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What's New in Greener DDR3 Server Memory

Low Voltage "DDR3L" server memory is expected to significantly grow in market share in 2011, as DRAM manufacturers increase their DDR3L output and lower their manufacturing costs. DDR3L memory may help you lower your server memory's power consumption by up to 15%. Read on for details on how DDR3L memory needs to be configured in your servers.

DDR3 memory runs at 1.5V and currently supports speeds ranging up to 1333MHz on servers. To allow for more-energy efficient servers, JEDEC also defined Low Voltage DDR3 memory, which runs at 1.35V; this memory is referred to as "DDR3L" and modules are classified as PC3L.

The following table shows all the key information for DDR3 and DDR3L memory:

It is important to note that DDR3L memory modules can run at either1.35V or the standard 1.5V. These modules are actually dual-voltage, and their configuration can get especially tricky on Intel Xeon 5600 series servers, as well as the AMD Opteron 6100 series servers.

Let's look at the Intel Xeon based servers specifically.

In older "Nehalem-EP" or Xeon 5500 series servers, DDR3L memory will only run at 1.5V, because those servers lack the hardware support needed to allow for 1.35V memory operation.

In newer "Westmere-EP" Xeon 5600 series servers, DDR3L memory can run at 1.35V but must meet specific configuration rules listed below:

  1. DDR3L memory should not be mixed with standard DDR3 memory: when there is a mix, all memory modules will default to 1.5V to accommodate the older memory modules which cannot run at 1.35V.
  2. Server manufacturers may enable 1333MHz memory performance modes that raise voltage to 1.5V. When this happens, the energy-saving benefit of DDR3L memory is traded off for increased bandwidth.

For example, our testing shows that HP ProLiant® G7 servers using Intel Xeon 5600 processors with 1333MHz memory controllers can support:

  • 1 DIMM per Channel at 1333MHz @ 1.35V
  • 2 DIMMs per Channel at 1333MHz @ 1.5V
  • 2 DIMMs per Channel at 1066MHz @ 1.35V

In the 2 DIMMs per Channel (also called "2DPC") case, the server's BIOS must be set to disable 1333MHz memory speed with processors that support it; if a processor that supports 1066MHz memory speed is used, there will be no need to disable the 1333MHz memory speed. So, 2DPC configurations must be run at 1066MHz to achieve the energy efficient mode of 1.35V DDR3L memory.

3DPC configurations get trickier and we did see differences in how OEM servers support it:

  • HP: 3DPC at 800MHz @ 1.35V
  • Dell: 3DPC at 800MHz @ 1.5V

Of course, OEMs could change these numbers via future BIOS updates, so you will need to make sure you are using the latest BIOS for your Xeon 5600 series servers when you install DDR3L memory. If you add Kingston DDR3L memory to your servers, you need to remove any pre-loaded server memory that may not be 1.35V Low Voltage DDR3L and follow the configuration rules for your server.

Author: Rich Kanadjian - Kingston

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