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2.2. What is migration?

Migration describes the process of moving a guest virtual machine from one host to another. This is possible because guests are running in a virtualized environment instead of directly on the hardware. There are two ways to migrate a virtual machine: live and offline.
Migration types
Offline migration
An offline migration suspends the guest virtual machine, and then moves an image of the virtual machine's memory to the destination host. The virtual machine is then resumed on the destination host and the memory used by the virtual machine on the source host is freed.
Live migration
Live migration is the process of migrating an active virtual machine from one physical host to another.
It is important to understand that the migration process moves the virtual machine's memory, and the disk volume associated with the virtual machine is also migrated. This process is done using live block migration.
In Fedora 19, shared storage is not necessary for storing guest images to be migrated. With live storage migration, a running virtual machine can be migrated from one host to another with no downtime. This capability can be used to optimize performance of virtual machines.

2.2.1. Benefits of migrating virtual machines

Migration is useful for:
Load balancing
When a host machine is overloaded, one or many of its virtual machines could be migrated to other hosts using live migration.
Upgrading or making changes to the host
When the need arises to upgrade, add, or remove hardware devices on one host, virtual machines can be safely relocated to other hosts. This means that guests do not experience any downtime due to changes that are made to any of the hosts.
Energy saving
Virtual machines can be redistributed to other hosts and the unloaded host systems can be powered off to save energy and cut costs in low usage periods.
Geographic migration
Virtual machines can be moved to another physical location for lower latency or for other special circumstances.
Shared, networked storage must be used for storing guest images to be migrated. Without shared storage, migration is not possible. It is recommended to use libvirt-managed storage pools for shared storage.


For more information on migration, refer to the Fedora Virtualization Deployment and Administration Guide.