One min design guide of 40, 2GB VM’s

Inspiration is coming from word “Let’s challenge”, If I know something then I must be in a position to explain it well and how much well I know, it would be seen in the comments. So here is design requirement, create virtual farm for 40 Virtual Machines with each 2GB RAM.This is strict requirement.

Total RAM Required

40 VM’s X 2GB (RAM) =80 GB RAM

Design best practice is to keep 4GB/Core (Reference: Advanced Technical Design Guide (Ron Oglesby, Scott Herold and Mike Laverick)

So we will need 20 Cores (80/4=20)

Hardware Selection

I chose 8 core machine i.e. 2 Way Quad Core machine (is explained in next blog)

8 Cores and 4GB/Core, I can easily host 32 GB RAM in total. So hardware will be 2 Way Quad Core and with 32 GB RAM

Total Number of Virtual Machines

Taking 4GB/Core and 2GB requirement for each VM, We can host only 2 VM’s/core.

So  2(VM’s/Core) X 8(Core) =16 VM

Out of total compute capacity let’s reserve 10% for service console, VMKernel and other host management agent.

10% of 32GB RAM comes to =3.2 GB RAM approx 4GB (I know very high amount)

So we are left to manipulate only 28 GB RAM (32 – 4 ) i.e. 4GB RAM less which means 2 VM’s less.

So by this calculation we can host safely(Very Very) 14 VM’s/Host.

Total Number of Host

We are asked to host 40VM’s.

40 (VM’s)/14 (VM’s /Host)=2.8 approx #3 Hosts

14 (VM’s/Host)  X 3 (Hosts) =42 Total virtual machines

And to consider a failover capacity of one host, we can add one more host.

Total number of host will be 4 with one host failover capacity. Failover capacity will also take care of maintenance activity like patches


So total Memory resources =32GB RAM X 4 host =128 GB RAM

Total VM’s=40

VM’s/ Host=40/4=10 VM

If one host fails, 3 host will be able to accommodate 40 virtual machines i.e. 40/3=13 VM’s host.(We designed 14/host, which means we can add 2 more VM’s)

This is one min design guide. There are still many parameters to consider. Definitely if we need to consider over committed benefit through TPS,Ballooning, I think we can add more VM to this 4 node cluster. But how many more VM, that is topic for next blog.

7 comments on “One min design guide of 40, 2GB VM’s

  1. Pingback: So why 2 Way Quad core ? « VCP4.0 Prep Blog

  2. Pingback: Few thoughts on Cluster size « Designing vSphere

  3. Hello,

    Good write-up.

    But one comment. The Vcpu requirement is missing. Although the design satisfies the memory requirement, the processor is under utilized. The general rule of thumb for deciding VM density is – 3-5 Vcpus per core. Even if your VMs require 2 Vcpus, from CPU point of view, with 3 Vcpu/core, you could host close to 12 VMs and with 5 VPCus/core, you could host upto 20 VMs.

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