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commguy23
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Contributor

Impact on guest OS of changing clock speeds from host to host

I'm not able to find any solid documentation on this so appreciate any input.

An example to spell out my question. Let's say we have a cluster with 6 hosts and have the budget to buy 2 more hosts. The existing 6 hosts are running 16 cores a 3.2GHz (about 50GHz per host), and the 2 new hosts are running 48 cores at 2.7Ghz (about 130GHz per host). We opt for this lower clock speed with more cores since we license based on socket count, not core count and can run many more VM's on one host with higher core count options.  We enable EVC and add the 2 new hosts to existing cluster.

How do the different clock speeds impact the VM performance within the Guest OS, say Windows Server 2016 VM with 2 cores for example. It's keeping it's 4 vCPUs when moving from host 6 (old) to host 7 (new), but the clock speed drops in total from 6.4GHz to 5.4GHz for that VM.

Obviously, slower clock speed, but let's say that VM only consumes about 60% CPU at peaks, or about 3.8GHz. Moving down to the new host / lower clock speed would have no negative impact on that VM, correct? Of course the newer hosts are new gen CPU's and lower clock speed may actually perform better, but trying to get an idea here and make sure it's not a mass negative impact.

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a_p_
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Whether you will see an impact basically will likely depend on the applications, which are running on the VMs.

Unless you are running (old) single-threaded applications, I think you will not see a huge difference, and if there is one, it may be an option to change that VM's settings, and add more vCPUs.

André

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