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Arseni Mourzenko
Asked: 2015-07-28 16:20:41 +0800 CST

I shouldn't use both RDIMMs and LR-DIMMs on the same motherboard, right?

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I tried to use on the same motherboard at the same time:

  • Three LR-DIMM memory sticks,

  • Four RDIMM memory sticks.

The server refused to start; when I use either three LR-DIMM sticks or four RDIMM sticks, the server starts successfully. However, I haven't tested different configurations and different slots to be perfectly sure that it's the mix of two types which prevents the server from starting.

According to the manual of this motherboard (Asus Z9PR-12):

You may install 1GB, 2GB, 4GB, 8GB, 16GB and 32GB* RDIMMs or 1GB, 2GB, 4GB and 8GB* UDIMMs or 8GB, 16GB and 32GB* LR-DIMMs into the DIMM sockets using the memory configurations in this section

I'm not sure whether the usage of “or” in the manual shows that one cannot and shouldn't mix memory of different types, such as, in my case, LR-DIMM with RDIMM.

Questions:

  • In general, is it common for server motherboards to support only one type at the same time? Could it be possible that in my case, the server wasn't starting for some other reason, such as wrong slots being used?

  • Technically speaking, what is preventing the motherboard from using memory of different types at the same time? If two CPUs are used, can I put LR-DIMM memory to the slots associated with CPU 1, and RDIMM memory to the slots associated with CPU 2?

  • Was I at risk of damaging either the motherboard or the memory by trying to use both types at the same time?

memory

3 Answers

  1. Best Answer

    According to Crucial, a RAM brand, the system would refuse to start up with mixed types.

    http://forum.crucial.com/t5/tkb/articleprintpage/tkb-id/dram@tkb/article-id/86

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  2. I have mixed RDIMM with LRDIMM in Dell Server PowerEdge T420 model then server showing mixing error,finally server not started. So don't try to mix in this way.

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  3. This is correct. You should NOT mix RDIMMs and LRDIMMs. They are different technologies.

    RDIMMs have a "register" which is like a partial "buffer", but does not fully buffer the signal like LRDIMMs.

    LRDIMMs fully buffer the signal with what is called, you guessed it, a "buffer"!!!

    This causes two different communication styles between these two different varieties of RAM, although the goal is the same: reliability. If you want reliable RAM, you cannot mix these different technologies.

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