Changing Processors on Virtual Machine

[bwaldon]

Hello Everyone,

I have a question about changing processors on a virtual machine.

Enviornment: vSpere 4.0, ESX server has 2 quad core processors, with hyperthreading enabled. Virtual Machine (1 of 😎 is Server 2008 x64. VM currently has 2 CPUs. Virtual Machine is stored on iSCSI SAN. this virtual machine was built as a virtual machine - it's not a conversion. Single ESX host, 8 virtual machines, all 8 stored on a SAN.

So, I have 8 Machines running against 16 logical processors. Between all of the machines, I have 19 processors provisioned (1 VM has 4, 1 VM has 1, the rest has 2). I am also using reservations to prioritize the most important VMs.

Problem: HIgher than expected processor utilization is affecting server response time on a particular VM.

Solution: Provision 2 or 6 more processors for that VM. ( I am actually looking into some high consuming processes seperately, but am posting this discussion to address the feasibility of adding more processors).

However, The vSphere Client tells me "Warning: Changing the number of virtual processors after the guest OS is installed may make your virtual machine unstable".

So my questions are:

1. What is that all about? I thought that a major benefit to virtualization is the ability to provision resources dynamically.

2. Is there any particular factors that might make the VM more or less unstable be increasing the processors?

3. Is this something that might only apply to reducing the number of processors?

4. If it does make my virtual machine unstable, can I change it back to make it stable again?

5. If it does make my virtual machine unstable, is this something that I am going to know right away?

6. Is there anyway to mitigate this risk?

7. Given the above enviornment, do you see a problem with provisioning VMs with 4 or 8 processors?

(I am thinking that I should avoid provisioning a single VM with 16 processors or provisioning 32 processors as a whole).

8. Any other questions / comments / concerns?

9. (seperate but might be related matter) One of my other virtual machines is a conversion and occasionally locks up out of the blue. the conversion took the OS from seeing 1 processor to seeing 4. Does the above warning apply to conversions? And might that be the cause of the locks ups.

Thanks in advance. I look forward to your feedback.

[weinstein5]

1) Yes you can provision dynamically but it is still going to depend on the operating systems ability to accdept those dynmic changes -

2) yes the operating sysytem -

3) no can apply to increasing as well -

4) I would think so but depends on how unstable it became - if it blue screened no -

5) I would think so - but am not sure on this one

6) understanding the o/s you are using -

7)first off you can not provision a VM with 16 virtual CPUs - the maximum with the enterprise plus license is 8 vritual CPUs - yes I do see a potential issue best practice is to start off with single vCPU - this makes it easier for the vMkernel to schedule the virtual cpu of the VM

😎 My recommendation would be to drop alll your VMs to a single vCPU and check performance - I think will find it acceptable -

9) confirm that the HAL is set to the multi-proc HAL - but I would try running it with a single vCPU first -

If you find this or any other answer useful please consider awarding points by marking the answer correct or helpfulSo my questions are:

[AntonVZhbankov]

>Solution: Provision 2 or 6 more processors for that VM.

If 2 CPUs are enough for VM then you should consider using reservations and give more shares to this VM. Adding more CPUs to vSMP machine on CPU-loaded host can bring you to lower performance.

>However, The vSphere Client tells me "Warning: Changing the number of virtual processors after the guest OS is installed may make your virtual machine unstable".

I suppose you want to Hot-Add more CPUs, without powering VM down first. Windows requires reboot after that anyway, so you can power VM off, add more CPUs if you want, and power on. This way will NOT bring you to any instability. But as I already said it can bring you to lower performance.

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MCSA, MCTS, VCP, VMware vExpert '2009

http://blog.vadmin.ru

[bwaldon]

Thank you,

Just to clarify, I do not intended on adding them "hot". I expect to shut the VM down, then add the additional processors, and then power it on.

Maybe the use of the word "dynamic" was misplaced.

The VM currently has 2 CPUs, so it should already have a Multi-CPU HAL.

I am going to check into the shares. I currently have 1000Mhz reserved, but maybe I need to reserve more for this VM.

[AntonVZhbankov]

So then no problems, but be aware of vSMP performance issues.

---

MCSA, MCTS, VCP, VMware vExpert '2009

http://blog.vadmin.ru