Migrate volumes¶
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:
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 get-pools
+----------+----------------------------------------------------+
| Property | Value |
+----------+----------------------------------------------------+
| name | server1@lvmstorage-1#lvmstorage-1 |
+----------+----------------------------------------------------+
+----------+----------------------------------------------------+
| Property | Value |
+----------+----------------------------------------------------+
| name | server2@lvmstorage-2#lvmstorage-2 |
+----------+----------------------------------------------------+
Note
Block Storage API supports cinder get-pools since V2 version.
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 thevolume_name_templateconfiguration parameter). For example, ifvolume_name_templateis kept as the default value (volume-%s), your first LVM back end has a logical volume namedvolume-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 thename_idattribute.Note
If you plan to decommission a block storage node, you must stop the
cindervolume 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.
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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