Error msg : Virtual Machine has 8 virtual CPUs, but the Host only supports 4. The Number of virtual CPUs may be limited by the guest OS selected for the Virtual Machine.

[sunder143]

Hi All,

Good Day to ALL,

Hope you guys already had this question previously,but i hear i came accross to verify and get an idea of how to overcome this error...

• Powering on a virtual machine configured with 8 vCPUs fails
  
• You see the error:
  
  Error msg : Virtual Machine has 8 virtual CPUs, but the Host only supports 4. The Number of virtual CPUs may be limited by the guest OS selected for the Virtual Machine.
  

i have attached my configurations and screenshots..

Background:

I am try to a build a VM with 8 cpus on this envirnoment to deploy exchange 2010 apps,i got the above window stating the error.

So can any one help me to go through this with any tweaks or need to upgrade of licences ..etc...

Question:if i upgrade my environement from ESX4.0(Now) to ESXi5.0(Vsphere5) with same licences mode of Enterprise,will i get the same error while creating a VM with same configurations..??

[AWo]

Thread moved to the ESX 4 forum.

4.0 supports a maximum of 4 vCPUs per guest: http://pubs.vmware.com/vsphere-4-esx-vcenter/topic/com.vmware.ICbase/Welcome/welcome.htmlhttp://pubs.vmware.com/vsphere-4-esx-vcenter/topic/com.vmware.vsphere.installclassic.doc_40_u1/insta...

4.1 supports up to eight vCPUs.

How many cores does your host have in summary?

Do you really want to go with 8 vCPU's if you (may) have 8 cores at all? I would start with as less vCPU's as possible.

AWo

[Lee_Sargeant]

Hi,

vSphere 4 Enterprise will only allow 4 vCPUs whereas vSphere 5 Enterprise will allow 8 vCPUs.  Be careful as moving to 5 could introduce problems for your licensing if you currently use more than 64GB of RAM per licensed physical CPU.

However as AWo has already stated it is always worth starting with fewer vCPUs and increasing later if you have performance issues.  Starting with too many vCPUs can introduce performance issues not just for other guests running on the host but also for the guest with too many CPUs.

Regards,

Lee

[sunder143]

Hi Awo & Lee,

Thanks for the reply..

we have Licensed vSphere 4 Enterprise for 2 physical CPUs (1-6 cores per CPU),as of now we planned to move the VM to be build on 4 cpus as we dont have option to run on 8 cpus as mentioned in the links for enterprise mode licences.

going further if  ESX4.0 enterprise plus or ESX5.0 supports 8 cpus,can we increase the CPU's for existing VM(to which vmware has warning like changing the configuration for an existing server will make unstable...etc)...or do we have any specific procedure to be followed to increase the cpus to be perfectly set and work fine.

[Lee_Sargeant]

Changing from 4 to 8 should be absolutely fine just by powering down and changing the drop down.  Changing from multiple CPUs to a single CPU can cause problems for a Windows Server if it has never had a single CPU before due to changes in the registry.  Single CPU to multiple CPU or changing the number of multiple CPUs is fine.

Regards,

Lee

[AWo]

However, I would recommend to test if you really get a performance improvement, when you switch to 8 vCPU's. If you have 2 cores and one guest has 8 vCPU's, even if ESX has optimized the multiprocessorig, you might face situations, where alle 8 vCPU's must be free before the guest can be scheduled and that leaves to only 4 cores free for all the rest at a given point of time.

I wouldn't use 8 vCPU's just because it is possible and because it might improve performance on a physical box. Things are a little bit different when you run virtualized servers.

AWo