HyperV virtualization

Hello Friends,

I have a question on Hyper V. We have server with two processor each with Intel Xeon 6 core 2.4 GHz processor. So a total of 12 cores available. Right now I have installed 128 GB ram with planned 16 VM's. What is the maximum number of VM's we can go for? What is the max recommended ram for that server considering 8 GB ram per Guest VM? Somewhere I saw the total VM's could be 4 times the total cores available provided with sufficient Memory and Storage but not sure if thats the calculation?

Looking forward for expert advise

Thanks and Best Regards
Sri MCEOAsked:
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David Johnson, CD, MVPOwnerCommented:
That is probably about the maximum but it is a it depends upon how many iops your disk subsystem can handle and the nature of the tasks running on each vm.. for instance an office worker can pretty much use a low end machine, where a productivity worker that is running large excel tasks, compiling code needs more resources, a video editor needs maximum disk i/o and cpu..
Tom CoxCommented:
I'm not sure there is a truly accurate calculation. To give you an idea I have 3 HP Gen8 with 16 cores and 196GB of RAM each and run a total of 40 VM's. When updating I'll put 20 VM's on one host and that is fine but I think too many more would push it.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
We work on 5-6 VMs per Core on the physical processor.

BUT CPU will not be your bottleneck, Memory will be the bottleneck.

So how much memory will you be assigned to each VM. If you assign 4GB to each VM, that's 32 VMs.
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Lee W, MVPTechnology and Business Process AdvisorCommented:
Unless you overcommit.

This is another question that I don't understand how one can give any reasonably  accurate answer to.  It depends on your environment and usage.  If these are web servers, maybe x number... but if they are heavily used web servers, then maybe y number.  Are you using dynamic memory?  On all systems?  I would say EVERY install WILL DEPEND on usage.  If you boot up 100 linux VMs off ISO images from DVD, your disk IOPS are not nearly as important.  What are the VMs running...

There's no way, in my opinion, that a reasonable person can answer a question on how many VMs a system can handle (at that scale) when reasonably detailed usage isn't known.  Everyone, in my opinion, is guessing based on what they believe your usage will be but only YOU can know with any certainty what it will and COULD be.
Sri MCEOAuthor Commented:
Hi Lee,

I do understand one cannot advise accurately however on a average we can guess. I was trying to check if there is a benchmark to gauge this.

Thank you all for sharing your views and appreciate it

Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
5-6 VMs per Core of physical processor, as a guide.
Lee W, MVPTechnology and Business Process AdvisorCommented:
I would suggest your best answer is one you arrive at by yourself - by studying your system usage currently, the performance increases expected from the hardware you're migrating to (or starting with), and setup test environments to determine what is appropriate for you.  Some people would say I'm crazy to run 8 VMs on 32 GB of RAM with a mirror and 8 AMD cores... and depending on usage, I might be... or it might be JUST FINE with room to spare.  *I* know my loads and what I can expect AND what is acceptable performance.  You need to establish that for yourself through experience, analysis of existing systems, and trial and error in my opinion.
Lee W, MVPTechnology and Business Process AdvisorCommented:
And keep in mind, CPU allocation and load will greatly affect the overall performance.  If you provision every VM with 8 logical processors, you might slow everything down significantly than if you provisioned all systems with only 2 logical processors.
Philip ElderTechnical Architect - HA/Compute/StorageCommented:
That depends.

When we deploy a solution we look at:
 + VM IOPS Requirements
 + VM vRAM Requirements
 + VM vCPU Requirements

In that order.

The disk subsystem tends to be the first bottleneck. We'd be at 16 to 24 10K SAS drives at the minimum for this setup with a 1GB hardware RAID controller (non-volatile or battery backed cache). RAID 6 is our go-to for array configuration. Depending on workloads one can look at Intel's DC S3500 series SSDs or the higher endurance models to get more IOPS out of the disk subsystem.

Keep in mind that the physical RAM is split between the two processors so one needs to be mindful of how the vRAM is divvied up between the VMs. Too much vRAM on one or two VMs can cause the physical RAM to be juggled between the two physical CPUs (NUMA).

And finally, each vCPU within a VM represents a thread to the physical CPU. Every thread (vCPU) for a VM needs to be processed by the CPU's pipeline in parallel. So, the more vCPUs we assign to a VM the more the CPU's logic needs to juggle the threads to have them processed. More vCPUs is not always better.

I have an EE article on Some Hyper-V Hardware and Software Best Practices that should be of some assistance too.

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Sri MCEOAuthor Commented:
Thank you Andrew and Lee for the suggestions.

Thank you Philip I completely agree with you. I guess we can only come to a perfect solution by optimizing our hardware.
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