btw, I found a Gigabyte board on Taobao that is unlisted on their site: MZF2-AC0, costs $900. 2 socket Epyc and 10 PCIE slots, may be of interest. A case that should fit, with 2x 2000W Great Wall PSUs and PDU is 4050 RMB (https://www.toploong.com/en/4GPU-server-case/644.html). You still need blower GPUs.
Re: 3090 NVLink, that only allows pairs of cards to be connected. PCIe allows full fabric switch of many cards.
IMO the most painful thing is that since these hardware configurations are esoteric, there is no software that detects them and moves things around "automatically." Regardless of what people thing device_map="auto" does, and anyway, Hugging Face's transformers/diffusers are all over the place.
They're going to realize soon enough that they've basically just been told that the extremely shitty problem they thought they'd moved beyond is back with a vengeance and the next generation of gaming cards has the potential to make the past few rounds of scalping shit-shows look tame.
We chose 6 because we have 128 PCIe lanes, aka 8 16x ports. We use 1 for NVMe and 1 for networking, leaving 6 for GPUs to connect them in full fabric. If we used 4 GPUs, we'd be wasting PCIe, and if we used 8 there would be no room for external connectivity aside from a few USB3 ports.