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I have a Windows Server 2008 R2 Datacenter with the Hyper-V role enable. I am planning to run 11 VMs (each with 4 vCPU) on this host. There is 1 CPU installed on the host computer. I would like to find out the number of virtual processors that can be supported on this host using this CPU, Intel Xeon Processor E5-2690

  • CPU count = 1
  • Core count (AKA logical processors) = 8
  • Virtual processors = 64?
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The reason for asking was to make sure the one cpu I'm buying is enough :) –  Hossein Aarabi Jul 24 '12 at 7:41
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2 Answers

up vote 2 down vote accepted

There is no hard proportion between virtual and physical cores. Of course, the very idea behind virtualization is that you overcommit resources (especially CPUs) to prevent vacancies of expensive and power-consuming hardware, but how many vCPUs you would be able to run on your hardware would depend on your load.

Start with an overcommit factor of 4-8, monitor the load and migrate virtual machines away as you see average usage values climbing over 70% of your total CPU capacity for prolonged periods of time (15-30 minutes) as this would indicate a CPU bottleneck.

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This post also explains it good: windowsitpro.com/article/virtualization2/… –  Hossein Aarabi Jul 24 '12 at 7:38
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Pretty much as many as you need. There is a hard limit, but it is quite high and wil lbe removed in the next Hyper-V generation (which is due in a week or two).

At the end, what you ask makes no sense because the limit is - IF you are limited - more one of performance, which does actually depend on how the CPU's are USED. if your physical CPU hits it's performance limit, then the number of virtual CPU's you can support is useless. You wont get more performance out.

You are a LOT more likely to be limtid by RAM or - most of the time - IO, than by a number like this which has no real meaning.

if you still want it ;) You are limited to 12 virtual processors per logical processor when all g host is is R2 Sp1 Sp1 and the guests run windows 7, otehrwise the limit is 8.

http://technet.microsoft.com/en-us/library/ee405267%28v=ws.10%29.aspx

That would put your limit to 64, or 96, depending on the clients. But again - this is a useless number most of the time.

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