At this point it's not really consumer-grade hardware though, so I wouldn't really classify it under "off the shelf".
These systems (from SuperMicro or Lenovo, for example) still fit in a standard 19" rack, and still take standard CPUs, standard DIMMs, and standard PCIe cards.
The main limitation is that the complete "barebones" system is indivisible, unlike with most 1S-2S systems, where motherboards can be mixed and matched among a wider variety of chasses. This limitation also applies to high-density offerings like blades and SM's "Twin" line. I have yet to encounter a situation where such a limitation has ever even entered the conversation.
Agreed. I was merely trying to convey that they wouldn't be the normal [E]ATX form factor boards.
The parent was asking for something "off the shelf", and I took this to mean purchasing a motherboard by itself. You could get something like that from SM[0], but I doubt you'd be able to find a compatible chassis for the motherboard from anyone other than SM. At that point it probably makes more sense to just buy the system[1].
[0] http://www.supermicro.com/products/motherboard/Xeon/C600/X10...
[1] http://www.supermicro.com/products/system/2U/2048/SYS-2048U-...
The parent did ask about a mobo, specifically, but I think it's a stretch that the question was about the component on its own, at least in the context of the broader conversation of feasibility.
I took the "off the shelf" question to be synonymous with "commodity". That is, the opposite of (truly) custom. One doesn't have to be Google or Facebook and design a board layout to get 8 sockets. One can just order it from the same vendor as all the other commodity hardware.
> it probably makes more sense to just buy the system
I'm asserting that this is what every commercial purchaser does, anyway.