Tuesday, June 2, 2009

New memory hot add feature in vSphere 4

In the new vSphere, there is a new feature - hot add memory and CPU. This is very cool as downtime can be avoided which is, at least in my company, very helphul during daily operations. To enable these features, do the following:
  1. Make sure the virtual machine hardware is upgraded to version 7 (right click the VM, choose 'Upgrade Virtual hardware', see below).
  2. Go to edit settings for the VM, then the options tab -> Memory/CPU hotplug and enable the two features (see below).
This is all very fine. However, for the features to work, you need a compatible VM OS. And Win2k3 Standard isn't supported. For Windows, it is primarily Enterprise Edition and Datacenter Edition that will work for win2k3. For Win2k8 at least hot add memory will work for all editions 32-bit and 64-bit. See the VMware Compability Guide or for compatible guest operating systems.

Update 2013.07.23: The CPU hot add will auto register the new CPUs in the guest OS for win2k8 R2 Datacenter and Enterprise edition (at least in vSphere 5), see compability chart here. Not for Standard edition. See demonstration here.


  1. Nice tip, but they don't tell you that you have to "enable" the options while the vm is powered down. I guess this should be in the step process when you upgrade the HW tools.

  2. Ok it works howevet it seems like it is buggy. When I start a server with 768MB I can upgrade to whatever memory size, it works. However if I select 1GB (or 2GB...) the only option is to increase from 1 to 2GB (or 3 or 4) but I cannot put 1280MB. Only workaround I found is to start with for instance 768MB then if I want 1GB I have to set 1028MB to stay in MB. Odd... (on vsphere 4.1)


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