I already posted this in another thread on Mac Pro (2019) RAM upgrades, but figured I'd drop it here as well. If you're not savvy with upgrading memory on Intel's recent server platforms, there's a lot to digest.
Apple's pages on
memory specifications and
installing or replacing memory in the 2019 Mac Pro leave much to be desired. The illustrations have been simplified to the point where they no longer depict the way in which the DIMM slots are actually connected to the memory controllers. Although it shows a 2S server,
this illustration from Thomas-Krenn makes the layout of the DIMM slots, memory channels, and memory controllers readily apparent and easy to comprehend. And while Apple provides plenty of do's and don'ts, for the sake of brevity they avoid explaining any of the reasons why. The best document I've seen thus far on the subject is actually
a white paper published by Lenovo. They also include a handy table which shows the relative impact on memory bandwidth due to populating the DIMM slots differently.
Intel performs platform memory validation and
publishes the results on their site. The results for Cascade Lake Xeons are in two separate documents:
one for RDIMMs and
another for LRDIMMs. I took the time to pare these lists down to the unique/orderable part numbers for what is currently available in the market and cross reference them with pricing from various sources. I also limited the list to CL=21 modules that use conventional parts and don't employ the more expensive 3DS die stacks made using TSVs.
288-pin DDR4-2933 (PC4-23400) CL=21 1.2V ECC Registered RDIMMs
288-pin DDR4-2933 (PC4-23400) CL=21 1.2V ECC Load Reduced LRDIMMs
Like other OEMs, Apple buys finished modules from Micron, Samsung and SK Hynix (the three companies that produce >95% of all DRAM chips). Although they may differ aesthetically (different PCB color, screen printing, stickers, etc.) and include an OEM part number, they almost always retain the part number from the original component manufacturer. It is highly unlikely that one would encounter any intentional functional differences between SDRAM modules bearing the same part number. Now that the 2019 Mac Pros are shipping, it shouldn't take long for owners to report the part numbers from their particular builds, either by checking System Information or looking at the stickers on the modules. I listed Apple's pricing (price/DIMM or upgrade kit price/2) next to the modules I suspect they're using. The photos accompanying the Apple Store listings for the RDIMM based upgrade kits are somewhat identifiable, but the LRDIMM kits are only paired with generic images.
The big three memory manufacturers generally sell their product through OEM channels rather than retail. Apple, Dell, HP, and Lenovo in turn all resell modules from Micron, Samsung, and SK Hynix with their own branding. Additionally, Micron sells to retail and direct to consumer under their Crucial brand, and Kingston assembles their own modules based on DRAM chips sourced from Micron and SK Hynix which they sell to retail and direct to consumer.
The problem with buying from OWC or iFixit is that you generally have no idea what you're actually getting or how thoroughly it was tested. These companies cater to the DIY market and provide fine service, but their pricing is usually not very competitive and the product can at times be questionable. They're like the Napa or O'Reilly of computer parts. They're convenient and you'll pay less than going to the dealership, but...
