VM Processor SIzing Best Practices (Full Version)

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tmiller@aviktech.com -> VM Processor SIzing Best Practices (24.May2012 12:41:34 PM)

I've been having a discussion with one of the other engineers in my office related to how many processors to assign to a virtual machine. Of course, this discussion is generic since ultimately it depends on what the VM is used for but...

All other things being equal, how many processors should a VM be given? My practice is to start with just a single virtual processor unless the workload requires more. I figure if a particular virtual server is running just fine with a single processor (say 5 to 20% processor utilization) without any performance issues that adding more processors to it will have zero impact on that VM's ability to perform. Only giving it what it needs will preserve my flexibility and scalability in the future for other VM's. If I need to add proc later, I can always do so.

My co-worker takes the approach that if a host has plenty of processor resources that we should go ahead and assign a minimum of two processors per VM (assuming of course the guest VM supports multi-processors) as a "just in case" configuration. While I understand this approach, it seems to me to be stuck in the physical world where it was difficult and expensive to increase processors after the fact. This is not a limitation in the virtual world.

What does everyone else think? What is VMware's "best practices" guidance on this topic?

Scott Lowe -> RE: VM Processor SIzing Best Practices (27.May2012 8:46:43 PM)

I'm of the mindset that if a VM requires only a single processor that it gets only a single processor. Once you add that second vCPU to a virtual machine, that virtual machine's behind the scenes processor scheduling duties become more complex, potentially impacting performance of both the virtual machine and the hypervisor.

If something needs more CPU, have at it since there is a need, but if not, stick with a single vCPU.


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