Oversubscription in thin provisioning

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.

This parameter also can be set as max_over_subscription_ratio=auto. When using auto, Cinder will automatically calculate the max_over_subscription_ratio based on the provisioned capacity and the used space. This allows the creation of a larger number of volumes at the begginning of the pool’s life, and start to restrict the creation as the free space approaces to 0 or the reserved limit.


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.

Setting this value to ‘auto’. The values calculated by Cinder can dinamically vary according to the pool’s provisioned capacity and consumed space.

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


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.


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'


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.

Volume replication extra specs

OpenStack Block Storage has the ability to create volume replicas. Administrators can define a storage policy that includes replication by adjusting the cinder volume driver. Volume replication for OpenStack Block Storage helps safeguard OpenStack environments from data loss during disaster recovery.

To enable replication when creating volume types, configure the cinder volume with capabilities:replication="<is> True".

Each volume created with the replication capability set to True generates a copy of the volume on a storage back end.

One use case for replication involves an OpenStack cloud environment installed across two data centers located nearby each other. The distance between the two data centers in this use case is the length of a city.

At each data center, a cinder host supports the Block Storage service. Both data centers include storage back ends.

Depending on the storage requirements, there can be one or two cinder hosts. The administrator accesses the /etc/cinder/cinder.conf configuration file and sets capabilities:replication="<is> True".

If one data center experiences a service failure, administrators can redeploy the VM. The VM will run using a replicated, backed up volume on a host in the second data center.

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.

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