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Josh Brower
Asked: 2011-04-01 19:56:26 +0800 CST

Hyper-V Cluster: More memory or faster memory

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I am working on pricing out 4 Dell R710s for a new Hyper-V cluster. I have a choice between the following two configurations, for the same pricing:

128GB of 800Mhz RAM per node

Or

96GB of 1333Mhz RAM per node.

CPUs are dual Xeon E5645.

It would be more ideal if I could go with the 128GB option, but I don't want it to come at a huge performance cost...

In this type of a situation, would the difference in the RAM clock speed produce noticeable performance gains or losses in a typical VM environment?

I could get away with the 96GB for at least 2 years.... So is the performance boost worth it?

hyper-v

3 Answers

  1. Best Answer

    I would go with 96GB, not for the reasons stated, but due to the limitations and configuration options of the R710 server. For best performance, you will want to use what Dell calls "Optimized Mode" for your memory and fill all three channels per processor. By utilizing all three channels, each memory controller can be independently processing different requests.

    However, if you fill all three DIMMs in a channel, the bus speed will automatically be clocked down to 800Mhz. In order to keep a faster bus speed, you can only fill two DIMMs per channel, and even in that configuration, the memory will be clocked down to 1066Mhz. 1333Mhz is only available with a single DIMM on each channel which would limit total capacity to 48GB.

    With this in mind, I would install 12x8GB 1066Mhz dual-ranked DIMMs, placing 2 DIMMs in each channel on each processor. The DIMM slots used would be A1 through A6 on processor 1, and B1 through B6 on processor 2.

    The last benefit of this configuration is that you could easily add another 48GB to the server down the road.

    Sources:
    Dell 11g Memory Whitepaper
    Dell PowerEdge R710 Owners Manual

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  2. The 128GB will not only be slower per-stick, it will also not be tri-channel configuration. The faster RAM + tri-channel will probably make a noticeable difference.

    That said, you have to size based on your VM load. If you will not have enough free memory to run all VMs on 3 hosts (in case of a host failure), you should go with the 128GB. Otherwise, go for the speed!

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  3. I agree with some of the previous posters. The biggest question you need to ask is if you are going to run low on RAM. Also using more than %80 of physical RAM for normal use is dicey. Of course peak usage is one thing, but while faster RAM is better if you're not going to have enough that is a big issue.

    Also where is your bottleneck going to be? Are you using a SAN or DAS? If you max out your storage arrays IO then the faster RAM is not going to do you that much good, and the extra RAM will matter more.

    If you aren't going to max out your RAM and you're running an enterprise SAN (LSI, EMC etc) then the faster RAM would be a better bet.

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