How to Perform Cold and Live KVM VM Migration with NFS Shared Storage

Introduction

When a KVM host (vm1) runs a virtual machine and another host (vm2) mounts the VM's disk directory via NFS, hot migration can cause disk corruption. Using a dedicated NFS or other shared‑storage server for all KVM hosts prevents this issue.

Prerequisites

All KVM hosts (vm1…vmn) must have the same KVM version installed.

Bridge networking: create a bridge interface (e.g., br0) and attach eth0 to it.

Mount points for the shared storage must be identical on every host.

Cold Migration

Drawbacks :

The VM must be shut down, which interrupts services (or may cause temporary MAC address conflicts if left running).

Migration can take a long time, especially for large disk images.

Steps:

Copy the VM's XML definition and disk image to the target host:

scp /etc/libvirt/qemu/qianyi-bridge-vm.xml kafka-2:/etc/libvirt/qemu/qianyi-bridge-vm.xml</code><code>scp /opt/kvm/vm/qianyi-bridge-vm.qcow2 kafka-2:/opt/kvm/vm/qianyi-bridge-vm.qcow2

Define the VM on the target host: virsh define /tmp/<vm-name>.xml Start the VM and verify it runs:

virsh list --all</code><code>virsh start <vm-name>

Test network access. Use arping to check for duplicate MAC addresses; if two MACs appear, shut down one VM to leave a single MAC.

Live Migration

Install NFS on a dedicated server (e.g., vm3) and ensure all hosts mount the same shared directory. df -h Typical df -h output shows the NFS mount 192.168.64.12:/opt/kvm on both source and destination hosts.

Execute live migration with virsh migrate:

# Live migration without shared storage</code><code>virsh migrate --live --verbose domain qemu+ssh://192.168.64.11/system --unsafe</code><code>virsh migrate --live --verbose test2-nat-network-2 qemu+ssh://192.168.64.11/system --unsafe   # migrate</code><code>virsh migrate --live --verbose test2-nat-network-2 qemu+ssh://192.168.64.14/system --unsafe   # reverse migrate</code><code>virsh migrate --live --persistent --undefinesource --verbose myvm qemu+ssh://[email protected]/system --unsafe

Parameter explanations: --live: perform online migration. --verbose: show detailed progress. qemu+ssh://IP/system: target host connection using SSH; system refers to the libvirt system instance. --unsafe: allow migration without shared storage (useful for testing). --undefinesource: remove the VM definition from the source after migration, but keep the disk files. --persistent: make the VM definition persistent on the destination so it survives host reboots.

Important: The disk element in both source and destination XML files must point to the shared storage directory.

Host Name Resolution

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4 kafka-1</code><code>::1         localhost localhost.localdomain localhost6 localhost6.localdomain6</code><code>192.168.64.14 kafka-1</code><code>192.168.64.11 kafka-2</code><code>192.168.64.12 kafka-3

Images

Network and MAC address observations are illustrated by the following screenshots: