When you want to move an instance from one compute host to another,
you can migrate the instance. The migration operation, which is also
known as the cold migration operation to distinguish it from the live
migration operation, functions similarly to the resize operation </user/resize> with the
main difference being that a cold migration does not change the flavor
of the instance. As with resize, 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 cli-os-migrate-cfg-ssh.

To list the VMs you want to migrate, run:

$ openstack server list

Once you have the name or UUID of the server you wish to migrate,
migrate it using the openstack server migrate command:

$ openstack server migrate SERVER

Once an instance has successfully migrated, you can use the openstack
server migrate confirm command to confirm it:

$ openstack server migrate confirm SERVER

Alternatively, you can use the openstack server migrate revert command to revert
the migration and restore the instance to its previous host:

$ openstack server migrate revert SERVER

Example

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

#!/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 migrate 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."

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

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.