VMware vSphere Hot Add and Hot Plug

KB ID 0000527 


I was trying to hot add some memory to a VM the other day, and found the option grayed out. Normally I’d just down the VM, add the memory, then bring it back up. But it was a production server and I was pretty sure the OS supported it.

A quick Google search told me why it was grayed out, but it also transpired there was little to no information on what version of Windows hot add and hot plug would work with.


I’m not going to argue the semantics of the differences between “hot add” and “hot plug”, if I’m taking about hot add I’m talking about memory, if I’m talking about hot plug I’m talking about adding CPUs. You also need to be aware that to date Few OS’s support hot remove or hot unplug. If you try you will see the following;

vSphere version 6 or 6.5 (Hot Unplug )

It simply wont let you lower the value;

Hot Remove Memory vSphere

Note: With a Supported O,S (i.e. Server 2016 and 2019) you CAN hot remove CPU.

vSphere version 5.0 or 5.5

hot remove

Hot Add Memory/ CPU in vSphere 6 & 6.5

As with earlier version of vSphere, to enable hot plug or hot remove, the machine has to be shut down. Then the option can be enabled. Select the VM > Edit Settings.

Memory: Virtual Hardware > Memory > Tick ‘Memory Hot Plug’ > Save.

Hot Add Memory vSphere 6

CPU: Virtual Hardware > CPU > Tick ‘Enable CPU Hot Add’ > Save.

Hot Plug, Hot Add  in the vSphere HTML5 Client

Hot Plug CPU Memory vmware HTML5

Hot Add Memory/ CPU in vSphere 5 & 5.5

As for memory and CPU settings you will probably see what I was seeing. Both the options are not changeable.

memory and cpu grayed out

2. Sorry but to enable this feature you need to power off the client machine, then when you edit its settings > Options > Advanced > Memory/CPU_Hotplug > You can enable hot add and hot plug > OK . Power the VM back on again.

enable hot add and hot plug

3. Now you will see you have the option to hot add memory and hot plug CPUs.

hot add and hot plug enabled

What Operating Systems support this?

Hot Plug Windows Supported

Like I said above, I did some testing because information is thin on the ground, this is what I was actually able to make work.

With 2008 R2 Standard

1. As you can tell from the table memory hot add will work but to add a CPU will need a reboot. Before I started I had 2 CPUs and 4GB of memory.

hot add mem

2. Lets add more memory and CPUs.

hot plug cpu

3. For all machines I tested there was a lag, sometimes as little as 3-5 seconds, other times as long as 15-20 seconds, during this time you will see some processor and memory usage spikes. But as shown the memory eventually becomes available.

hot add memoryupdate

4. Post reboot, your extra CPUs will appear.

powered on

With 2008 R2 Enterprise and Datacenter

1. Note I’m using Datacenter here, but Enterprise is the same. I increased the memory from 4 to 5 GB, And added a further 3 CPUs.

powered on

2. It does work, you simply need to restart the “Task Manager” to reflect the increased CPU count.

powered on

3. Finished.

powered on

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Author: Migrated

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