livemigration

LiveMigration is a virtualization capability that moves a running virtual machine from one physical host to another with minimal disruption to services. It enables maintenance without downtime, dynamic load balancing, and smoother hardware upgrades by keeping the VM operational during the transfer.

How it works: The VM’s memory, CPU state, and device state are transferred from the source to the destination while the guest continues to run. In the common pre-copy method, most memory pages are copied while the VM executes; pages modified during transfer are tracked and re-copied in subsequent iterations. When the amount of dirty memory falls below a threshold, the VM is briefly paused, the final state is synchronized, and execution resumes on the destination. Some implementations employ post-copy, where the VM starts on the destination quickly and remaining memory pages are fetched on demand from the source.

Requirements and caveats: LiveMigration typically requires shared storage for virtual disks or compatible remote storage, a fast and reliable inter-host network, and hardware feature compatibility between hosts. Certain devices, such as PCI passthrough GPUs, may complicate migration. The guest and host operating systems must support migration, and licensing or security constraints may apply. Network connections and I/O may experience brief disruption during the final synchronization.

Platform variants: Major virtualization platforms implement live migration with platform-specific terminology. KVM with libvirt supports pre-copy and post-copy migrations; VMware vSphere uses vMotion; Microsoft Hyper-V offers Live Migration; Xen, and OpenStack Nova can orchestrate live migrations in cloud deployments.

Limitations and considerations: LiveMigration is most effective for workloads with sufficient available memory and compatible CPU features across hosts. Large memory footprints, devices with direct hardware access, or workloads requiring ultra-low latency I/O may limit or prevent live migration.