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Can I put 12 x 4gb memory in Dell R520 rack server

Posted on 2016-10-13
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Last Modified: 2016-11-22
Hi

It is not clear in the manual.

In sample configurations, with 2 processors.
For 48 gb it says use 6 x 8gb.

But I have got
8 x Crucial 4GB DDR3L-1600 ECC UDIMM
4 x 4GB DDR3 - Synchronous Unregistered (Unbuffered) (came in system)

There are 12 slots and it says on Crucial, Max Unbuffered DDR3 SDRAM:48GB

if not what can I put where to get maximum ram?

Thanks Terry
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Question by:terrybuck9
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by:garycase
ID: 41842362
You can't install more than 6 modules if you're using unbuffered RAM.

If you use registered (buffered) modules, you can populate all 12 slots.

So ... NO, you can't install 12 4GB unbuffered modules.
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by:garycase
ID: 41842364
... as I suspect you already know, buffered modules are FAR more reliable than unbuffered modules => so if your goal is to maximize the installed RAM, I'd switch to registered modules.   You can NOT, however, mix the RAM types.
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by:andyalder
ID: 41843410
Yes, you can have 2 UDIMMs per channel meaning all 12 slots can be populated with UDIMMs if both CPUs are fitted.
Since you already have the RAM why don't you try it, you'll get an error message if it's an unsupported configuration, it won't blow the server up.

http://www.dell.com/downloads/global/products/pedge/en/server-poweredge-11g-white-paper.pdf (out of date since it says max UDIMM size is 2GB)
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by:terrybuck9
ID: 41846350
I do have 2 processors. But I had already bought all new ram before Andy's comment came on. I will test it on my test server when I get time.

Garycase do you agree with Andyalder?

Thanks Terry
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by:garycase
ID: 41846391
Since each of the CPU's has 3 memory channels, you can indeed put 2 DIMMs per channel for a total of 12 DIMMs.

Dell's specs, however, indicate that they support a maximum of 48GB of unbuffered modules -- and these were published well after the white paper Andy noted ... the same specs indicate up to 384GB of registered modules, so clearly the specs reflect modern module sizes (since the RDIMMs would have to be 32GB modules to reach the max capacity.   So you are limited to 4GB modules.

With unbuffered modules, the concern is the bus loading ... and higher density unbuffered modules may cause problems.   If you've already ordered registered modules, that's a FAR better choice => there's are no loading issues with RDIMMs, since they only present one load/module instead of one/load/chip (typically 18-36 loads/module).
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by:terrybuck9
ID: 41852875
Hi Gary

Yes I have ordered 6 x 8gm RDIMMS.  Is that ok?

Thanks Terry
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by:andyalder
ID: 41852905
Waste of money if the UDIMMs which you bought worked (which they should). RDIMMs are preferable because you can fit a lot more RAM in a server using them rather than UDIMMs but for your required config UDIMMs are fine (if you ignore the FUD).
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by:terrybuck9
ID: 41852909
Please re-read my message as I amended it.
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by:andyalder
ID: 41852917
If you have ordered RDIMMs you'll have to take the original UDIMMs out since as Gary mentioned earlier you cannot mix DIMM technology types although you can mix speed and capacity.
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by:garycase
ID: 41852940
Using 6 8GB RDIMMs will work fine -- you could actually use up to 32GB modules with registered modules.    This will still leave 6 additional slots that you can add more RDIMMs to should you ever need to increase the memory capacity.

I would never consider it a "waste of money" to buy registed RAM ... it's far more reliable than unbuffered modules.   Note that a fully populated system, with 12 RDIMMs, will present less loading on the address and data buses than ONE unbuffered module.
\
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by:andyalder
ID: 41853061
It's a waste of money if they've already bought the UDIMMs which is what they said they had done. Also that system doesn't take 32GB RDIMMs, only 32GB LRDIMMs.

Having a family of people in your SUV will be FOUR times the loading of a single person behind the wheel, so I'd recommend letting your kids walk to school. (That's FUD obviously, your car is designed for that many people same as your server is designed for 2 UDIMMs per channel)
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by:garycase
ID: 41853127
Putting 4 people in an SUV doesn't change the road you're traveling on ... indeed it most likely results in a smoother ride, since SUVs (just like the trucks their chassis is based on) ride better when they're reasonably loaded.

