Change Memory Hot Add Settings in the vSphere Web Client
Note on reservations:
Assigning a reservation to a virtual machine that is larger than its configured memory is wasteful.
The vSphere Web Client prevents you from making such an assignment on the Resources tab.
If you give a virtual machine a large reservation and reduce its configured memory size on the Hardware tab,
the reservation is reduced to match the new configured memory size.
Memory hot add lets you add memory resources for a virtual machine while the machine is powered on.
Enabling memory hot add produces some extra memory overhead on the ESXi host for the virtual machine.
Power off the virtual machine.
Ensure that the virtual machine has a guest operating system that supports memory hot add functionality.
Ensure that the virtual machine is using hardware version 7 or later.
Ensure that VMware Tools is installed.
Select a virtual machine.
In the VM Hardware panel, click Edit Settings.
Click Virtual Hardware.
Click the Memory triangle to expand the memory options.
Select Enable to enable adding memory to the virtual machine while it is powered on.