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Overview

Live resize of vCPU and memory lets you adjust compute resources for a running instance within the bounds configured on the instance’s flavor. The Dashboard provides a slider-based dialog for intuitive resource adjustment while the instance stays online. Live resize can increase or decrease vCPU and memory when the guest operating system, guest tools, and virtualization drivers permit the change. Guest behavior matters: some operating systems support both directions cleanly, while others may require drivers, manual CPU online steps, or a reboot before the new resource state is fully visible.
Prerequisites
  • Instance must be in Active status
  • The instance must not be locked
  • The flavor must have live resize enabled (hw:cpu_min or hw:mem_min extra specs) — if you do not see Adjust Resources in the actions menu, the flavor does not support live resize
  • The guest operating system must have the required guest tools and drivers for CPU and memory changes

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Resize vCPU or Memory

1

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 only appears for instances in Active status whose flavor has live resize enabled. If you do not see it, contact your administrator to configure a live-resize-enabled flavor.
2

Adjust resources with sliders

The dialog shows the current state and adjustment controls:The dialog fetches the live instance status to determine the current and allowed resource ranges. Color-coded hints indicate the scaling method:
The vCPU slider step aligns to the CPU thread count configured in the flavor. For example, with 2 threads per core, vCPUs adjust in steps of 2.
3

Apply the change

Click Confirm. Polystack applies the requested live resize while the instance remains running, subject to guest operating system and driver support.
Reducing memory while applications are using it may cause out-of-memory events inside the guest. Check application memory usage before reducing memory.
4

Verify inside the instance

SSH into the instance and confirm the new resources:
Check vCPU count
Check available memory
The guest OS shows the updated vCPU count and memory after the live resize is applied.

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Comparison: Live Resize vs. Flavor Resize

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Troubleshooting

Adjust Resources not visible in the menu

The instance’s flavor does not have live resize enabled. The flavor must have hw:cpu_min or hw:mem_min extra specs set. Contact your administrator to configure a live-resize-enabled flavor, or resize to a flavor that supports it.
The requested value is outside the allowed range. The minimum and maximum are determined by the flavor’s hw:cpu_min / hw:mem_min (minimum) and the flavor’s vCPUs / memory (maximum). Try a value within the slider range.
On older Linux kernels (before 4.15), new CPUs may need to be brought online manually:
Bring hotplugged CPUs online
Modern kernels (4.15+) bring CPUs online automatically.
The guest OS memory driver may not be loaded or may not support the requested direction. Check inside the guest:
Check balloon driver
If not loaded: modprobe virtio_balloon and add virtio_balloon to /etc/modules for persistence. Some guest operating systems require virtio-mem, ballooning, DIMM hotplug support, or a reboot before memory changes are fully visible.

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Next Steps

Admin: Enable Live Resize

Configure flavors with hot-add extra specs and resource bounds

Resize Instance

Standard flavor resize — change vCPU, memory, and disk with a reboot

Flavors

View available flavors and their live resize capabilities

Launch an Instance

Create an instance with a live-resize-enabled flavor