Generally speaking, LRDIMMs are a lot more attractive than their quad ranked RDIMM counterparts with the same capacity. Due to the capacitive load of memory chips on the signal integrity of a memory channel, the clock speed and the number of chips in a channel are limited. To make this more clear, we described the relation between DPC (DIMMs Per Channel), CPU (Sandy Bridge and Ivy Bridge), DIMM type, and DIMM clock speed in the following table. We based this table on the technical server manuals and recommendations of HP, Dell, and Cisco. Low voltage DDR3 works at 1.35V, "normal" DDR3 DIMMs work at 1.5V.

Memory type: 2DPC (SB) 2DPC (IVB) 3DPC (SB) 3DPC (IVB)
Dual Rank RDIMM - 1600 1600 1600 1066 1066/1333 (*)
Dual Rank RDIMM - 1866 1600 1866 1066 1066/1333 (*)
Quad Rank RDIMM - 1333 1333 1333 N/A N/A
LRDIMM - 1866 1600 1866 1333 1333
LV 16GB RDIMM - 1333 (1.35V) 1333 1333 N/A N/A
LV 16GB LRDIMM - 1600 (1.35V) 1600 1600 1333 1333

(*) Some servers support 1333 MHz, others limit speed to 1066 MHz

The new Ivy Bridge CPU supports 1866 MHz DIMMs—both LRDIMMs and RDIMMS—up to 2DPC. The load reduced DIMMs support up to 3DPC at 1333 MHz. In most servers, RDIMMs are limited to 1066 MHz at 3DPC. However, the main advantage of LRDIMMs is still capacity: you get twice as much capacity at 1866 MHz. Dual ranked RDIMMs are limited to 16GB while LRDIMMs support 32GB with the same load. 64GB LRDIMMs are now available, but currently (Q4 2013) few servers seem to support them. Notice also that only LRDIMMs support Low Power DIMMs at 3 DPC.

The quad ranked 32GB RDIMMs support only 2DPC and are limited to 1333 MHz. With 40% more speed at 2DPC and the same capacity, and 50% more capacity (3DPC) in your server, the LRDIMMs are simply a vastly superior offering at the same cost. So we can safely forget about quad ranked RDIMMs.

Server Memory Worth the Price Premium?
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  • subflava - Thursday, December 19, 2013 - link

    Great article...look forward to more enterprise/IT professional based articles from Anandtech in the future. This is very timely for me as my company is just about to pull the trigger on a server upgrade. Interesting stuff.
  • JohanAnandtech - Friday, December 20, 2013 - link

    Thanks for sharing! :-)
  • DERSS - Friday, December 27, 2013 - link

    You guys are seriously super-cool; thanks.
  • wsaenotsock - Thursday, December 19, 2013 - link

  • blaktron - Thursday, December 19, 2013 - link

    Good article, although as an enterprise architect, I can tell you the one true benefit to LRDIMMS is in 2 and 4 socket vhost builds, because the double density RAM gives you the freedom to turn off NUMA spanning and still get near-ideal guest density.

    Almost nobody runs caching servers that big, although at almost double performance over a 256GB build (the 100k + concurrent user norm) its kind of attractive to run 2 of these per DC instead of 6 smaller ones (which would actually be the real world comparison with those kind of deltas).
  • mexell - Thursday, December 19, 2013 - link

    Real-world pricing, at least in the enterprise context, is quite a bit off from your numbers. In my employer's price bracket, we regularily buy similar servers as your 24*16GB config for about the same price (13k€) - but including a 3 year subscription VMWare Enterprise license, which is about 6 to 7 k€ on its own. No one pays list price on that kind of hardware.
  • JohanAnandtech - Friday, December 20, 2013 - link

    Are you sure that there is not a big discount on the VMware license? And smaller enterprises will pay something close to the list price. I know that the typical discount is 10-20% for smaller quantities, not more.
  • blaktron - Friday, December 20, 2013 - link

    Depends on the country Johan. The partner channel managers get to decide discounts on partner orders (which he is describing). Also, the bundling discount doesn't happen everywhere, but I could buy that server for like $15k CDN.

    The VMware license cost seems out of this world to me too, because we license our hosts for anyone from 2500 to 5k CDN, depending on their agreement with VMware.
  • mexell - Saturday, December 21, 2013 - link

    I don't really know where exactly the discount is applied, as the licenses are OEM and we don't get line-item pricing. In our market segment (large enterprise with Dell, medium-to-large with HP) we usually see at least 40% off on list prices, in some cases (networking equipment) up to 75%.

    VMWare, on the other hand, is especially rigid with their pricing structure. Two years ago, when we negotiated for a 100 host branch office deployment, they referred to their list pricing. For them, we are not even big enough to speak directly to us.
  • dstarr3 - Thursday, December 19, 2013 - link

    Wow. With 768GB of memory, I bet you could run Crysis.

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