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System:

  • PowerEdge R430
  • Intel Xeon E5-2440 (dual)
  • 32GB RAM
  • Hyper-V 2012 R2

VMs:

  • DC - 8GB RAM
  • APP - 19GB RAM

The VM's are not configured for dynamic memory.

The client was complaining about poor APP server performance. In looking at system resources (Host)I noticed that the available RAM was only 71MB. The available RAM used on the APP server was only 4GB.

I attempted to look for documentation indicating how much RAM should be allocated (best practice) to the host OS but only found hits on configuring RAM for VMs.

Before making any changes, I am trying to see if I can find more concrete information on this.

Please let me know if I am missing any information and I will be happy to edit my post.

Thank you.

  • Before you start blaming this on memory, have you collected data on CPU, Network, Disk IO, etc.? – EEAA Nov 10 '15 at 16:30
  • Not directly blaming memory - just trying to find information on best practice on allocating memory to the host OS. The CPU utilization was under 10% and the disk IO was low (ie <1 and response times under 10). Network was also 200Kbps. – Mike66350216 Nov 10 '15 at 16:54
  • Is sr-iov enabled? – Jim B Nov 10 '15 at 17:40
  • A DC with 8GB of RAM? That seems like overkill. You could probably do with 1 to 2GB (assuming by DC that you mean Domain Controller). Does the APP server really need/require 19GB of RAM? It sounds like you're over-provisioning your VM's. – joeqwerty Nov 10 '15 at 18:04
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I would leave 2-4gb of that machine unallocated.

That said, to answer the question: you can NOT overallocate with the exception of something like the initial startup memory. But no, generally not.

If the app server has 19gb ram allocated, it uses a little more than that on the host, and it does so all the time.

Now, the main question is why the app server has only 4gb memory. Can it be someone installed the app server as 32 bit operating system? It would not really matter how much you give the VM in that case ;)

And obviously please check disc IO. Discs are generally a weak point in computrs - and a terrible weak point once you run multiple computers against the same discs. Basically somene needs to do a baseline performance profile from within the app server to see where it hurts.

But that is not part of your question. your question was specific: Possible over allocation to guest VMs? - and the answer is no.

  • I apologize if I was not clear. The APP server was configured with 19GB of non dynamic memory but was only using 4GB (Server 2012R2 is the guest OS). I am looking for a best practice document on how much RAM to reserve for the host OS. I gave an example as to why I am looking for this but perhaps that was more confusing than helpful. – Mike66350216 Nov 10 '15 at 16:58
  • This is what I discovered. The wandering ServerFault question/answer is here. Essentially, once you get over 24GB RAM on a host, you have to start paying attention to RAM lost to the Nonpaged pool. – Granger Nov 18 '16 at 3:53

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