Dynamic Local Disk Allocation & Resource Overcommit in KubeVirt using OpenEBS‑LVM

In the previous articles we covered KubeVirt installation, basic usage, and migration from oVirt, leaving two tasks: dynamic local‑disk allocation and fixed IP assignment. This article resolves the first task by replacing local‑storage with OpenEBS‑LVM , which adds capacity limits, dynamic PV provisioning, and resize capabilities.

Enable Overcommit Ratios

When creating a StorageClass, enable volume expansion ( allowVolumeExpansion: true ) and set the binding mode to WaitForFirstConsumer .

Configure the KubeVirt overcommit settings by editing the KubeVirt custom resource:

<code>kubectl -n kubevirt edit kubevirts.kubevirt.io kubevirt</code>

<code>spec:
  configuration:
    developerConfiguration:
      cpuAllocationRatio: 2   # CPU overcommit 2x
      memoryOvercommit: 200   # Memory overcommit 2x
</code>

Example: set a VM spec to 8 CPU cores and 16 GiB memory without declaring a request. After creation, the Pod’s request values are half of the specified limits, confirming the overcommit behavior.

<code>spec:
  domain:
    cpu:
      cores: 8
    memory:
      guest: 16Gi
</code>

After the VM starts, the actual resources observed inside the VM match the expected 8c/16g.

If a Request value is declared, it takes precedence over the overcommit calculation.

CPU/MEM Upgrade

Modify the CPU and memory values in the VM spec and restart the VM:

<code>virtctl restart &lt;vmi&gt;</code>

KubeVirt also supports hot‑plug upgrades (CPU/Memory) without downtime, subject to kernel version and storage mode constraints.

Online Disk Expansion

Enable the

ExpandDisks

feature gate in the KubeVirt configuration:

<code>spec:
  configuration:
    developerConfiguration:
      featureGates:
        - ExpandDisks
</code>

Check the current data‑disk size (e.g., 500 Gi) and expand it to 600 Gi by editing the associated PVC size; OpenEBS‑LVM will automatically resize the volume.

<code>spec:
  resources:
    requests:
      storage: "600Gi"
</code>

After expanding the PVC, resize the filesystem on the node:

<code>resize2fs /dev/vdb   # ext4 filesystem, data disk is vdb</code>

Fixed IP and Node Affinity

While fixed IP is not yet implemented, node affinity can be used to keep a VM on a specific node to avoid IP changes. Annotate the VMI with the desired pod IP and set

nodeSelector

to the target node hostname.

<code># View node block allocations
kubectl get blockaffinities.crd.projectcalico.org
# Choose an unused IP
calicoctl ipam show --ip=$(ip_addr)
# Set pod IP annotation
cni.projectcalico.org/ipAddrs: '["pod-ip"]'
# Node selector
nodeSelector:
  kubernetes.io/hostname: "nodename"
</code>

Complete VM YAML Example

<code>apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
  labels:
    kubevirt.io/vm: <vmi-name>
  name: <vmi-name>
spec:
  running: false
  template:
    metadata:
      labels:
        kubevirt.io/vm: <vmi-name>
      annotations:
        cni.projectcalico.org/ipAddrs: '["pod-ip"]'
    spec:
      nodeSelector:
        kubernetes.io/hostname: "nodename"
      domain:
        cpu:
          cores: 8
        memory:
          guest: 16Gi
        devices:
          disks:
          - disk:
              bus: virtio
            name: datavolumedisk1-sys
          - disk:
              bus: virtio
            name: datavolumedisk1-data
        interfaces:
        - name: default
          bridge: {}
      networks:
      - name: default
        pod: {}
      volumes:
      - dataVolume:
          name: demo-1-sys
        name: datavolumedisk1-sys
      - dataVolume:
          name: demo-1-data
        name: datavolumedisk1-data
      dataVolumeTemplates:
      - metadata:
          name: demo-1-sys
        spec:
          storage:
            storageClassName: openebs-lvm
            accessModes:
            - ReadWriteOnce
            resources:
              requests:
                storage: 80Gi
          source:
            http:
              url: <img_url>
      - metadata:
          name: demo-1-data
        spec:
          storage:
            storageClassName: openebs-lvm
            accessModes:
            - ReadWriteOnce
            resources:
              requests:
                storage: 500Gi
          source:
            blank: {}
</code>

Reference Links

Node overcommit: https://kubevirt.io/user-guide/compute/node_overcommit/#node-overcommit

CPU Hotplug: https://kubevirt.io/user-guide/compute/cpu_hotplug/#cpu-hotplug

Memory Hotplug: https://kubevirt.io/user-guide/compute/memory_hotplug/#memory-hotplug

Disk expansion: https://kubevirt.io/user-guide/storage/disks_and_volumes/#disk-expansion

Node assignment: https://kubevirt.io/user-guide/compute/node_assignment/#node-assignment