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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?

  • No, load reduced DIMMs and registered DIMMs cannot be mixed. – Michael Hampton Jul 28 '15 at 0:27
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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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    To be more precise, Crucial states that the system will refuse to start up "properly." Some machines may start up "improperly." For example, some systems might spit out warnings but still boot, with the mismatched RAM being unaddressable. Mind you, even if a system boots, leaving unaddressable RAM plugged in is a bad idea (and such warnings may stop or slow remote server reboots). – Brian Sep 7 '16 at 13:43
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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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    Actually, the registered and load-reduced variants don't address reliability but scalability. The buffering relieves the RAM controller line drivers and enables them to drive more chips. – Zac67 Apr 7 '18 at 8:46
  • I can't believe that someone actually came along and actually said "Actually, the registered and load-reduced variants don't address reliability but scalability" as if the ability to scale successfully had nothing to do with the reliability of the technology. – Hypocritus Apr 8 '18 at 16:45
  • Care to elaborate? Exactly how does scalability directly improve reliability? – Zac67 Apr 8 '18 at 17:02
  • My answer (bold for importance of purpose and message): Do not mix "registered" DDR4 RAM (RDIMMs) with "buffered" DDR4 RAM (LR-DIMMs). Not only will you not be able to scale, your RAM will be completely unreliable (until someone invents a controller that will drive both at the same time). The goals of the technologies are the same. But don't mix them. They are incompatible. This is my answer, and this is my point. Is this clear enough or should I add a caveat to every sentence I write in order to avoid being "actually"ed? – Hypocritus Apr 8 '18 at 17:29
  • And "No" I don't have a personality disorder. I don't know what you are talking about. – Hypocritus Apr 8 '18 at 17:36
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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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