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Co-Relation between vCPU & Server cores

Posted on 2011-02-22
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Hi.

Can some one please explain the relationship between Server cores & vCPUs.

The servers which we use for virtualizaton are Dual core or Quad core or 2x Quad Core.

When we create a VM on the Hypervisor, it asks for vCPU.

How do we best optimally use the hardware.

For example, if a server requires Quad core processor & if i have to virtualize the server, how many vCPUs do i allcate so as to not to impact the performance.

Is there any relationship between them?

Regards,

Rohan
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Question by:Rohan_Singnapurkar
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Lee W, MVP earned 2000 total points
ID: 34957822
Each Core is a CPU.  Virtualization doesn't provide multiple cores to the VMs, it sees them as individual CPUs.  So if you have a server/app that wants 4 CPUs (or a quad core), you have to assign 4 cores.

Put another way, a multi-Core processor is really just one "chip" with multiple CPUs on it.  So Virtualizations products should allow you to assign up to the number of cores you have (viewing each core as a CPU)
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by:Rohan_Singnapurkar
ID: 34958063
can you please elaborate on the same by providing one example.
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by:Lee W, MVP
ID: 34958275
I have provided two.  If you don't understand what I'm saying, please clarify what you find unclear and I can try to elaborate on that.
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Author Comment

by:Rohan_Singnapurkar
ID: 34958361
Does this mean that in Virtualization, each core is treated as a CPU. If so then wot is the co relation between a CPU & a vCPU.

For example an Hyper V supports 6 - 8 VMs per core. Does this mean that 1 CPU = 8vCPU.

Also if I have to create a server which in a physical environment requires Quad core, should i then be assigning 32 vCPUs to the Server as we cannot assign CPUs in a virtual environment.
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by:Lee W, MVP
ID: 34958410
> Does this mean that in Virtualization, each core is treated as a CPU.
My response started with "Each Core is a CPU."

you seem to be thinking of the "chip" installed in the computer as the CPU.  It's not.  It's more of an "add-in" card that contains 2, 3, 4, 6 or more CPUs - these are generally referred to as "cores".

a vCPU is a virtual CPU - a single virtual core.

Total Physical Cores = Total Virtual CPUs
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Expert Comment

by:ACourvoisier
ID: 37854834
Hi,

A large explanation there, in english. Very well explained (but I don't access to the images):

http://complaintsincorporated.com/2011/02/21/vcpu-pcpu-lcpu-ht-and-you/

of 21 feb. 2011.

In matter, I understand that VMware recommanded to allow 4 vCPU max per core.
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