BUILD UDIMM vs RDIMM

[qwertymodo]

I understand the basic conceptual differences between UDIMM vs RDIMM RAM, but are there any particular details or "gotchas" with respect to choosing one vs the other in a FreeNAS/ZFS system? Previously, I understood the differences between ECC and non-ECC RAM, and thought I'd be ok without ECC, until I started reading into some of the issues specific to non-ECC RAM in conjunction with ZFS, so I want to be sure I don't run into any more unpleasant surprises as I'm re-thinking my build.

It may be a moot point, as the board I'm looking at, the Asrock C2550D4I, only indicates support for UDIMM, but I just want to know what I'm getting in to here.

 

[DrKK]

Sir:

Generally speaking, the motherboard supports UDIMMs *or* RDIMMs. The electrical properties are very very different. I have heard that you will blow your motherboard to bits if you use RDIMMs in a UDIMM board, and vice versa. Most people using SuperMicro boards (which is most of us using home FreeNAS's) will be using UDIMMs.

Whether or not the RAM is registered has nothing to do with its ECC capabilities. One isn't "better" ECC than the other. It's all about whether the server motherboard desires buffered or unbuffered properties. Like I said, I actually think every single person I've helped set up a server mobo required unregistered/unbuffered RAM.

 

[qwertymodo]

The servers at my work most certainly support both (though you have to choose one or the other and you can't mix), and they will list 2 separate RAM capacities for the machine based on which you choose. I wasn't implying the difference between UDIMM/RDIMM was in any way related to ECC/non-ECC, I was merely comparing my current lack of understanding on the one to my previous misunderstanding on the other.

 

[DrKK]

Interesting. OK, well, most of the boards I deal with will blow if you put RDIMMs in them.

What is your question then? Or was it just a comment?

 

[qwertymodo]

The question was, assuming the board I go with supports both, is there any particular reason to choose one over the other? Or alternatively, if I can't find a board that supports both, should I be choosing the board based on support for one type over the other? And finally, as I'm currently leaning towards the Asrock C2550D4I, can anybody confirm whether or not it even supports RDIMM, because if not, my initial question may be moot (though then I'd be looking at the second question instead).

 

[DrKK]

As far as I know, there is absolutely no reason to choose one over the other. Some larger sized DIMMs are, for electrical reasons, always "registered". I see no mention in the spec sheet or any other materials for the board you're talking about that refer to any kind of memory other than UDIMM.

And, like I said, just about 100% of us are running UDIMMs at home for our FreeNAS servers.

 

[qwertymodo]

Works for me

Sent from my Galaxy Nexus using Tapatalk

 

[survive]

Hi qwertymodo,

A RDIMM (registered DIMM) has a special chip on the DIMM that goes between the memory controller & the memory chips that acts as a sort of middleman to decrease the load on the memory controller. That's why you see the max unbuffered (UDIMM) memory supported on most (all) motherboards listed as 32GB. If you look at a registered DIMM (check google image search) you should see a single extra little chip on one side of the DIMM, that's the register chip. This register is what allows a processor to access the monster amounts of RAM that some high-end server boards allow. The memory controller talks to the register chip, which can then talk to the memory chips proper. The register usually adds a hint of latency to each access, but in most cases it's well worth the trade off to get at all that extra RAM.

Registered DIMMs can usually be purchased cheaper because there are a boatload more of them made. 4GB RDIMMs can be had for next to nothing off ebay from all the shops that are swapping there sticks for 8 or 16GB modules. That said, I took a look at the manual & memory QVL list for that board and I don't see any indication that it uses RDIMMs so it's not something I suspect you need to worry about. Ideally you would get some sticks off the list here:

http://www.asrockrack.com/general/productdetail.asp?Model=C2550D4I#Memory QVL

and you shouldn't have any problems with your RAM.

Be sure to come back and let us know how the board works out for you, those Avoton boards look really slick!

-Will

 

[DrKK]

Survive:

Can you comment on what happens when RDIMMs are placed into a motherboard that does not desire/expect the extra register chip? I was told these combinations blow boards, all the time. Was I misinformed, or is that true?

 

[jgreco]

Hi qwertymodo,

A RDIMM (registered DIMM) has a special chip on the DIMM that goes between the memory controller & the memory chips that acts as a sort of middleman to decrease the load on the memory controller. That's why you see the max unbuffered (UDIMM) memory supported on most (all) motherboards listed as 32GB.

[sigh] Close but misses the mark just a bit.

The E3 (and by extension the other socket 1155/1150 chips) only support UDIMM's. They also happen to be limited to 32GB.

UDIMM's were intended to be cheaper to build and possibly make faster operation possible (since in some designs RDIMM can take an extra cycle). As a result they are the only memory option for the 1155/1150 product offerings.

Due to the 1155/1150 32GB limit, UDIMM's had been limited in capacity to 8GB because there was basically no use for larger ones. The Avoton and I think an AMD product line have changed that.

The statement

max unbuffered (UDIMM) memory supported on most (all) motherboards listed as 32GB

is misleading; you see that due to the 32GB max addressable memory on those boards. E5's can address more. With 8GB modules, the E5-26xx can take 64GB UDIMM. See for example:

http://www.supermicro.com/products/motherboard/Xeon/C600/X9SRH-7TF.cfm

But the E5 is really designed for large gobs of registered memory, which as it stands tends to be a little less expensive as well, at least in the lower densities. I had no trouble picking up 128GB for a box here for about $1000 a year ago.

 

[survive]

Hi jgreco,

Indeed, I did word that awkwardly\incorrectly....it would certainly be better to say that current Intel processors are limited to 16GB of UDIMMs per channel. I was clearly more focused on the Ivy\Haswell boards we tend to see folks asking about rather than the socket 2011 gear you get to mess with!

The memory QVL list for the Avoton board the OP asked about does indeed list 16GB DIMMs from an outfit called "Memphis", I believe one of their guys passed through here a few months back posting about their upcoming modules.

-Will

 

[jgreco]

That's correct. I'm toying with the idea of putting together a VM datastore on an Avoton, either 32GB with SSD pool or 64GB with HDD pool and 256GB L2ARC...

 

[no_connection]

Is the power draw for RDIMM still higher than UDIMM?
Or was that just a result of older hardware? (My ML350G5 sips around 5W per stick)

 

[jgreco]

I believe it is still generally correct, though the RDIMM will also have higher density if you look at it on a watt-per-GB basis.

 

[cyberjock]

Technically, RDIMMs do use more power. But you are talking about fractions of a watt. It's not something you should be considering in the purchase of RAM.