2.3. Migration

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Migration describes the process of moving a guest virtual machine from one host to another. This is possible because the virtual machines 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. Note that this is not possible between all Red Hat Enterprise Linux releases. Consult the Virtualization Administration Guide for details.

2.3.1. Benefits of Migrating Virtual Machines

Migration is useful for:
Load balancing
When a host machine is overloaded, one or more of its virtual machines could be migrated to other hosts using live migration. Similarly, machines that are not running and tend to overload can be migrated using offline migration.
Upgrading or making changes to the host
When the need arises to upgrade, add, or remove hardware devices on a 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 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 other physical locations for lower latency or for other reasons.
When the migration process moves a virtual machine's memory, from Red Hat Enterprise Linux 6.3, the disk volume associated with the virtual machine is also migrated. This process is performed using live block migration.
Shared, networked storage can be used to store guest images to be migrated. When migrating virtual machines, it is recommended to use libvirt-managed storage pools for shared storage.


For more information on migration, refer to the Red Hat Enterprise Linux 6 Virtualization Administration Guide.
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