[liquidgecka]

> > CDUs exchange heat between coolant liquid and the facility-level water supply.

Oh interesting I missed that when I went through in the first pass. (I think I space bared to pass the image and managed to skip the entire paragraph in between the two images so that’s on me.

I was running off an informal discussion I had with a hardware ops person several years ago where he mentioned a push to unify cooling and eliminate thermal transfer points since they were one of the major elements of inefficiency in modern cooling solutions. By missing that as I browsed through it I think I leaned too heavily on my assumptions without realizing it!

Also, not all chips can be liquid cooled so there will always be an element of air cooling so the fans and stuff are still there for the “everything else” cases and I doubt anybody will really eliminate that effectively. The comment you quoted was mostly directed towards the idea that Cray-1 had liquid cooling, it did, but it transferred to air outside of the server which was an extremely common model for most older mainframe setups. It was rare for the heat to be kept liquid along the whole path.

[matt-p]

The CDUs are essentially just passive water to water heat exchangers with some fancy electronics attached. You want to run a different chemical mix outside to the chillers as you do on the internal loop, it also helps regulate flow/pressure and leak detection with auto cutoff is all fairly essential.

Running direct on facility water would made day to day operations and maintenance a total pain.

[avar]

    > not all chips can be
    > liquid cooled.

Why not? It's just a heatsink except with water running through cavities within it, instead of a fan sitting on top of the heatsink.

[bri3d]

One of the biggest problems with water cooling, especially on boards that weren’t designed for it, can be passive components which don’t usually have a heatsink and therefore don’t offer a good surface for a water block, but end up in a thermal design which requires airflow - resistors and FETs are common culprits here. Commodity assemblies are also a big problem, with SFPs being a huge pain point in designs I’ve seen.

The problem is often exacerbated on PCBs designed for air cooling where the clearance between water cooled and air cooled components is not high enough to fit a water block. Usually the solution when design allows is to segment these components into a separate air cooled portion of the design, which is what Google look to have done on these TPU sleds (the last ~third of the assembly looks like it’s actively air cooled by the usual array of rackmount fans).

[michaelt]

If you're blasting enough air around to cool a 600W GPU, you don't care if your GPU's power connector dissipates 10W under certain circumstances - the massive airflow will take care of it.

Take that airflow away and you have to be a good deal more careful with your connector selection, quality control and usability or you'll risk melted connectors.

Water-cooling connectors and cables isn't common, outside of things like 250kW EV chargers.