=================
Migrate instances
=================

.. note::

   This documentation is about cold-migration. For live-migration usage, see
   :doc:`live-migration-usage`.

When you want to move an instance from one compute host to another, you can use
the :command:`openstack server migrate` command. The scheduler chooses the
destination compute host based on its settings. This process does not assume
that the instance has shared storage available on the target host. If you are
using SSH tunneling, you must ensure that each node is configured with SSH key
authentication so that the Compute service can use SSH to move disks to other
nodes. For more information, see :ref:`cli-os-migrate-cfg-ssh`.

#. To list the VMs you want to migrate, run:

   .. code-block:: console

      $ openstack server list

#. Use the :command:`openstack server migrate` command.

   .. code-block:: console

      $ openstack server migrate VM_INSTANCE

#. To migrate an instance and watch the status, use this example script:

   .. code-block:: bash

      #!/bin/bash

      # Provide usage
      usage() {
          echo "Usage: $0 VM_ID"
          exit 1
      }

      [[ $# -eq 0 ]] && usage
      VM_ID=$1

      # Show the details for the VM
      echo "Instance details:"
      openstack server show ${VM_ID}

      # Migrate the VM to an alternate hypervisor
      echo -n "Migrating instance to alternate host "
      openstack server migrate ${VM_ID}
      while [[ "$(openstack server show ${VM_ID} -f value -c status)" != "VERIFY_RESIZE" ]]; do
          echo -n "."
          sleep 2
      done
      openstack server resize --confirm ${VM_ID}
      echo " instance migrated and resized."

      # Show the details for the migrated VM
      echo "Migrated instance details:"
      openstack server show ${VM_ID}

      # Pause to allow users to examine VM details
      read -p "Pausing, press <enter> to exit."

.. note::

   If you see the following error, it means you are either running the command
   with the wrong credentials, such as a non-admin user, or the ``policy.json``
   file prevents migration for your user::

     Policy doesn't allow os_compute_api:os-migrate-server:migrate to be performed. (HTTP 403)

.. note::

   If you see the following error, similar to this message, SSH tunneling was
   not set up between the compute nodes::

     ProcessExecutionError: Unexpected error while running command.
     Stderr: u Host key verification failed.\r\n

The instance is booted from a new host, but preserves its configuration
including instance ID, name, IP address, any metadata, and other properties.
