How to determine ESXi Server VM Capacity?

Posted on 2012-09-07
Last Modified: 2013-03-19
Greetings Experts!

We are currently running two ESXi 4.1 Servers with 13 VM's spread across both servers. We would like to determine how many more VM's we can add to the servers before hardware performance is maxed out. Is there any way to do this through vSphere Client or perharps some other tool out there that will help us in this regard?

Thanks in advance!
Question by:scfowler
    LVL 116

    Assisted Solution

    by:Andrew Hancock (VMware vExpert / EE MVE)
    The scientific way is to use VMware VMMark


    Download vCenter Operations Manager or VKernel Operations vOps Server both are FREE trials, and based on current performance and baselines after approx two weeks of monitoring will PREDICT, how many VMs you can host with resources left.

    You can also run What If simulations....
    LVL 40

    Accepted Solution

    The tools provided by @hanccocka are good, and as he states "scientific", but as a good 'guesstimation', you can figure about 4-6 VMs per CPU *CORE* (not CPU socket, but CPU core). Then of course there is RAM consideration. You need 2GB of host RAM to run ESXi, then more or less it's a 1-to-1 ratio of RAM for VMs. So, if you configure a VM to have 2GB RAM, then 2GB physical host RAM is used for that VM. There is also RAM "overhead" that needs calculated, but honestly, it's fairly miniscule (anywhere from 123MB to 740MB RAM per VM based on vCPUs configured for the VM). So, if you want a 'quick' guess, just figure 4-6 VMs per CPU core, then just keep in mind your memory is a 1-for-1 ratio. You would find in guides that 1 vCPU configured for a VM is equal to 1 CPU core, but it doesn't fully utilize that core all the time or even part of the core. So, you also find that you can run about 4-6 VMs per core (and btw, that's a guess for a single socket, quad-core CPU; so anywhere from 16-24 VMs on your host for a single quad-core CPU). You would have slightly less VMs with more cores per CPU socket because the VMs share the CPU bus. (about 2-4 VMs per core on 6-8 core CPU sockets).


    Author Comment

    Thanks for the replies and excellent information!

    coolsport00 - What if you configure your VM's to use multiple vCPU's? Can you still figure on about 4 - 6 VM's per core, or would it then be less?
    LVL 116

    Expert Comment

    by:Andrew Hancock (VMware vExpert / EE MVE)
    Exactly the same 5-6 VMs per core, if if multiple vCPUs in use.
    LVL 40

    Expert Comment

    The same...4-6/core, as mentioned above.


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