Bdtyktl

Perhaps I may be overlooking some aspect that is an important cause as to why these don't exist, but I feel that having RAM expansions through PCIe would be perfectly feasible. I know that a lot of operating systems use virtual memory and store some lower priority items on hard disks, but considering the lower speeds I feel that we could use some bonus not-quite-as-fast memory.

6Gb/s SATA ~= 800MB/s

PCIe 2.0 = 500MB/s per lane. 16 Lane ~= 8GB/s

Sure, 8GB/s isn't as fast as actual RAM, but it's 10x the speed of SATA. Why not have a PCIe board with a couple of RAM slots for use with the old RAM that you just replaced with that recent upgrade? PCIe has the benefit of being on almost every motherboard out there. One 'adapter' PCIe RAM Expansion Board would be (supposedly) compatible most PCs.

What am I missing since this hasn't been done yet?

This is a complex issue, that is highly dependent on exactly what you want to do with that RAM.

In most cases, it is cheaper and better to simply replace the motherboard with a new motherboard that supports the amount of RAM that you require. I have a motherboard here in front of me that can take 16 memory modules. The largest module available is 32-Gig. That's a total of 512 Gigabytes in a single machine. (Never mind that 16 modules of that size would cost about US$14,000, or that the MoBo also has dual 8-core CPU's on it.)

Having the RAM on the MoBo means that it is the highest speed possible. You can use it for both a RAM-Disk as well as normal program and data storage. The best of both worlds.

But in your question you keep comparing it to SATA storage, so I am thinking that you'd want to use this extra RAM as a RAM-disk and not for general CPU RAM. This is a valid use, and years ago people did have PCI cards with lots of RAM on it specifically for this purpose. Those cards looked like another disk drive, and not just more CPU RAM. Often these cards had an external power connector on them so you could give them some sort of backup power in case the main power failed.

These types of cards have largely gone away. They were obsoleted mainly by three things: 1. Motherboards now can have much more RAM on them than in the past. 2. There are more modern solid-state drives using Flash memory and PCIe (some with large RAM caches) that work better. and 3. They were just too expensive for what limited advantages it gave.

There are other reasons why you might want to have a PCIe card with lots of RAM, but all of them are applications where the card is doing something other than just storing data. Like Video cards, or data acquisition cards. These things do not apply here.

This has been done; many years ago you could buy ISA cards (pre-PCI) with RAM on, which presented to your PC as either "extended" or "expanded" memory. This was a way to get past the 1MB limit of the original PC.

Modern PCs have a section of extra RAM attached to the video card, separate from main memory.

The reason why you don't get RAM expansion cards nowadays is that latency is a serious problem. There isn't really any provision in the OS for preferred versus non-preferred RAM, so you'd have to use it as a swap disk / pagefile.

Modern server systems achieve up to 75GB/sec between CPU and main memory and even mid-grade systems can support up to 768GB total DRAM capacity. Any requirement to scale beyond that with faster-than-SATA speeds is covered by FLASH PCIe solutions that boast x8 PCIe speeds and many TB of capacity without the data volatility issues associated with DRAM.

Adding to pjc50's reasons, it wasn't successful with PCI cards either, see this discussion for more details.

With main memory being larger, there is also less of a need.

i don't see how this is not possible to get close to ram speeds from nvme's
i test triple channel ddr3 in a benchmark to 22GB/s

32x pci-e bandwidth isnt far from that. but

the ram has NANO seconds of latency while the ssd would have Millaseconds
but thats also not a limitation of pci-e, thats purely current affordable retail
storage.
there is specialty non-volatile storage that could achieve this but the with the 50k price tag, i don't think retail would see this for atleast 10-15 years.

although, if you literally used ram modules on a custom board and the motherboard firmware would allow it to detect it as memory. technically, it would work to extend memory. the underlying tech is there for it to work.
"<3 pci-e"