DDR3 memory technology

architectures beginning with HP ProLiant G6 and G7 servers eliminated the need for an FBDIMM

solution with DDR3.

Load Reduced DIMMs

Load Reduced DIMMMs (LRDIMMs) are a new type of DDR3 memory to go alongside of the

Unbuffered (UDIMM) and Registered (RDIMM) DDR3 memory. LRDIMMs buffer the memory address

bus and the data bus to the memory controller by adding a full memory buffer chip to the DIMM

module. Unlike the Fully Buffered DIMMs (FBDIMMs) from DDR2 days, Load Reduced DIMMs deliver

data to the memory controller in parallel rather than using a high-speed serial connection.

Because LRDIMMs are completely buffered, they have both advantages and disadvantages when

used in a server. They include:

• Support for the maximum amount of memory per channel. You can install as many as three

LRDIMMs on a single memory channel. More important, with LRDIMMs you can now install three

quad-ranked DIMMs on the same channel. You can only install two quad-rank RDIMMs.

• Increased power consumption. The addition of the buffering chip and the higher data rates increase

DIMM power consumption.

• Latency. For equivalent memory speeds, buffering requires adding clock cycles to memory reads or

writes, increasing the latency of LRDIMMs relative to single and dual-rank RDIMMs. This condition is

offset by the fact that on fully populated memory channels LRDIMMs operate at a higher speed than

quad-rank RDIMMs, resulting in a comparatively lower overall latency.

Table 1 compares the operating characteristics of a quad rank LRDIMM with those of an equivalent

RDIMM.

Table 1: Operating characteristics of Quad-rank LRDIMM versus RDIMM

32 GB Low Voltage

Quad-rank RDIMM

(ProLiant G7 )

32 GB Low Voltage

Quad-rank LRDIMM

(ProLiant Gen8)

LRDIMM

Advantage

Max. DIMMs per channel 2 3 50%

Max. Capacity per channel 64 GB 96 GB 50%

Max. Bandwidth per

Channel

6.4 GB/s (@ 800 MT/s) 8.5 (@ 1066 MT/s) 33%

Idle Latency 80 ns 74 ns 7%

Max. Power 6 W 9W -50%

LRDIMMs allow you to configure large capacity systems with faster memory data rates than quad-rank

RDIMMs. ProLiant Gen8 servers support LRDIMMs as well as UDIMMs and RDIMMs, although you

cannot mix DIMM types within a single server. G6 and G7 servers do not support LRDIMMs.

LRDIMMs and rank multiplication

Load reduced DIMMs are the first DDR3 DIMMs to support operation of three quad-ranked DIMMs on

a memory channel. DDR3 memory architecture can only natively support a maximum of eight ranks

per channel, not the 12 ranks required for three quad-rank DIMMs. LRDIMM architecture solves this

issue using by using a design known as rank multiplication.