Configure a multiple-storage back-end

To enable a multi back-end configuration, 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]).

	Note
	The configuration group name is not related to the volume_backend_name.

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:

# a list of back-ends that are served by this compute node
enabled_backends=lvmdriver-1,lvmdriver-2,lvmdriver-3
[lvmdriver-1]
volume_group=cinder-volumes-1
volume_driver=cinder.volume.drivers.lvm.LVMISCSIDriver
volume_backend_name=LVM_iSCSI
[lvmdriver-2]
volume_group=cinder-volumes-2
volume_driver=cinder.volume.drivers.lvm.LVMISCSIDriver
volume_backend_name=LVM_iSCSI
[lvmdriver-3]
volume_group=cinder-volumes-3
volume_driver=cinder.volume.drivers.lvm.LVMISCSIDriver
volume_backend_name=LVM_iSCSI_b

In this configuration, lvmdriver-1 and lvmdriver-2 have the same volume_backend_name. If a volume creation requests the LVM_iSCSI 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.

You must enable the filter_scheduler option to use multi back-end. Filter scheduler acts in two steps:

The scheduler uses the filtering and weighing process to pick the best back-end to handle the request, and explicitly creates volumes on specific back-ends through the use of volume types.

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

$ cinder --os-username admin --os-tenant-name admin type-create lvm

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

$ cinder --os-username admin --os-tenant-name admin type-key lvm set 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:

$ cinder --os-username admin --os-tenant-name admin type-create lvm_gold

$ cinder --os-username admin --os-tenant-name admin type-key lvm_gold set volume_backend_name=LVM_iSCSI_b

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

	Note
	To list the extra-specifications, use this command: $ cinder --os-username admin --os-tenant-name admin extra-specs-list

	Note
	If a volume type points to a volume_backend_name that does not exist in the Block Storage configuration, the filter_scheduler returns an error that it cannot find a valid host with the suitable back-end.

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.

$ cinder create --volume_type lvm --display_name test_multi_backend 1

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

$ cinder create --volume_type lvm_gold --display_name test_multi_backend 1

This second volume is created on lvmdriver-3.