When you configure multiple-storage back ends, you can create several
back-end storage solutions that serve the same OpenStack Compute
configuration and one cinder-volume is launched for each
back-end storage or back-end storage pool.

In a multiple-storage back-end configuration, each back end has a
name (volume_backend_name). Several back ends can have the
same name. In that case, the scheduler properly decides which back end
the volume has to be created in.

The name of the back end is declared as an extra-specification of a
volume type (such as, volume_backend_name=LVM). When a
volume is created, the scheduler chooses an appropriate back end to
handle the request, according to the volume type specified by the
user.

Enable multiple-storage back
ends

To enable a multiple-storage back ends, you must set the enabled_backends flag in the
cinder.conf file. This flag defines the names (separated by
a comma) of the configuration groups for the different back ends: one
name is associated to one configuration group for a back end (such as,
[lvmdriver-1]).

$ cinder-manage volume update_host --currenthost CURRENTHOST --newhost CURRENTHOST@BACKEND

The options for a configuration group must be defined in the group
(or default options are used). All the standard Block Storage
configuration options (volume_group,
volume_driver, and so on) might be used in a configuration
group. Configuration values in the [DEFAULT] configuration
group are not used.

These examples show three back ends:

enabled_backends=lvmdriver-1,lvmdriver-2,lvmdriver-3
[lvmdriver-1]
volume_group=cinder-volumes-1
volume_driver=cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name=LVM
[lvmdriver-2]
volume_group=cinder-volumes-2
volume_driver=cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name=LVM
[lvmdriver-3]
volume_group=cinder-volumes-3
volume_driver=cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name=LVM_b

In this configuration, lvmdriver-1 and
lvmdriver-2 have the same volume_backend_name.
If a volume creation requests the LVM back end name, the
scheduler uses the capacity filter scheduler to choose the most suitable
driver, which is either lvmdriver-1 or
lvmdriver-2. The capacity filter scheduler is enabled by
default. The next section provides more information. In addition, this
example presents a lvmdriver-3 back end.

[3par]
use_multipath_for_image_xfer = true
volume_driver = cinder.volume.drivers.hpe.hpe_3par_fc.HPE3PARFCDriver
volume_backend_name = 3parfc

[emc]
use_multipath_for_image_xfer = true
volume_driver = cinder.volume.drivers.emc.emc_smis_fc.EMCSMISFCDriver
volume_backend_name = emcfc

Configure shared volume
driver backends

When configuring multiple volume backends, common configuration
parameters can be shared using the [backend_defaults] section. As an example:

[DEFAULT]
enabled_backends=backend1,backend2,backend3

[backend_defaults]
image_volume_cache_enabled = True
volume_clear = none
target_helper = tgtadm
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver

[backend1]
volume_group = cinder-volume-1
image_volume_cache_enabled = False

[backend2]
volume_group = cinder-volume-2

[backend3]
volume_group = cinder-volume-3

In this configuration, backend2 and
backend3 have the same
image_volume_cache_enabled as it is defined in the
backend_defaults section. In other words,
backend2 and backend3 have enabled the image
cache features. image_volume_cache_enabled in
backend1 is False, that means any overwritten configuration
in a volume backend will ignore the original value in
backend_defaults.

Configure
Block Storage scheduler multi back end

You must enable the filter_scheduler
option to use multiple-storage back ends. The filter scheduler:

1. Filters the available back ends. By default,
   AvailabilityZoneFilter, CapacityFilter and
   CapabilitiesFilter are enabled.
2. Weights the previously filtered back ends. By default, the CapacityWeigher option is enabled. When this
   option is enabled, the filter scheduler assigns the highest weight to
   back ends with the most available capacity.

The scheduler uses filters and weights to pick the best back end to
handle the request. The scheduler uses volume types to explicitly create
volumes on specific back ends. For more information about filter and
weighing, see filter_weigh_scheduler.

Volume type

Before using it, a volume type has to be declared to Block Storage.
This can be done by the following command:

$ openstack --os-username admin --os-tenant-name admin volume type create lvm

Then, an extra-specification has to be created to link the volume
type to a back end name. Run this command:

$ openstack --os-username admin --os-tenant-name admin volume type set lvm \
  --property volume_backend_name=LVM_iSCSI

This example creates a lvm volume type with
volume_backend_name=LVM_iSCSI as extra-specifications.

Create another volume type:

$ openstack --os-username admin --os-tenant-name admin volume type create lvm_gold

$ openstack --os-username admin --os-tenant-name admin volume type set lvm_gold \
  --property volume_backend_name=LVM_iSCSI_b

This second volume type is named lvm_gold and has
LVM_iSCSI_b as back end name.

$ openstack --os-username admin --os-tenant-name admin volume type list --long

Usage

When you create a volume, you must specify the volume type. The
extra-specifications of the volume type are used to determine which back
end has to be used.

$ openstack volume create --size 1 --type lvm test_multi_backend

Considering the cinder.conf described previously, the
scheduler creates this volume on lvmdriver-1 or
lvmdriver-2.

$ openstack volume create --size 1 --type lvm_gold test_multi_backend

This second volume is created on lvmdriver-3.