Configure multiple-storage back ends¶
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]).
Note
The configuration group name is not related to the volume_backend_name.
Note
After setting the enabled_backends flag on an existing cinder
service, and restarting the Block Storage services, the original host
service is replaced with a new host service. The new service appears
with a name like host@backend. Use:
$ cinder-manage volume update_host --currenthost CURRENTHOST --newhost CURRENTHOST@BACKEND
to convert current block devices to the new host 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:
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.
Note
For Fiber Channel drivers that support multipath, the configuration group
requires the use_multipath_for_image_xfer=true option. In
the example below, you can see details for HPE 3PAR and EMC Fiber
Channel drivers.
[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 Block Storage scheduler multi back end¶
You must enable the filter_scheduler option to use multiple-storage back ends. The filter scheduler:
- Filters the available back ends. By default,
AvailabilityZoneFilter,CapacityFilterandCapabilitiesFilterare enabled. - 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 Configure and use driver filter and weighing for 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.
Note
To list the extra-specifications, use this command:
$ openstack --os-username admin --os-tenant-name admin volume type list --long
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.
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.
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