Memory not at full speed in Poweredge R920

[chrgrose]

I have a Poweredge R920 with 4x 4980 v2. My goal is to fill it with 96 16gb dimms running at 1600 MHz. So far I've installed 32 dimms (all whites slots) which are a mix of SK Hyinx 2Rx4 PC3-12800R and low power Samsung 2Rx4 PC3L-12800R dimms. I expected that mixing these modules would result in all modules running at 1600 MHz, but at normal 1.5 voltage. The only other difference among these modules is that the samsungs are a mix of 11-13-E2 and 11-11-E2 timing modules. I don't think that matters.

Unfortunately, the R920 only runs these modules at 1333MHz (in 'Performance mode'). I've tried to sort this out with the R920 manual but I am confused by the supported memory configurations tables (pg. 42 in the manual). I don't see an RDIMM 16GB 1.35V configuration, for example. It also seems that when I eventually populate all channels the operating frequency might only be 1066 MHz? Really?

Is anyone knowledgeable in this area? Given my mix of dimms, can I get the R920 to operate at 1600 MHz with 96 16GB modules? Or did I waste my money paying the premium for higher speed RDIMMs?

Thanks!

R920 manual: https://topics-cdn.dell.com/pdf/poweredge-r920_owners-manual_en-us.pdf

 

[Evan]

Yes populating all slots with RDIMM and not LRDIMM has a dramatic drop in speed often in those generations.

You really can’t mix memory.

 

[chrgrose]

The manual also says that it supports octal rank dimms, but there are no octal rank cases in the memory configuration table. It only has up to 'quad rank physical' listed. Looking at the market I can only find octal rank 64GB DDR3 LRDIMMs. Would these still work? I know I cant mix RDIMM and LRDIMM.

 

[Evan]

Never seen an 8R regular dimm. Only ever huge LRDIMM as you have found.
Afraid no idea at all if they would work.

 

[acquacow]

I've never seen a server that runs the memory at full speed with all dimms populated, or at max dram capacity. Usually it's about half the max speed.

 

[Evan]

I've never seen a server that runs the memory at full speed with all dimms populated, or at max dram capacity. Usually it's about half the max speed.

Scalable onwards mostly no issues where it’s configured as 2 bands of 6 per cou only, but the 3rd bank (dimm’s 9-12 per cpu) on e5 systems your right, they all take a performance hit.