What is the difference between 2Rx4 and 2Rx8. Also why is there a flood of 2Rx4 memory on ebay for cheep


Hi guys im currently building myself a server and looking at RDIMMS I see a ton of 2Rx4 memory for cheep and not much 2Rx8 memory at all for decent prices. I am getting a Intel S2600CP2J Motherboard with 2 Xeon E5-2670’s and I was going to use it as an ESXi server. from what ive read there is a negligible performance impact going with x4 vs x8 memory, though i heard it depends on the chipset and processor. Also is it possible to mix them in a server? (a few 2Rx4’s and 2Rx8’s) if not on one CPU can one cpu have 2Rx4 and one have 2Rx8?

I was thinking of getting THIS

Thanks for the help.

x8 is rare. Most ram on ebay is from decommissioned or upgraded servers.
2 to 5 years old hardware gets recycled in most large organizations.
For best compatibility, try to find a spec sheet from the motherboard.
Rank 4 dims dont work in some systems where rank 2 almost always work.
Depending on the motherboard you might be able to mix ranked dimms.
Although the dimms might need to be placed in the specific socket.

Thanks for the informative reply. It helped alot.

Sorry to resurect something this old but I feel the question I have would serve other who are searching better if it existed alongside this explanation.

I just bought a Dell r710. I got it for $350 w/ 2 CPUs and all the drives are 300GB 15k drives.

It was advertised with 72GB of ram but when I pulled it out I found that it only has 48GB.

When I bought it I really didn’t know if it was going to be dual triple or quad but it’s triple which I have some experience with almost a decade ago with the 1366 CPUs.

Anyways long story short is this.

I contacted the seller and he says it was a mistake and that he had 2 710s one with 4 and 1 with 6 drives and one with 48 and the other with 72. He accidentally labeled the 4 drive one as 48 (and I did see that) and the 6 drive one as 72.

He said he was willing to resolve the issue. I told him he needs to give me 3 matching identical sticks of memory. The communication on his end got very short and he stopped answering all of my questions and started answering only certain questions.

Finally I find out the following.

He wants me to bring my 48GB of ram and exchange it for the 72GB. I said no way on Earth is that happening before he even had a chance to try to say anything else. I knew immediately that the ram I had must be more valuable.

That brings me to this. I have 6 8GB Crucial PC3L-12800R 1rx4 dimms he wants to trade me for 18 4GB Hynix PC3L-10600R 2rx8 dimms.

Thanks to this post I clearly understand the 1r vs 2r and its impact in the system.

I don’t understand the impact of x4 vs x8. I understand what it is referring too but not what it does to my machine.

I’m especially concerned with loading in 18 2r dimms because that’s now like having 36 pieces of memory vs the 6 I have now.

Another concern of mine is can the system handle 36 or can it only use those 3rd sticks as spares? Also my understanding is that with 18 sticks the system will be limited to 800 speed instead of 1333 or whatever.

Can someone explain the implications of filling all 3 slots of each channel with 2r instead of 1r and can anyone explain to me what impact the x4 vs x8 has.

At this point I would sooner just sell the 48GB of memory I have now and buy 96GBs of udimm. It seems as if that is totally plausible.

The machines expected use is as a lab, a router, a VM machine, and to plug my 16 bay dual 10gbit NAS into it so that it can run the software since the NAS doesn’t have a normal PC CPU in it. It sometimes struggles to transcode Plex and record the houses cameras at the same time. So I think for what I want it for I can get away with udimms instead of rdimms.

So a recap

Could you guys give me an idea of:

what the x4 and x8 impacts

Can the server use all 18 4GB dimms in triple channel mode or does the 2r become an issue at that point. If I stick with 8GB I’d only ever populate 2 sockets per channel to get to 96GB.

Is there a major impact to the ECCs ability between the rdimms and udimm.

Is 18 4GB 2r rdimms better than say 12 8GB udimms?


Even though your reply bumped the thread more than 6 months, it seems to have been allowed to stand and I’ve got some answers so I’ll continue the discussion. If it needs to be split off into its own discussion, then that works too. :slight_smile:

The Dell servers of that era had to deal with triple channel memory cpus, and their memory requirements get very, very funky. I have an r410 and it has 4 slots and 3 channels on each cpu. Yep, that should have your head scratching because how can each channel be the same size/ranking and yet there are 4 slots? So the answer to these puzzling questions is to read Dell’s excellent documentation on the server which does explain a lot of this in detail (love enterprise docs!).

It will vary from model to model too, so don’t assume what works for one will work for another. Good luck!

Oh, and the same applies for HP servers of the same era. :wink:

1 Like

The thread was properly bumped so I wouldn’t worry about it.

1 Like

Thank you! Always want to play by the rules, so it’s good to know I was within them. :smiley:

1 Like

Hey, necrodude here. Seeing the question never got answered I’ll answer it myself.
The reason why eBay has too many 2Rx4 rams because they have compatibility issues with older Intel motherboards (which only supports x8 or x16 chips). Seemingly, 2Rx4 rams are cheaper to manufacture and people have trouble using it on their Intel systems that’s why it still floats around eBay.

1 Like

Happy to have found peoples with some views on this issue.
I have a Dell Precision T5500 (mobo: 0D883F, xeon E5520) with 3 modules 4Gb 2Rx8 PC3 10600R-09-10-B0-D2. Can I add three 2Rx4 modules for example: 4Gb 2Rx4 PC3-10600R -09-10-E1-D2 ?

Thank you for your help.