openstack-manuals/doc/admin-guide-cloud/source/blockstorage_over_subscription.rst

Oversubscription in thin provisioning

OpenStack Block Storage enables you to choose a volume back end based on virtual capacities for thin provisioning using the oversubscription ratio.

A reference implementation is provided for the default LVM driver. The illustration below uses the LVM driver as an example.

Configure oversubscription settings

To support oversubscription in thin provisioning, a flag max_over_subscription_ratio is introduced into cinder.conf. This is a float representation of the oversubscription ratio when thin provisioning is involved. Default ratio is 20.0, meaning provisioned capacity can be 20 times of the total physical capacity. A ratio of 10.5 means provisioned capacity can be 10.5 times of the total physical capacity. A ratio of 1.0 means provisioned capacity cannot exceed the total physical capacity. A ratio lower than 1.0 is ignored and the default value is used instead.

Note

max_over_subscription_ratio can be configured for each back end when multiple-storage back ends are enabled. It is provided as a reference implementation and is used by the LVM driver. However, it is not a requirement for a driver to use this option from cinder.conf.

max_over_subscription_ratio is for configuring a back end. For a driver that supports multiple pools per back end, it can report this ratio for each pool. The LVM driver does not support multiple pools.

The existing reserved_percentage flag is used to prevent over provisioning. This flag represents the percentage of the back-end capacity that is reserved.

Note

There is a change on how reserved_percentage is used. It was measured against the free capacity in the past. Now it is measured against the total capacity.

Capabilities

Drivers can report the following capabilities for a back end or a pool:

thin_provisioning_support = True(or False)
thick_provisioning_support = True(or False)
provisioned_capacity_gb = PROVISIONED_CAPACITY
max_over_subscription_ratio = MAX_RATIO

Where PROVISIONED_CAPACITY is the apparent allocated space indicating how much capacity has been provisioned and MAX_RATIO is the maximum oversubscription ratio. For the LVM driver, it is max_over_subscription_ratio in cinder.conf.

Two capabilities are added here to allow a back end or pool to claim support for thin provisioning, or thick provisioning, or both.

The LVM driver reports thin_provisioning_support=True and thick_provisioning_support=False if the lvm_type flag in cinder.conf is thin. Otherwise it reports thin_provisioning_support=False and thick_provisioning_support=True.

Volume type extra specs

If volume type is provided as part of the volume creation request, it can have the following extra specs defined:

'capabilities:thin_provisioning_support': '<is> True' or '<is> False'
'capabilities:thick_provisioning_support': '<is> True' or '<is> False'

Note

capabilities scope key before thin_provisioning_support and thick_provisioning_support is not required. So the following works too:

'thin_provisioning_support': '<is> True' or '<is> False'
'thick_provisioning_support': '<is> True' or '<is> False'

The above extra specs are used by the scheduler to find a back end that supports thin provisioning, thick provisioning, or both to match the needs of a specific volume type.

Capacity filter

In the capacity filter, max_over_subscription_ratio is used when choosing a back end if thin_provisioning_support is True and max_over_subscription_ratio is greater than 1.0.

Capacity weigher

In the capacity weigher, virtual free capacity is used for ranking if thin_provisioning_support is True. Otherwise, real free capacity will be used as before.