I wonder how the cost compares to a Tinybox. $25k for 6x 4090 or $15k for 6x 7900XTX. Of course that's the full package with power supplies, CPU, storage, cooling, assembly, shipping, etc. And a tested, known good hardware/software configuration which is crucial with this kind of thing.
Well, if you run out of VRAM, you drop off a performance cliff. That's a whole order of magnitude slower than just using a slower GPU but fitting everything into VRAM.
You can build a setup like in the OP for somewhere around $10k, depending on several factors, the most important of which are the price you source your GPUs at ($700 per 3090 is a reasonable going rate) and what CPU you choose (high core count, high frequency Epyc CPUs will cost more).
