm inquiring with Michael's and Sons and looking at other options. I'm hopeful my normal electricians, Absolute Electric, may be willing to offer suggestions too.