Chapitre 12. Migrating virtual machines
If the current host of a virtual machine (VM) becomes unsuitable or cannot be used anymore, or if you want to redistribute the hosting workload, you can migrate the VM to another KVM host.
12.1. How migrating virtual machines works
The essential part of virtual machine (VM) migration is copying the XML configuration of a VM to a different host machine. If the migrated VM is not shut down, the migration also transfers the state of the VM’s memory and any virtualized devices to a destination host machine. For the VM to remain functional on the destination host, the VM’s disk images must remain available to it.
By default, the migrated VM is transient on the destination host, and remains defined also on the source host.
You can migrate a running VM using live or non-live migrations. To migrate a shut-off VM, you must use an offline migration. For details, see the following table.
| Migration type | Description | Use case | Storage requirements |
|---|---|---|---|
| Live migration | The VM continues to run on the source host machine while KVM is transferring the VM’s memory pages to the destination host. When the migration is nearly complete, KVM very briefly suspends the VM, and resumes it on the destination host. | Useful for VMs that require constant uptime. However, VMs that modify memory pages faster than KVM can transfer them, such as VMs under heavy I/O load, cannot be live-migrated, and non-live migration must be used instead. | The VM’s disk images must be located on a shared network, accessible both to the source host and the destination host. |
| Non-live migration | Suspends the VM, copies its configuration and its memory to the destination host, and resumes the VM. | Creates downtime for the VM, but is generally more reliable than live migration. Recommended for VMs under heavy I/O load. | The VM’s disk images must be located on a shared network, accessible both to the source host and the destination host. |
| Offline migration | Moves the VM’s configuration to the destination host | Recommended for shut-off VMs. | The VM’s disk images do not have to be available on a shared network, and can be copied or moved manually to the destination host instead. |
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