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I am currently building a new server. I have options between say

32GB Memory for 2 CPUs, DDR3, 1066MHz (8x4GB Dual Ranked RDIMMs)

and

36GB Memory for 2 CPUs, DDR3, 1066MHz (18x2GB Dual Ranked RDIMMs)

Both at the same price.

Should I go for the higher ram amount or the fewer chips?

This will be for a Dell PowerEdge R710 with two Intel® Xeon® E5530, 2.4Ghz, 8MB Cache, 5.86 GT/s QPI, Turbo, HT

Thanks

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It would be very helpful to know what processors you're planning on using in this system, ideally the server's make and model too please –  Chopper3 May 20 '09 at 6:53
    
Good point Chopper, have edited. –  Alex Andronov May 20 '09 at 7:06
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4 Answers

up vote 19 down vote accepted

Thanks for the update Alex. Most new 'Nehalem'-based servers such as the model you're looking at use triple-channel RAM, notice how they have 'divisible-by-three' memory slots (3, 6, 9, 12, 18 etc) where previous models had 'devisible-by-two' versions (2, 4, 8, 16 etc). You can put less than three modules in at a time but you're likely to miss out on the full performance of the new 'QPI'-based memory architecture.

Also of great importance with these new chips is to balance your memory across processors - if you only have a single processor then you shouldn't put memory in the slots associated with the empty processor slot, only with the first processor - this effectively halves the memory capability of single-processor servers. If you have two processors you need to ENSURE that they both have the same amount AND type/size of memory as each other.

Now onto your actual question. With these new servers the more physical memory chips you use the slower they get - they're still faster than the previous generation of chips/memory even at their slowest but you should aim to use less of the most capacious modules that make sense to you financially.

Looking at the specific machine you want I would urge you to go with either 48GB Memory (12x4GB) or 24GB Memory (6x4GB) based on your requirements. Best of luck.

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I would go for fewer chips, the reasoning being:

  1. This may leave some slots available for more RAM at a later date if necessary.
  2. When you decommission the server 3+ years from now, the 4 GB chips are more likely to be useful for salvaging than the 2 GB ones.

I doubt having lots of chips would give you any noticeable performance improvements in most scenarios.

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Higher density has nothing but benefits

  • Upgrade easily
  • More reliable
  • less power used (those few Watts tend to accumulate...)
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The impact of triple vs. dual channel will not be so outspoken. However, within a triplet, your memory should be matched, so starting with 8 dimms now and expanding later might be more challenging unless you would be prepared to lose 2 dimms at that time. Do you need the memory right now? Then take the 18x2GB. Is it a luxury for now, and you are comfortable with upgrading later, go with 9x4GB if you can.

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