OpenStack has the ability to migrate volumes between back ends which
support its volume-type. Migrating a volume transparently moves its data
from the current back end for the volume to a new one. This is an
administrator function, and can be used for functions including storage
evacuation (for maintenance or decommissioning), or manual optimizations
(for example, performance, reliability, or cost).

These workflows are possible for a migration:

1. If the storage can migrate the volume on its own, it is given the
   opportunity to do so. This allows the Block Storage driver to enable
   optimizations that the storage might be able to perform. If the back end
   is not able to perform the migration, the Block Storage uses one of two
   generic flows, as follows.

2. If the volume is not attached, the Block Storage service creates
   a volume and copies the data from the original to the new volume.

   Note

   While most back ends support this function, not all do. See the driver documentation </configuration/block-storage/volume-drivers>
   for more details.

3. If the volume is attached to a VM instance, the Block Storage
   creates a volume, and calls Compute to copy the data from the original
   to the new volume. Currently this is supported only by the Compute
   libvirt driver.

As an example, this scenario shows two LVM back ends and migrates an
attached volume from one to the other. This scenario uses the third
migration flow.

First, list the available back ends:

# cinder get-pools
+----------+----------------------------------------------------+
| Property |                       Value                        |
+----------+----------------------------------------------------+
|   name   |           server1@lvmstorage-1#lvmstorage-1        |
+----------+----------------------------------------------------+
+----------+----------------------------------------------------+
| Property |                      Value                         |
+----------+----------------------------------------------------+
|   name   |           server2@lvmstorage-2#lvmstorage-2        |
+----------+----------------------------------------------------+

You can also get available back ends like following:

# cinder-manage host list
server1@lvmstorage-1    zone1
server2@lvmstorage-2    zone1

But it needs to add pool name in the end. For example,
server1@lvmstorage-1#zone1.

Next, as the admin user, you can see the current status of the volume
(replace the example ID with your own):

$ openstack volume show 6088f80a-f116-4331-ad48-9afb0dfb196c

+--------------------------------+--------------------------------------+
| Field                          | Value                                |
+--------------------------------+--------------------------------------+
| attachments                    | []                                   |
| availability_zone              | zone1                                |
| bootable                       | false                                |
| consistencygroup_id            | None                                 |
| created_at                     | 2013-09-01T14:53:22.000000           |
| description                    | test                                 |
| encrypted                      | False                                |
| id                             | 6088f80a-f116-4331-ad48-9afb0dfb196c |
| migration_status               | None                                 |
| multiattach                    | False                                |
| name                           | test                                 |
| os-vol-host-attr:host          | server1@lvmstorage-1#lvmstorage-1    |
| os-vol-mig-status-attr:migstat | None                                 |
| os-vol-mig-status-attr:name_id | None                                 |
| os-vol-tenant-attr:tenant_id   | d88310717a8e4ebcae84ed075f82c51e     |
| properties                     | readonly='False'                     |
| replication_status             | disabled                             |
| size                           | 1                                    |
| snapshot_id                    | None                                 |
| source_volid                   | None                                 |
| status                         | in-use                               |
| type                           | None                                 |
| updated_at                     | 2016-07-31T07:22:19.000000           |
| user_id                        | d8e5e5727f3a4ce1886ac8ecec058e83     |
+--------------------------------+--------------------------------------+

Note these attributes:

• os-vol-host-attr:host – the volume’s current back
  end.

• os-vol-mig-status-attr:migstat – the status of this
  volume’s migration (None means that a migration is not currently in
  progress).

• os-vol-mig-status-attr:name_id – the volume ID that
  this volume’s name on the back end is based on. Before a volume is ever
  migrated, its name on the back end storage may be based on the volume’s
  ID (see the volume_name_template configuration parameter).
  For example, if volume_name_template is kept as the default
  value (volume-%s), your first LVM back end has a logical
  volume named volume-6088f80a-f116-4331-ad48-9afb0dfb196c.
  During the course of a migration, if you create a volume and copy over
  the data, the volume get the new name but keeps its original ID. This is
  exposed by the name_id attribute.

  Note

  If you plan to decommission a block storage node, you must stop the
  cinder volume service on the node after performing the
  migration.

  On nodes that run CentOS, Fedora, openSUSE, Red Hat Enterprise Linux,
  or SUSE Linux Enterprise, run:

  # service openstack-cinder-volume stop
  # chkconfig openstack-cinder-volume off

  On nodes that run Ubuntu or Debian, run:

  # service cinder-volume stop
  # chkconfig cinder-volume off

  Stopping the cinder volume service will prevent volumes from being
  allocated to the node.

Migrate this volume to the second LVM back end:

$ openstack volume migrate 6088f80a-f116-4331-ad48-9afb0dfb196c \
  --host server2@lvmstorage-2#lvmstorage-2

You can use the openstack volume show command to see the status of
the migration. While migrating, the migstat attribute shows
states such as migrating or completing. On
error, migstat is set to None and the host attribute shows
the original host. On success, in this example, the output
looks like:

$ openstack volume show 6088f80a-f116-4331-ad48-9afb0dfb196c

+--------------------------------+--------------------------------------+
| Field                          | Value                                |
+--------------------------------+--------------------------------------+
| attachments                    | []                                   |
| availability_zone              | zone1                                |
| bootable                       | false                                |
| consistencygroup_id            | None                                 |
| created_at                     | 2013-09-01T14:53:22.000000           |
| description                    | test                                 |
| encrypted                      | False                                |
| id                             | 6088f80a-f116-4331-ad48-9afb0dfb196c |
| migration_status               | None                                 |
| multiattach                    | False                                |
| name                           | test                                 |
| os-vol-host-attr:host          | server2@lvmstorage-2#lvmstorage-2    |
| os-vol-mig-status-attr:migstat | completing                           |
| os-vol-mig-status-attr:name_id | None                                 |
| os-vol-tenant-attr:tenant_id   | d88310717a8e4ebcae84ed075f82c51e     |
| properties                     | readonly='False'                     |
| replication_status             | disabled                             |
| size                           | 1                                    |
| snapshot_id                    | None                                 |
| source_volid                   | None                                 |
| status                         | in-use                               |
| type                           | None                                 |
| updated_at                     | 2017-02-22T02:35:03.000000           |
| user_id                        | d8e5e5727f3a4ce1886ac8ecec058e83     |
+--------------------------------+--------------------------------------+

Note that migstat is None, host is the new host, and
name_id holds the ID of the volume created by the
migration. If you look at the second LVM back end, you find the logical
volume volume-133d1f56-9ffc-4f57-8798-d5217d851862.