Are there limits to PCIe bifurcation? I know there are cards that split an x16 slot into 4 x4; could you do 8 SSDs connected as x2 or 16 as x1?
It's throwing away a lot of bandwidth (you don't fill it with the fastest SSDs, probably), you still need enough area to hold all the SSDs, and if you have trace length matching/a maximum trace length and a layout holding lots of m.2 sticks, that may complicate things.
But if it is possible, it still gets you the capacity, without PLX chips or similar.
I believe the bifurcation fractions are hard wired on the PCB, but somebody could come up with a card that had 4 or 8 M.2 slots that only had 2 or 1 lanes actually connected for each.
The main challenge would be the BIOS/OS who have to probe, negotiate and configure PCIe ports. Even with the current normal variants of bifurcation, support for it isn't always available even if any 16x root port should need to support 16 x1 devices to pass certification.
Coming to think of it, I believe there might even be crypto miner hardware left over somewhere, which did in fact connect 16 GPUs with only a single lane for each: perhaps not bootable with M.2 SSDs, but a PnP OS might eventually get it right.
Of course, I'd still prefer switching variants, if only they didn't sell them at these crazy prices.
In the case of AMD's AM5, there are "budget" variants in a way from ASmedia which they use for the "PCH".
"budget" because they are cheaper than many other PCIe-only switches with similar capabilities, yet not really that cheap as current high-end finished AM5 mainboard with dual chips make painfully obvious.
I find it hard to stomach, that the IOD in the cheapest Zen 3/4 CPUs is essentially a rather capable PCIe++ switch chip with 6 CPU cores thrown in for free at €100 or less, but a much less capable PCIe only-switch will cost an arm and a leg. Even the first ASmedia switch on the mainboard offers much more for less and it's only with the cascaded fully fledged ones that they try to top PCIe switch vendor pricing.
These IOD/ASmedia chips are essentially multi-protocol PCIe switches that also support SATA and USB and I've always thought that I'd prefer those on a x4 PCIe 5.0 slot, so that I can do the allocation myself by chosing an appropriate board that has the mix of USB/M.2/SATA/NIC which fits to my use case (and extension needs).
And with M.2 -> PCIe x4 slot cables you can even physically cascade them to support some really crazy custom levels someone might find useful who today is thinking about hooking up M.2 SSDs to an x1 slot and then a Thunderbolt cable should actually allow even more flexibility (and some chaos).
The basic flexibility is all there and exactly what has made Personal Computers so attractive for four decades, but sometimes vendors seem to concentrate more on market segmentation than supporting it.
P.S. Actually the IOD supports even Infinity Fabric and RAM, potentially even some variants of CXL... Of course the signal speed and mainboard trace lengths pose challenges that won't fit into desktop MB budgets.
336tb of data my have one year of work...
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