Chapter 15. KVM Migration

Migration works by sending the state of the guest virtual machine's memory and any virtualized devices to a destination host physical machine. It is recommended to use shared, networked storage to store the guest's images to be migrated. It is also recommended to use libvirt-managed storage pools for shared storage when migrating virtual machines.

Migrations can be performed both with live (running) and non-live (shut-down) guests.

In a live migration, the guest virtual machine continues to run on the source host machine, while the guest's memory pages are transferred to the destination host machine. During migration, KVM monitors the source for any changes in pages it has already transferred, and begins to transfer these changes when all of the initial pages have been transferred. KVM also estimates transfer speed during migration, so when the remaining amount of data to transfer reaches a certain configurable period of time (10ms by default), KVM suspends the original guest virtual machine, transfers the remaining data, and resumes the same guest virtual machine on the destination host physical machine.

In contrast, a non-live migration (offline migration) suspends the guest virtual machine and then copies the guest's memory to the destination host machine. The guest is then resumed on the destination host machine and the memory the guest used on the source host machine is freed. The time it takes to complete such a migration only depends on network bandwidth and latency. If the network is experiencing heavy use or low bandwidth, the migration will take much longer. Note that if the original guest virtual machine modifies pages faster than KVM can transfer them to the destination host physical machine, offline migration must be used, as live migration would never complete.

Migration is useful for: