Both of the supported motherboards support installation of 2TB of DRAM.

... and if you have the money, you can use 3 out of 4 PCIe5 slots for CXL expansion. So that could be 2TB DRAM + 1.5TB DRAM-over-CXL, all cache coherent thanks to CXL.mem.

I guess there are some use cases for this for local users, but I think the biggest wins could come from the CXL shared memory arrays in smaller clusters. So you could, for example, cache the entire build-side of a big hash join in the shared CXL memory and let all other nodes performing the join see the single shared dataset. Or build a "coherent global buffer cache" using CPU+PCI+CXL hardware, like Oracle Real Application Clusters has been doing with software+NICs for the last 30 years.

Edit: One example of the CXL shared memory pool devices is Samsung CMM-B. Still just an announcement, haven't seen it in the wild. So, CXL arrays might become something like the SAN arrays in the future - with direct loading to CPU cache (with cache coherence) and being byte-addressable.

https://semiconductor.samsung.com/news-events/tech-blog/cxl-...

You havent seen the price of 128GB DDR5 RDIMMs, they are maybe $1300 each.

A lot of the initial use cases of CXL seem to be to use up lots of older DDR4 RDIMMs in newer systems to expand memory, eg cloud providers have a lot.

Yeah I saw the same. I've been keeping an eye on the CXL world for ~5 years and so far it's 99% announcements, unveilings and great predictions. But the only CXL cards a consumer/small business can buy are some experimental-ish 64GB/128GB cards that you can actually buy today. Haven't seen any of my larger clients use it either. Both Intel Optane and DSSD storage efforts got discontinued after years of fanfare, from technical point of view, I hope that the same doesn't happen to CXL.

That is pretty hilarious. I wonder what’s the reason behind this. Maybe they wanted plausible deniability in case someone tried to buy it (“oh the phone lines were down, you’ll have to go there to buy one”).

Yup, for best results you wouldn't just dump your existing pointer-chasing and linked-list data structures to CXL (like the Optane's transparent mode was, whatever it was called).

But CXL-backed memory can use your CPU caches as usual and the PCIe 5.0 lane throughput is still good, assuming that the CXL controller/DRAM side doesn't become a bottleneck. So you could design your engines and data structures to account for these tradeoffs. Like fetching/scanning columnar data structures, prefetching to hide latency etc. You probably don't want to have global shared locks and frequent atomic operations on CXL-backed shared memory (once that becomes possible in theory with CXL3.0).

Edit: I'll plug my own article here - if you've wondered whether there were actual large-scale commercial products that used Intel's Optane as intended then Oracle database took good advantage of it (both the Exadata and plain database engines). One use was to have low latency durable (local) commits on Optane:

https://tanelpoder.com/posts/testing-oracles-use-of-optane-p...

VMware supports it as well, but using it as a simpler layer for tiered memory.

Huh, 200ns is less than I imagined; even if it is still almost 100x slower than regular RAM, it's still around 100x faster than NVMe storage.

I'd have rather a question why we had single (or already dual) core CPUs with dual-channel memory controller and now we have 16-core CPUs but still with only dual-channel RAM.

Such things have existed for quite a long time...

For example:

https://en.wikipedia.org/wiki/I-RAM

(Not a unique thing, merely the first one I found).

And then there are the more exotic options, like the stuff that these folk used to make: https://en.wikipedia.org/wiki/Texas_Memory_Systems - iirc - Eve Online used the RamSan product line (apparently starting in 2005: https://www.eveonline.com/news/view/a-history-of-eve-databas... )

I do remember similar products existing forever honestly. As in the Apple 2 had RAM expansion cards, the ISA bus days for 286-486’s had ISA RAM expansion cards, the PCI era had them and there’s ones before this on PCIe too.

The latency was always a pain though. At some point you realize an SSD with lots of cache gives just as good results since the only downside to an SSD is latency (bandwidth can always be increased in parallel).

RAM controllers are expensive enough that it's rarely worth pairing them with old RAM lying around.

(S)ATA or PCI to DRAM adapters were widely available until NAND became cheaper per bit than DRAM, at which point the use for it kind of went away.