Why aren't there PCIe RAM expansions?

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?

agweber

Posted 2013-02-01T16:56:06.583

Reputation: 309

Would be great to have like a "L2 RAM" -- something the OS could page stuff into if it needed -- i.e. for your DDR4 system, you could have a PCIe card that took your old DDR3 DIMMs and exposed them as an L2 RAM to the system... – BrainSlugs83 – 2015-09-14T15:43:06.553

http://www.sandisk.com/about-sandisk/press-room/press-releases/2012/sandisk-launches-pci-express-based-products-for-enterprise-applications/ – None – 2013-02-01T17:00:48.883

@rawbrawb, I think that is a SSD, not RAM. – Brian Carlton – 2013-02-01T17:52:03.727

@BrianCarlton mea culpa! – None – 2013-02-01T19:17:04.723

3Note: Almost no PC have PCI-X. It was only found in server market and tops out at around 1 GB/s. Furthermore it's obsolete. It was development based on classical PCI and is not related to PCI Express (often abbreviated PCIe). – AndrejaKo – 2013-02-01T20:31:01.660

There are cards like this: http://www.fusionio.com/products/iodrive2/

– Bill Lynch – 2013-02-02T07:12:41.710

Thank you for pointing that out AndrejaKo, I just assumed eXpress since I had seen PCIx before. – agweber – 2013-02-04T16:09:02.970

Answers

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.

David Kessner

Posted 2013-02-01T16:56:06.583

Reputation:

Probably another factor deprecating RAM cards was the move from 32-bit processors to 64-bit processors. More recently, PCIe flash (usually with a DRAM cache) has taken a similar role. – Paul A. Clayton – 2015-07-06T11:07:36.957

@David - I'm sure that motherboard was very expensive though -- not to mention buying all that current gen RAM -- for the enthusiast consumer who always has last gen RAM laying around, having an "L2 RAM" card that the OS could use as swap (so it's slower, and maybe even only accessible serially, seriously not asking for much here -- but being able to reuse our old lastgen chips to speed our systems up would be great...) – BrainSlugs83 – 2015-09-14T15:48:06.883

Heck -- even if it was RAM the OS couldn't directly address, it'd still be worth it (think of products like PrimoCache and Primo RamDisk that can use RAM the OS can't address, for example). -- You buy one card, and suddenly a bunch of extra RAM that you already have is available for third party programs to use to speed up your system... – BrainSlugs83 – 2015-09-14T15:50:15.637

@KevinCathcart I agree -- one feature that would be killer is if had some flash rom on it, and you could program into it the "bad" areas, so you could use failed RAM for example -- sort of like what scandisk used to do for bad clusters (like, I have a couple of 8 GB sticks that have just 1 MB of failed area each -- add them all up, oh look, 31.9 GB...) :-) -- People always say, this would be too expensive of a card to build because it would need 2 layers per slot -- ridiculous -- if you expose it as a disk and access it serially, there's no need for that... (just use chip select). – BrainSlugs83 – 2015-09-14T15:55:33.367

2Interesting. I've had a similar idea floating in the back of my head. The concept was a PCIe or perhaps SATA3 based "drive" that uses inexpensive sticks of last-generation ram, for volatile-only use. Common cases would be /tmp, swap, TempDB, and similar. It should be possible to obtain SSD-like read speeds, with much faster write speeds, and by using last-gen sticks it would cost substantially less than adding more general RAM. I suppose people find SSDs good-enough in most cases. – Kevin Cathcart – 2013-02-01T20:14:39.187

@KevinCathcart But if you could put that memory on the Mobo, and not have the expense of making a PCIe card, then you could buy the latest gen RAM and still save money. And you could use that memory as /tmp space, or just more RAM, and it would be a lot faster than anything done with PCIe. – None – 2013-02-01T21:52:29.480

2Sure if putting that ram on the MOBO is viable. In a non-server environment, ram slots are often very scarce, and motherboard replacements are frequently non-viable. In a server environment things are rather different. Such a device would definitely have less impact there. I'll also admit that this would work better if prices for old generation RAM dropped faster than they actually do. – Kevin Cathcart – 2013-02-01T22:10:28.973

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.

pjc50

Posted 2013-02-01T16:56:06.583

Reputation: 5 786

Yeah, those ISA cards sat right on the Processor's IO bus though -- no Northbridge / Southbridge -- it was basically as directly connected to the CPU as the onboard RAM was (at least, it was in my 286). -- I'm thinking, in a modern computer you could use PCI-e RAM as some kind of L2 RAM -- like you mention for swapping, etc. -- the OS doesn't even have to be aware of it (though, rightfully, it should). – BrainSlugs83 – 2015-09-14T15:59:41.543

5you could use RAM as a (non-permanent) disk too, as well as for swap. – Brian Carlton – 2013-02-01T17:56:07.337

I remember having a 64k add-in card for my Apple IIe that doubled the available RAM to 128k. There was no separate video memory, so the add-on also enabled 80 column text and double-resolution 'graphics'. – HikeOnPast – 2013-02-02T00:13:18.903

Yep: it's not the throughput that the problem. It's the latency. – Joel Coehoorn – 2013-02-04T16:25:21.113

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.

HikeOnPast

Posted 2013-02-01T16:56:06.583

Reputation: 131

Your "midgrade system" is a rack mounted server. Come on guys, we're talking enthusiast class. -- Current gen RAM is always super expensive, and we always have a bunch of last gen RAM sitting around. -- If someone built this product, they would make a damn killing... – BrainSlugs83 – 2015-09-14T16:01:59.010

2Nitpick: PCIe, not PCIx. They're two different standards. – Bryan Boettcher – 2013-02-01T23:06:26.703

Valid. Corrected. – HikeOnPast – 2013-02-02T00:08:03.670

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.

Brian Carlton

Posted 2013-02-01T16:56:06.583

Reputation: 721

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"

Ryan

Posted 2013-02-01T16:56:06.583

Reputation: 1

The most recent incarnation of PCI RAM cards was produced by Gigabyte in 2005 and 2006 under the names i-RAM and GC-RAMDISK, respectively. They supported 4 slots totaling 4 GiB of DDR-400. The transfer rate was that of SATA, 150 Mb/s, and the latency of 0.1 ms.

landric

Posted 2013-02-01T16:56:06.583

Reputation: 1