红帽全球峰会第 12 章 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
You can migrate a running virtual machine (VM) without interrupting the workload, with only minor downtime, by using a live migration. By default, the migrated VM is transient on the destination host, and remains defined also on the source host. The essential part of a live migration is transferring the state of the VM’s memory and of any attached virtualized devices to a destination host. For the VM to remain functional on the destination host, the VM’s disk images must remain available to it.
To migrate a shut-off VM, you must use an offline migration, which copies the VM’s configuration to the destination host. 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, for VMs that modify memory pages faster than KVM can transfer them, such as VMs under heavy I/O load, the live migration might fail. (1) | The VM’s disk images must be accessible both to the source host and the destination host during the migration. (2) |
| Offline migration | Moves the VM’s configuration to the destination host | Recommended for shut-off VMs and in situations when shutting down the VM does not disrupt your workloads. | The VM’s disk images do not have to be accessible to the source or destination host during migration, and can be copied or moved manually to the destination host instead. |
(1) For possible solutions, see: Additional virsh migrate options for live migrations
(2) To achieve this, use one of the following:
- Storage located on a shared network
-
The
--copy-storage-allparameter for thevirsh migratecommand, which copies disk image contents from the source to the destination over the network. - Storage area network (SAN) logical units (LUNs).
- Ceph storage clusters
For easier management of large-scale migrations, explore other Red Hat products, such as:
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