So you’ve upgraded your ESX 3.x servers to 4.0 and you’ve upgraded your vCenter server, now you want to access the shiny new hot-add feature to upgrade some running server’s memory. Except you can’t, the feature is no-where to be seen. Something to bear in mind though, your OS needs to support hot-add, so you’ll need a Windows Enterprise or Datacenter edition.
Here’s how to enable it:
Upgrade the VM’s tools, if you haven’t already. This requires a re-boot, but don’t reboot, shut the server down down, otherwise you’ll require another reboot to add the feature. The tools can be upgraded from the system tray icon.
Once the VM is shut down, upgrade the Virtual Hardware (again, assuming you haven’t already!)
Once your virtual machine is fully up to date with VMTools and Hardware, you can edit the Advanced options (again, while the machine is shut down) and enable the feature.
Now with the server booted, you can add Memory and CPU without the need for a reboot. In my tests changing the Memory from 1GB to 2GB caused a single dropped ping, and changing from 1 vCPU to 2 vCPUs caused a ping to drop.