Live vCPU & RAM Scaling

Overview

Live vCPU and RAM scaling adjusts an instance’s compute resources while it is running — no reboot required. This is distinct from the standard resize operation, which requires a reboot. Live scaling uses a combination of CPU hotplug, memory balloon, DIMM hotplug, and virtio-mem to adjust resources within the bounds defined by the instance’s flavor.

Prerequisites

An instance in Active status with a hot-add-enabled flavor
The flavor must have hw:cpu_min or hw:mem_min extra specs set
The instance must not be locked

Adjust Resources (Dashboard)

Dashboard
CLI

Open the Resource Adjustment dialog

Navigate to Compute > Instances. Click the More dropdown on the instance row, then select Adjust Resources under the Configuration Update group.

This action is only available for instances in Active status whose flavor has hot-add enabled (hw:cpu_min or hw:mem_min extra specs). If the flavor does not support hot-add, this action does not appear.

Adjust vCPU and memory

The dialog shows:

Field	Description
Instance	Instance name (read-only)
Current Info	Current vCPU count and memory allocation (read-only)
vCPUs	Slider input — adjust between minimum and maximum vCPUs
Memory (GiB)	Slider input — adjust between minimum and maximum memory (0.25 GiB steps)
Make permanent	Checkbox — persist the configuration after a soft reboot (default: checked)

The vCPU slider step aligns to the CPU thread count configured in the flavor. The memory slider uses 0.25 GiB increments.The dialog fetches the live instance status to determine current and allowed resource ranges. Color-coded hints indicate the scaling method:

Color	Meaning
Green	virtio-mem — fully adjustable in both directions
Yellow	No balloon driver detected — limited scaling
Blue	DIMM hotplug information

Apply the adjustment

Click Confirm. The resources are adjusted immediately while the instance continues running.

Instance vCPU and memory updated in real-time. Verify from the instance detail page or by running nproc / free -h inside the guest.

Source credentials

source openrc.sh

Check current live resource status

openstack server show <INSTANCE_ID> -c "polystack-status"

Adjust vCPU count

curl -X POST -H "X-Auth-Token: $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"current_vcpus": 4}' \
  "$NOVA_ENDPOINT/v2.1/os-polystack-adjust/<INSTANCE_ID>"

Adjust memory

curl -X POST -H "X-Auth-Token: $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"current_memory_mb": 8192}' \
  "$NOVA_ENDPOINT/v2.1/os-polystack-adjust/<INSTANCE_ID>"

Live resource adjustment uses the platform Nova API extension. The openstack CLI does not have a native command for this operation.

Configure Hot-Add Flavors

To enable live scaling, create flavors with hot-add parameters:

Dashboard
CLI

When creating a flavor (see Flavor Management), enable the Hot-Add toggle in the Step 1 (Params Setting) section. Configure:

Field	Description
Enable Hot-Add	Set to Yes
Minimum CPU	Starting vCPU count at boot (can scale up to flavor’s max vCPUs)
Minimum Memory (GiB)	Starting memory at boot (can scale up to flavor’s max memory)

The instance boots with the minimum resources and can be scaled up to the flavor’s maximum without rebooting.

Create a hot-add enabled flavor

openstack flavor create \
  --vcpus 8 --ram 16384 --disk 0 \
  --public \
  hotadd.large

openstack flavor set \
  --property hw:cpu_min=2 \
  --property hw:mem_min=4096 \
  hotadd.large

This creates a flavor where instances boot with 2 vCPUs / 4 GiB RAM and can scale up to 8 vCPUs / 16 GiB RAM while running.

Next Steps

Resize an Instance

Change flavors entirely (requires reboot) for capacity beyond hot-add range

Flavors

Create and manage flavors with hot-add configuration

Launch an Instance

Launch instances with hot-add enabled flavors

Troubleshooting

Resolve live scaling failures and balloon driver issues

Core Services

Other Services

Overview

Adjust Resources (Dashboard)

Open the Resource Adjustment dialog

Adjust vCPU and memory

Apply the adjustment

Configure Hot-Add Flavors

Next Steps

Resize an Instance

Flavors

Launch an Instance

Troubleshooting

​Overview

​Adjust Resources (Dashboard)

Open the Resource Adjustment dialog

Adjust vCPU and memory

Apply the adjustment

​Configure Hot-Add Flavors

​Next Steps

Resize an Instance

Flavors

Launch an Instance

Troubleshooting

Overview

Adjust Resources (Dashboard)

Configure Hot-Add Flavors

Next Steps