VM Hot Add CPU Or Memory

Private Cloud Director supports hot-add of CPU and / or memory resources to a running virtual machine. Using this feature, you can increase CPU cores or memory allocated to a running VM, without having to power-cycle it. This is valuable for production VMs that need more resources to meet workload demands but that can not be powered off.

Pre-requisites

• Read more here about Zero-Size Flavor. Using zero-size flavor when creating your VM is a requirement for hot-add. Before you can hot-add CPU or memory resources to a VM, the VM must have been created using zero-size flavor.

Hot-add CPU or Memory to a Running VM

1. Select a powered-on VM that was created with a zero-size flavor.
2. On the VM grid view, navigate to ▷ Other Actions ▷ Hotplug.
3. Enter the new vCPU and/or memory value for the VM. The values cannot exceed the Hotplug vCPUs Maximum and Hotplug RAM Maximum set when the VM was created.
4. Click Hotplug VM.

Limitations

• Hot add can only be used to increase the current CPU or memory allocation of a running VM. Reducing currently allocated CPU or memory is supported only while the VM is stopped.
• You can not use the hot-add feature to resize the disk of a running VM today.
• Hot-add-capable VMs that boot from a persistent volume and were created before the PCD 2025.10 release may fail to live migrate until a one-time corrective hotplug is applied. See Resolve Live Migration Failures for Legacy Boot-from-Volume Hotplug VMs.

Known Issues

Hot-add VMs Can Use More Host Memory Than Their Hotplug RAM Value

A hot-add VM starts with its full Hotplug RAM Maximum. The VirtIO balloon driver in the guest then returns the memory above the VM's Hotplug RAM value to the host. The return is gradual, and it never happens if the guest has no working balloon driver. The most common case is a Windows VM installed from an ISO without the virtio-win guest tools. Until the memory is returned:

• The VM can use host memory up to its Hotplug RAM Maximum, while the VM details still show the smaller Hotplug RAM value. A few such VMs can exhaust a host's memory and affect every VM on that host.
• Windows Server 2019, Windows Server 2022, and Windows 11 guests can report the Hotplug RAM Maximum as installed memory in Task Manager, System Information, and Performance Monitor. This can happen even with the balloon driver installed, and rebooting the VM does not reliably correct it.

To limit the impact:

• Set Hotplug RAM Maximum to the most memory you plan to hot-add. The Deploy New VM wizard pre-fills 65536 MiB (64 GiB). You cannot change the maximum after the VM is created. If the VM does not need memory hot-add, set Hotplug RAM Maximum to the same value as Hotplug RAM.
• When you create hot-add VMs through the CLI, API, or Terraform provider, set HOTPLUG_MEMORY_MAX and HOTPLUG_CPU_MAX in the VM metadata. If you omit them, the VM's maximum is the host's total memory and vCPU count.
• Install the virtio-win guest tools in every Windows VM that uses hot-add, preferably in your golden image. Confirm that VirtIO Balloon Driver appears under System devices in Device Manager.