Mixing DIMM sizes in UCS

[dani_bosch]

Hello,

As per B-200M2 Spec Sheet, is not recommended to mix DIMM sizes within the blade:

http://www.cisco.com/en/US/prod/collateral/ps10265/ps10280/spec_sheet_c17-644236.pdf

But as per C-200M2 Spec Sheet, it IS recommended to mix DIMM sizes...in fact one of the best-performing scenarios is actually mixing 4GB DIMMs and 8 GB DIMMs:

http://www.cisco.com/en/US/prod/collateral/ps10265/ps10493/c200m2_lff_specsheet.pdf

Taking into account that C-200 M2 and B-200 M2 are de facto the same server, these two sentences are contradictory...

Any insights on this?

Thanks,

[David Alpizar]

Hi ,

There are some facts that can be taken into consideration with combining DIMMs on the servers.

I have seems cases where the server where the server works with a combination of DIMMs.

Please notice the following:

DIMMs within the server should all be the same type and speed. If you mix different DIMM types or DIMMs with different clock rates in the same server, such configurations are supported; however, these configurations may cause the system to operate at less than optimum levels. Doing so will cause a significant loss of performance.

Your selected CPU(s) can have some effect on performance.

Each of these arrangements can provide different behavior.

http://www.cisco.com/en/US/partner/docs/unified_computing/ucs/c/hw/C200M1/install/replace.html#wp1080310

David

[dani_bosch]

David, I'm actually complaining that the info is contradictory...the document you say reads exactly "

DIMMs within the server should all be the same type, speed, and size"...that's totally contradictory with page 47 of this document:

http://www.cisco.com/en/US/prod/collateral/ps10265/ps10493/c200m2_lff_specsheet.pdf

And that's precisely what I'm asking for...who should I trust??

[Zaira Vega]

Hi,

Actually this document:

http://www.cisco.com/en/US/prod/collateral/ps10265/ps10493/c200m2_lff_specsheet.pdf says the same as the other one in page 46, and it makes reference to the table on page 47, where it shows the recommended DIMM configurations:

DIMMs within the server should all be the same type and speed. If you mix different DIMM types or DIMMs with different clock rates in the same server, such configurations are supported; however, these configurations may cause the system to operate at less than optimum levels. See Table 25 on page 47 for recommended DIMM combinations.

Basically, you cannot mix DIMMs with different clock rate and rank type.You can confirm this with that table. Each configuration shows the same clock rate, no matter the DIMM size.

In any way, I recommend you to follow this one:

http://www.cisco.com/en/US/prod/collateral/ps10265/ps10493/c200m2_lff_specsheet.pdf

because it is the most updated.(Reviewed on JULY 10, 2012)

http://www.cisco.com/en/US/prod/collateral/ps10265/ps10493/c200m2_lff_specsheet.pdf

[dani_bosch]

Zaira, I still don't see any coherence in all of this: you say I cannot mix DIMMs with different rank type, while in the PDF you mention there's one config (72 GB mixing 4GB single rank and 8GB dual rank DIMMs) that has maximum performance.

So again, there's some info missing here, or some parts are wrong and some parts are correct...

I think this is a very key design info that should be much clearer...

Any more insights?

[Robert Burns]

We've recently started to ship single rank 4GB DIMMs now, whereas 8GB DIMMs are still dual rank.  With today’s DIMMs, mixing 4GB and 8GB DIMMs in a configuration now means mixing single and dual-rank DIMMs, making this 72GB configuration a little less “sweet” than before.

The mixing of different ranks has more impact to performance than mixing DIMM sizes. 

I'm working to have our Memory Guides updated accordingly.

Regards,

Robert

[dani_bosch]

Now Robert, for the sake of good design, can we say the table 25 (page 47) in the C-200 M2 Spec Sheet would be 100% correct should the 4 GB DIMMs be dual rank instead of the currect single rank?

Thanks,

[Robert Burns]

100% - No.  Is it the best configuration to achieve 72GB - absolutely with all dual rank DIMMs.  I just wouldn't classify it as a "sweet spot" as different mixing density DIMMs will not perform as well as like densities (though the performance hit is very minor). 

In my opinion the only sweet spots would only be the evenly distributed, semetric population or exact DIMMs.

Robert