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Physical resource vs Virtual resource

Posted on 2011-09-16
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Last Modified: 2012-06-27
Hi EE,

When you add or allocated addition CPU or RAM from the virtual guest server, is that equavalent to adding extra physical CPU or RAM in the physical environment.

What I noticed for instance particularly in VMWare, even though I have only dual core CPU on the vmware host server, I can still create multiple vmware guest servers and allocate two virtual CPUs to several of my virtual guest servers.

If that is the case, having only dual core CPU (physical) on the vmware host server, how is it possible that you can allocate more virtual CPUs to several vmware guest servers when you only have 2 physical CPU?

I thought you can only allocate CPU or RAM based from your available physical resource?

Appreciate your detail explanation?
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Question by:mcse2007
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LVL 121
ID: 36551908
When you add or allocated addition CPU or RAM from the virtual guest server, is that equavalent to adding extra physical CPU or RAM in the physical environment.

Yes, it is, but one of the benefits with VMware, is you can overcommitt memory, which is adding memory you do not physical have.

vCPU:- The sophiscated VMware Hypervisor - time slices or schedules the vCPU resource, so it is not a one to one relationship with the number of processors or cores.
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by:mcse2007
ID: 36552017
Hi hanccocka,

Regarding " time slices or schedules the vCPU resource", is that means, the VMware hypervisor does joggling of CPU resources from one vm guest server to then allocates resources to another vm guest server when needed?

For instance, the vmware host server has 2 physical memory allocated and it is hosting 4 virtual guest servers, each vm guest server is allocated 1 vcpu...totalling 4 vcpus all together. Any vm guest servers that on idle mode, the hypervisor allocates any unuse resources to vm guest servers that are busy processing requests? What happens to all vm guest servers busy processing requests, how will the hypervisor handles the slicing and scheduling of the vCPU resource?

Sorry to ask these questions, i'm kind of intrigue how the hypervisor handles such complex tasks.

Note: Don't forget to add me to your mailing list, for future nomination  for vExpert, next time comes around. My e-mail: lpalad007@gmail.com.
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Andrew Hancock (VMware vExpert / EE MVE^2) earned 250 total points
ID: 36552108
Yes,  the Hypervisor schedules every 20ms (don't quote me!) it's fast. So A vCPU =  A Core, but it may not be the same core for the duration of the running of the VM, because the Hypervisor shifts it to a different core. That's why we pay the money for VMware's hypervisor algorithm.

Here we go, all you need to know (you asked!)
http://www.vmware.com/files/pdf/perf-vsphere-cpu_scheduler.pdf

Checkout the Configuration Maximums for VMware vSphere 4.1, you'll see the numbers of VMs per Core etc
http://www.vmware.com/pdf/vsphere4/r41/vsp_41_config_max.pdf
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by:mcse2007
ID: 36552177
thanks to those links, particularly the first one, and cheers for the email...got it.
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