VM vCPU And vRAM Hot Add&Hot Plug

[liangshuai ma]

我想知道cpu和内存热添加是不是推荐开启，那么最佳实践是什么

I wonder if CPU and memory hot add is recommended to be enabled, so what are the best practices

updated by @Jason McClellan Platform Admin - please post in english unless the board is designated otherwise.

[Francesco Grimaldi]

Hi, 

CPU and memory hot plug is definitely a very interesting feature. However, you need to consider an important factor: when adding CPU and RAM on the fly, not all operating systems are capable of recognizing the hot addition (so make sure to check the compatibility of the guest operating system). Moreover, and even more importantly, while most modern operating systems can detect the hot addition of hardware resources, it doesn't necessarily mean that your application will be able to utilize them. 

You need to verify whether the application running on the guest OS can actually make use of the newly added resources. For example, think of an application written in Java running on a JVM-are you certain that the JVM will be able to claim the new RAM? 

The recommendation is to always perform tests to confirm this behavior. I hope this explanation is clear.

[liangshuai ma]

Thank You

so make sure to check the compatibility of the guest operating system This Whether there is a URL connection？

How are the best practices distributed?

[BF_NSO]

Note that enabling CPU Hot Add essentially disables vNUMA capabilities.     Know your workload before enabling.

vNUMA is disabled if vCPU hotplug is enabled

[liangshuai ma]

This I understand, shut down vNUMA, then what are the best practices? Several sockets and several cores

[CSpreha]

HotAdd isn't so much about being able to add CPU/RAM if you undersize, it's for testing that sizing during development, then doing one last power down, fixing the cpu/ram, and disabling HotAdd.   Also, it's best if you are on vSphere8 to start reconfiguring your VMs to use the Configure at Startup option for the CPUs so that the vCPUs get configured most efficiently for the underlying cpu architecture (monolithic, chiplets, cores per chiplet, etc) and just let it configure itself.  Caveat of course if you need to configure specific socket/cores for a licensing issue.

[liangshuai ma]

Isn't it recommended to turn it on?

[CSpreha]

Just to turn it on in general? No.  It's for performance testing and then you should settle on a permanent VM design.  You can leave it on, but (for right now), you do lose vNUMA and processing efficiencies.  This is changing "in the next version" so you can keep vNUMA with HotAdd.  
Always try to right-size your VMs w/o using band-aids.  If you're on v8, then use VMX20 or 21, and use the Assign At Power-On CPU setup, and pick an appropriate but not overly large CPU and RAM setting.  If during testing the sizing was wrong, it's test, you can power-down the VM and make changes before going to UA and Production.  If you have a production box that's changing CPU requirements, well then, the development pipeline needed more testing.  You can turn HotAdd on, go thru a few months of production-testing, but then at some point it's still best to turn it off and live with a permanent design.