What Are Memory Ranks, Why Do We Have Them and Why Are They Important?

-written by Jeff Goldenbaum, Dataram Memory Blog Team Member

Comparing Quad-Rank and Dual-Rank Memory Modules

The term “rank” simply refers to a 64-bit chunk of data.  In its simplest form, a DIMM with DRAM chips on just one side would contain a single 64-bit chunk of data and would be called a single- rank (1R) module.  DIMMs with chips on both sides often contain at least two 64-bit chunks of data and are referred to as dual-rank (2R) modules.  Some DIMMs can have DRAM chips on both sides but are configured so that they contain two 64-bit data chunks on each side—four in total—and are referred to as quad-rank (4R) modules.  Quad-rank DIMMs run at a maximum PC3-8500 (DDR3-1066) speed in current architecture.

Why Have Quad-Rank DIMMs?

With each new generation of DRAM chip, quad-rank DIMMs are the least expensive way to achieve the highest density DIMM.  Today, a 16GB dual-rank DDR3 DIMM is built with thirty-six 4Gb (gigabit) chips.  A quad-rank 32GB DIMM will utilize thirty-six “dual-die” (essentially two DRAMs within each physical chip) 4Gb chips.  Until the next generation 8Gb chips become more mainstream and production costs come down, we won’t see a cost-effective dual-rank 32GB DIMM.  But when that does occur, you’ll not only see dual-rank 32GB DIMMs, but you’ll also see quad-rank 64GB DIMMs!

Quad-rank DIMMs actually perform best in Xeon 7500 CPU based systems as the memory BOB (buffer-on board) architecture delivers better performance compared to single- or dual-rank DIMMs of equal capacity.  The maximum speed is still PC3-8500 and is consistent regardless of whether the system has the minimum configuration or is fully loaded.

When to Consider Ranks

Today’s 2-way servers with Intel Xeon 5600 series processors that utilize DDR3 memory technologies have limitations when it comes to how many ranks of memory may be installed.  DIMM slots are configured as “channels” with either 3 channels of 2 DIMM slots each (server models with 12 DIMM slots), or 3 channels of 3 DIMM slots each (server models with 16 or 18 DIMM slots).   Each channel supports no more than 8 ranks of memory.  Simply put, no more than 12 quad-rank DIMMs can be installed in these servers.  To utilize all available slots in servers with 16 or 18 DIMM slots, the use of dual-rank DIMMs is required.

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