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LittleNickey
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Overhead when enabling hot-add/hot-plug - with modern OS?

A colleague told me the other day that using hot-add for memory/CPU on VMs wasn't recommended due to the overhead.

This had me puzzled since I've managed environments with several thousand VMs with hot-add enabled and never heard of any overhead before during my 5 years working with VMware.

So I immediately started researching and found some merit to the assertion via ​Duncan Eppings blog and Luca Sturleses blog, but these range back to 2012 and 2014 and I havn't found anything official material from VMware on the subject. These articles discusses ESXi 5.0 and 5.1 and Duncan refers to a MS KB which only seems to be regarding Windows 2003.

From reading the comment field in Duncans post it becomes apparent that there are divided meanings if the overhead is an issue or not.

The most recent community post I can find also references Duncans blog, but again this is an old post and regards old ESXi and Windows versions.

Are there any official references?

How can one calculate the overhead (I did not quite understand the relevance of the vSphere 5/5.1 configuration maximums mentioned in both blogs)? Duncan says it's in the range of percents, but what to consider when trying to calculate this?

Is this relevant with modern OS'es (ESXi 6.5/Windows 2012R2/2016) and with the amount of memory/cpu hosts today can use?

-- Oskar
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EvgeniyL
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commguy23
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I'm in the same boat. We have always had hot add in the templates for all of our VMs. I managed one environment with over a few hundred VMs all with the hot add CPU and RAM enabled; mostly RDS / terminal servers with many vCPUs. I have never seen any issues with overhead or performance, but again Google searches keep coming back to those two articles. I am not sure if lack of vNUMA performance impact is a huge deal in today's beastly hosts.

LittleNickey
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Hi EvgeniyL​,

thanks for the reply and a great article! I had missed this one and it explains it very well. Like I thought it doesn't seem to have any impact unless you violate the vNUMA rules.

-- Oskar
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5mall5nail5
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I will recommend hot-add/plug when doing load tests, but other than that it should be disabled for the reasons mentioned above

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