Migrate volumes

OpenStack has the ability to migrate volumes between back-ends. 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:

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.

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 in the OpenStack Configuration Reference for more details.

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

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

$ cinder show 6088f80a-f116-4331-ad48-9afb0dfb196c
+--------------------------------+--------------------------------------+
|            Property            |                Value                 |
+--------------------------------+--------------------------------------+
|          attachments           |                [...]                 |
|       availability_zone        |                zone1                 |
|            bootable            |                False                 |
|           created_at           |      2013-09-01T14:53:22.000000      |
|      display_description       |                 test                 |
|          display_name          |                 test                 |
|               id               | 6088f80a-f116-4331-ad48-9afb0dfb196c |
|            metadata            |                  {}                  |
|     os-vol-host-attr:host      |         server1@lvmstorage-1         |
| os-vol-mig-status-attr:migstat |                 None                 |
| os-vol-mig-status-attr:name_id |                 None                 |
|  os-vol-tenant-attr:tenant_id  |   6bdd8f41203e4149b5d559769307365e   |
|              size              |                  2                   |
|          snapshot_id           |                 None                 |
|          source_volid          |                 None                 |
|             status             |                in-use                |
|          volume_type           |                 None                 |
+--------------------------------+--------------------------------------+

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, RedHat 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:

$ cinder migrate 6088f80a-f116-4331-ad48-9afb0dfb196c server2@lvmstorage-2

You can use the cinder 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:

+--------------------------------+--------------------------------------+
|            Property            |                Value                 |
+--------------------------------+--------------------------------------+
|          attachments           |                [...]                 |
|       availability_zone        |                zone1                 |
|            bootable            |                False                 |
|           created_at           |      2013-09-01T14:53:22.000000      |
|      display_description       |                 test                 |
|          display_name          |                 test                 |
|               id               | 6088f80a-f116-4331-ad48-9afb0dfb196c |
|            metadata            |                  {}                  |
|     os-vol-host-attr:host      |         server2@lvmstorage-2         |
| os-vol-mig-status-attr:migstat |                 None                 |
| os-vol-mig-status-attr:name_id | 133d1f56-9ffc-4f57-8798-d5217d851862 |
|  os-vol-tenant-attr:tenant_id  |   6bdd8f41203e4149b5d559769307365e   |
|              size              |                  2                   |
|          snapshot_id           |                 None                 |
|          source_volid          |                 None                 |
|             status             |                in-use                |
|          volume_type           |                 None                 |
+--------------------------------+--------------------------------------+

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.

	Note
	The migration is not visible to non-admin users (for example, through the volume `status`). However, some operations are not allowed while a migration is taking place, such as attaching/detaching a volume and deleting a volume. If a user performs such an action during a migration, an error is returned.

	Note
	Migrating volumes that have snapshots are currently not allowed.

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