Server Buying Decisions: Memoryby Johan De Gelas on December 19, 2013 10:00 AM EST
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.