12.6. Migrating a virtual machine using the command-line interface

Conditions préalables

• The source host and the destination host both use the KVM hypervisor.
• The source host and the destination host are able to reach each other over the network. Use the ping utility to verify this.

• Ensure the following ports are open on the destination host.

  • Port 22 is needed for connecting to the destination host by using SSH.
  • Port 16509 is needed for connecting to the destination host by using TLS.
  • Port 16514 is needed for connecting to the destination host by using TCP.
  • Ports 49152-49215 are needed by QEMU for transfering the memory and disk migration data.
• For the migration to be supportable by Red Hat, the source host and destination host must be using specific operating systems and machine types. To ensure this is the case, see Supported hosts for virtual machine migration.
• The VM must be compatible with the CPU features of the destination host. To ensure this is the case, see Verifying host CPU compatibility for virtual machine migration.

• The disk images of VMs that will be migrated are located on a separate networked location accessible to both the source host and the destination host. This is optional for offline migration, but required for migrating a running VM.

  For instructions to set up such shared VM storage, see Sharing virtual machine disk images with other hosts.

• When migrating a running VM, your network bandwidth must be higher than the rate in which the VM generates dirty memory pages.

  To obtain the dirty page rate of your VM before you start the live migration, do the following:

  1. Monitor the rate of dirty page generation of the VM for a short period of time.

     # virsh domdirtyrate-calc vm-name 30

  2. After the monitoring finishes, obtain its results:

     # virsh domstats vm-name --dirtyrate
     Domain: 'vm-name'
       dirtyrate.calc_status=2
       dirtyrate.calc_start_time=200942
       dirtyrate.calc_period=30
       dirtyrate.megabytes_per_second=2

     In this example, the VM is generating 2 MB of dirty memory pages per second. Attempting to live-migrate such a VM on a network with a bandwidth of 2 MB/s or less will cause the live migration not to progress if you do not pause the VM or lower its workload.

     To ensure that the live migration finishes successfully, Red Hat recommends that your network bandwidth is significantly greater than the VM’s dirty page generation rate.

• When migrating an existing VM in a public bridge tap network, the source and destination hosts must be located on the same network. Otherwise, the VM network will not operate after migration.

• When performing a VM migration, the virsh client on the source host can use one of several protocols to connect to the libvirt daemon on the destination host. Examples in the following procedure use an SSH connection, but you can choose a different one.

  • If you want libvirt to use an SSH connection, ensure that the virtqemud socket is enabled and running on the destination host.

    # systemctl enable --now virtqemud.socket

  • If you want libvirt to use a TLS connection, ensure that the virtproxyd-tls socket is enabled and running on the destination host.

    # systemctl enable --now virtproxyd-tls.socket

  • If you want libvirt to use a TCP connection, ensure that the virtproxyd-tcp socket is enabled and running on the destination host.

    # systemctl enable --now virtproxyd-tcp.socket

Vérification

• On the destination host, list the available VMs to verify if the VM has been migrated:

  # virsh list
  Id Name                 State
  ----------------------------------
  10 wanderer1              running

  If the migration is still running, this command will list the VM state as paused.

Résolution de problèmes

• In some cases, the target host will not be compatible with certain values of the migrated VM’s XML configuration, such as the network name or CPU type. As a result, the VM will fail to boot on the target host. To fix these problems, you can update the problematic values by using the virsh edit command. After updating the values, you must restart the VM for the changes to be applied.

• If a live migration is taking a long time to complete, this may be because the VM is under heavy load and too many memory pages are changing for live migration to be possible. To fix this problem, change the migration to a non-live one by suspending the VM.

  # virsh suspend wanderer1