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.
- 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 Deployment and 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, the disk volume associated with the virtual machine is also migrated. This process is performed using live block migration.
For more information on migration, see the Red Hat Enterprise Linux 7 Virtualization Deployment and Administration Guide.
2.3.2. Virtualized to Virtualized Migration (V2V)
As a special type of migration, Red Hat Enterprise Linux 7 provides tools for converting virtual machines from other types of hypervisors to KVM. The
virt-v2vtool converts and imports virtual machines from Xen, other versions of KVM, and VMware ESX.
For more information on V2V, see the V2V Knowledgebase articles.
In addition, Red Hat Enterprise Linux 7.3 and later support physical-to-virtual (P2V) conversion using the
virt-p2vtool. For details, see the P2V Knowledgebase article.