Memory address and data buses, however, work best when they have clean "signalling" -- i.e. the square waves that control them are indeed square.   ... and the more electrical loading on those buses, the more degraded the signalling waveforms become.    Corsair has an excellent presentation that shows just why this is the case ... see Item #10 here:
http://www.xlrq.com/stacks/corsair/153707/index.html

That is why, of course, you can put 384GB of LRDIMM in the system, but only 48GB of unbuffered modules.
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by:andyalder
ID: 41853908
The SUV analogy I used was because you compared the loading of 12 RDIMMs to **one** UDIMM, in effect you only had the driver in your UDIMM setup and left the passenger seats empty whereas with RDIMMs you populated all the seats!  If you compare the loading of 12 RDIMMs to **six** UDIMMs it would have been better but you deliberately skewed it by only populating one memory channel in the configuration you didn't want used, FUD indeed.
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by:garycase
garycase earned 250 total points
ID: 41865367
Adding more passengers in an SUV actually IMPROVES the ride, as these are generally on truck-based chassis where a bit of weight provides better ride and handling.     Adding electrical load to a memory subsystem DEGRADES the signaling waveforms and decreases the reliability.    The impact of using UDIMM vs. Registered modules is the same vis-à-vis the electrical load on the power supply; but is dramatically different with regard to the loading of the IC's that drive the address and data buses -- causing notable degradation of the signaling waveforms.     I was simply noting that fully populating the system with registered modules actually provides LESS loading on the address and data buses than ONE unbuffered module, due to the loading differences.

This is shown nicely in Corsair's "Memory Basics" presentation -- see Item #10 in the link I provided in my last post.
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by:andyalder
andyalder earned 250 total points
ID: 41865832
Installing one UDIMM into this server provides the same memory loading per channel as installing SIX UDIMMs into it, that's where your FUD comes in, you've deliberately misbalanced the UDIMM solution to make the RDIMM solution sound better than it is.
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terrybuck9 earned 0 total points
ID: 41874546
Thank you both for you help, but I am still confused.

I sent back the Udimms so i cant now try them.  
I then bought Rdimms from Dell and they worked fine, even though twice the price.

So all's well that ends well.

Thanks Terry
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by:andyalder
ID: 41874608
The UDIMMs would have been fine, the memory controller in the CPU automatically down-clocks them to compensate for the higher bus loading but RAM clock speed isn't that important for performance anyway, it's quantity rather than quality that generally improves matters. You fell for FUD.
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by:garycase
ID: 41874635
The UDIMMs would have worked; but registered modules are always preferable for reliability.   Whether that's "FUD" or simply minimizing your risk is perhaps debatable => but what's absolutely true is your system's memory and data buses now have 1/18th of the loading they would have had with UDIMMs ... and you can freely increase the memory should you ever need to to 8 times the capacity you have now, which would not have been the case with UDIMMs.
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by:andyalder
ID: 41874823
Gary,  1/18th of the loading is a huge difference. Do you have any simple text based source for that or have you put 3 UDIMMs in each channel to belabour the point?

Not a video or a powerpiss presentation but a real PDF or TEXT document from a memory manufacturer that demonstrates your point would be preferable. The Corsair slideshow doesn't cut the mustard because it is a powerpiss presentation rather than a PDF.

Terry, I am confused as well because surely any "proof" would be available in simple text format rather than forcing us to download a slideshow. That's why I think it is FUD. If it was real facts then there would be text documents to prove the point rather than only slideshows. Quite frANKLY i'm scared that it's a link to malware,

I would rather use RDIMMs than UDIMMs as well but the justification provided so far is CRAP. No math, no sums, just a slodeshow with photos of smiling people.
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by:garycase
ID: 41874917
It's very simple math -- as shown in the Corsair presentation you for some reason don't trust (they are, of course, a major memory manufacturer).

With unbuffered ECC memory modules, there are typically 18 memory chips on a module.   Each of these chips puts a single TTL "load" on the bus when it's addressed or is transferring data.    So there are 18 loads per module.    Note that some high capacity modules actually have 36 chips ... so they would present 36 loads to the buses.

With registered modules, there is a buffer chip, which is the ONLY load placed on the address and data bus.   That's what buffering is all about.   The loads for the individual memory chips aren't passed through to the system address or data bus.   So there is 1 load per module, regardless of how many actual memory chips are on the module.

The impact of this loading on the bus signals is clearly shown in Corsair's presentation (Item #10).    Basically, the higher the load, the "dirtier" the signaling, as the square waves rapidly degrade due to the capacitance of the loads.    If you're old enough to remember the days of corded phones, you may have experienced this impact if you ever had a lot of phones plugged in and too many folks in the house picked up at the same time -- resulting in either very degraded line quality or even a completely dropped line.   A modern cordless base station that supports multiple phones is kinda the equivalent the buffered memory -- as it only present one load to the telephone line, even though there are multiple phones in use.   [As I suspect you know, a "load" in telephone terms is called a "ringer equivalent"]
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by:terrybuck9
ID: 41880239
Thanks
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