If a hardware malfunction or other error causes a cloud compute node
to fail, you can evacuate instances to make them available again.

To preserve user data on the server disk, configure shared storage on
the target host. When you evacuate the instance, Compute detects whether
shared storage is available on the target host. Also, you must validate
that the current VM host is not operational. Otherwise, the evacuation
fails.

There are two different ways to evacuate instances from a failed
compute node. The first one using the nova evacuate command can be used to evacuate a
single instance from a failed node. In some cases where the node in
question hosted many instances it might be easier to use nova host-evacuate to
evacuate them all in one shot.

Evacuate a single instance

The procedure below explains how to evacuate a single instance from a
failed compute node. Please be aware that these steps describe a post
failure scenario and should not be used if the instance is still up and
running.

1. To find a host for the evacuated instance, list all hosts:

   $ openstack host list

2. Evacuate the instance. You can use the
   --password PWD option to pass the instance password to the
   command. If you do not specify a password, the command generates and
   prints one after it finishes successfully. The following command
   evacuates a server from a failed host to HOST_B.

   $ nova evacuate EVACUATED_SERVER_NAME HOST_B

   The command rebuilds the instance from the original image or volume
   and returns a password. The command preserves the original
   configuration, which includes the instance ID, name, uid, IP address,
   and so on.

   +-----------+--------------+
   | Property  |    Value     |
   +-----------+--------------+
   | adminPass | kRAJpErnT4xZ |
   +-----------+--------------+

   Optionally you can omit the HOST_B parameter and let the
   scheduler choose a new target host.

3. To preserve the user disk data on the evacuated server, deploy
   Compute with a shared file system. To configure your system, see section_configuring-compute-migrations. The following
   example does not change the password.

   $ nova evacuate EVACUATED_SERVER_NAME HOST_B --on-shared-storage

   Note

   Starting with the 2.14 compute API version, one no longer needs to
   specify --on-shared-storage even if the server is on a
   compute host which is using shared storage. The compute service will
   automatically detect if it is running on shared storage.

Evacuate all instances

The procedure below explains how to evacuate all instances from a
failed compute node. Please note that this method should not be used if
the host still has instances up and running.

1. To find a host for the evacuated instances, list all hosts:

   $ openstack host list

2. Evacuate all instances from FAILED_HOST to
   TARGET_HOST:

   $ nova host-evacuate --target_host TARGET_HOST FAILED_HOST

   The option --target_host is optional and can be omitted
   to let the scheduler decide where to place the instances.

   The above argument FAILED_HOST can also be a pattern to
   search for instead of an exact hypervisor hostname but it is recommended
   to use a fully qualified domain name to make sure no hypervisor host is
   getting evacuated by mistake. As long as you are not using a pattern you
   might want to use the --strict flag which got introduced in
   version 10.2.0 to make sure nova matches the FAILED_HOST
   exactly.