Over Subscription in Thin Provisioning

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This proposal is to introduce a mechanism to allow over subscription in thin provisioning and also use reserved percentage to prevent over provisioning.

Problem description

There are a couple of problems in capacity reporting in thin provisioning:

  • Some drivers are still using ‘infinite’ and ‘unknown’ to report capacity. This could lead to over provisioning.
  • There’s no mechanism to allow over subscription in thin provisioning. Over subscription allows flexibility in storage allocation and usage and it is an important concept in providing thin provisioning support.


The following terminologies will be used in this spec:

total_capacity: This is an existing parameter already reported by the driver. It is the total physical capacity. Example: Assume backend A has a total physical capacity of 100G.

free_capacity: This is an existing parameter already reported by the driver. It is the real physical capacity available to be used. Example: Assume backend A has a total physical capacity of 100G. There are 10G thick luns and 20G thin luns (10G out of the 20G thin luns are written). In this case, free_capacity = 100 - 10 -10 = 80G.

free: This is calculated in the scheduler by substracting reserved space from free_capacity.

volume_size: This is an existing parameter. It is the size of the volume to be provisioned.

provisioned_capacity: This is a new parameter. It is the apparent allocated space indicating how much capacity has been provisioned. Example: User A created 2x10G volumes in Cinder from backend A, and user B created 3x10G volumes from backend A directly, without using Cinder. Assume those are all the volumes provisioned on backend A. The total provisioned_capacity will be 50G and that is what the driver should be reporting.

allocated_capacity: This is an existing parameter. Cinder uses this to keep track of how much capacity has been allocated through Cinder. Example: Using the same example above for provisioned_capacity, the allocated_capacity will be 20G because that is what has been provisioned through Cinder. allocated_capacity is documented here to differentiate from the new parameter provisioned_capacity. If a driver can guarantee that the backend is used solely for a specific Cinder installation, then provisioned_capacity could be the same as allocated_capacity. Only driver has knowledge of this and therefore driver can make a decision on how to report provisioned_capacity.

max_over_subscription_ratio: This is a new parameter. It is the float representation of the over subscription ratio when thin provisioning is involved. It is a ratio of provisioned capacity over total capacity. Default ratio is 1.0, meaning provisioned capacity cannot exceed the total capacity. Note that this ratio is per backend or per pool depending on driver implementation.

reserved_percentage: This is an existing parameter. It represents the percentage of backend capacity that is reserved. It ranges from 0 to 100. Default is 0. Note this refers to the real capacity. This is per backend or per pool depending on driver implementation.

reserved: This is a ratio of reserved_percentage over 100 calculated by the scheduler.

virtual_free_capacity: This is a parameter calculated based on other parameters. It refers to how much space a user can still provision apparently. Note that this is different from free_capacity. free_capacity is about the real available physical capacity. virtual_free_capacity is about apparent available virtual capacity. Example: Assume the total capacity of backend A is 100G. The max over subscription ratio is 2.0. If no volumes have been provisioned yet, the virtual_free_capacity is 100 x 2.0 = 200. If 50G volumes have already been provisioned, the virtual_free_capacity is 200 - 50 = 150.

Use Cases

Proposed change

New parameters in get_volume_stats

One new configuration options “max_over_subscription_ratio” will be added to cinder/volume/driver.py. This configuration option will be added to cinder.conf. It can be configured for each backend when multiple-backend is enabled.

Note: This configuration option is provided as a reference implementation and will be used by the LVM driver. However, it is not a requirement for a driver to use this option from cinder.conf. Driver can choose its own way if that makes more sense.

In scheduler/host_manager.py, the following additional information sent by each backend will be saved and will be used later by the scheduler to make decisions. Note these are reported for a backend or a pool, depending on driver implementation. In the LVM driver, these will be reported for a backend (which is treated as one pool).

  • provisioned_capacity
  • max_over_subscription_ratio

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

Note: The configuration option max_over_subscription_ratio added in cinder.conf is for configuring a backend. For a driver that supports multiple pools per backend, it can report these ratios for each pool in get_volume_stats.

  • Driver can get this option in cinder.conf and report the same ratio for the pools that belonging to the same backend.
  • Alternatively driver can choose to report different ratio for each pool in get_volume_stats, without using this option in cinder.conf.


Driver can report the following capabilities for a backend or a pool:

thin_provisioning_support = True (or False) thick_provisioning_support = True (or False)

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

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 backend that supports thin provisioning, thick provisioning, or both to match the needs of a specific volume type.

If an extra spec scope key “provisioning:type” is defined, it can be used by the driver to detemine whether the lun to be provisioned is thin or thick. The value of this extra spec is either “thin” or “thick”. Note this extra spec is not used by the scheduler to find a backend.

Capacity filter

In the capacity filter, the following will be evaluated in the decision making when choosing a backend that fits the criteria:

If (provisioned_capacity + volume_size) / total_capacity >= max_over_subscription_ratio, the backend will not be chosen to provision the volume. Note: This formula will be executed only if “thin_provisioning_support” is True and max_over_subscription_ratio >= 1.

If ((free_capacity - total_capacity * reserved) * max_over_subscription_ratio) < volume_size, the backend will not be chosen to provision the volume. Note: This formula will be executed only if “thin_provisioning_support” is True and max_over_subscription_ratio >= 1.

If (free_capacity - total_capacity * reserved) < volume_size, the backend will not be chosen to provision the volume. Note this check was already in the capacity filter, but the formula is changed to use total_capacity * reserved instead of free_capacity * reserved.

Capacity weigher

In the capacity weigher, virtual_free_capacity should be used for ranking if “thin_provisioning_support” is True. Otherwise, real free_capacity will be used as before. A change is made to measured reserved space against the total_capacity. virtual_free_capacity = total_capacity * max_over_subscription_ratio - provisioned_capacity - total_capacity * reserved

LVM driver

In the default LVM driver, changes will be made in get_volume_stats which periodically reports capabilities and the information will be received by the scheduler.

  • Changes will be made in the LVM driver to report provisioned_capacity. It makes calls to the LVM class in brick to retrieve volume information including capacities.
  • The LVM driver will also report max_over_subscription_ratio. This will be from the configuration parameters set in cinder.conf.
  • While other drivers need to report max_over_subscription_ratio, they are not required to read those ratios from cinder.conf.

Changes will also be made in the following LVM driver functions to make sure over provisioning will not happen even when a request didn’t go through the scheduler:

  • create_volume
  • extend_volume

The following will be evaluated in the above LVM driver functions:

  • If the ratio of the apparent provisioned capacity over real total capacity has exceeded the over subscription ratio, the operation will fail.
  • If the free space is smaller than the volume size, the operation will fail.

Use cases

The design of this feature will support the following use cases.

Use case 1: Each volume type has a separate backend or pool. For example, Gold volume type uses pool gold, Silver volume type uses pool silver, and Bronze volume type uses pool bronze. Each pool can have a different max over subscription ratio.

Use case 2: One volume type is associated with multiple backends or pools. For example, Silver volume type uses pool 1 and pool 2. Both pools can have the same max over subscription ratio. Note that capacities for each pool can be different at any given time.

Use case 3: One backend or pool is used by multiple volume types. For example, pool 3 is used by volume types Gold, Silver, and Bronze. Assume Gold volume type uses thick luns only, Silver volume type can have either thick or thin luns, and Bronze volume type has thin luns only. Because the over subscription ratio is calculated by the ratio of provisioned_capacity over total_capacity and all three volume types are sharing the same pool, the ratio will be the same for all volume types. Gold volume type can guarantee its space reservation by creating thick luns. The apparent size and the used size of a Gold volume will always be the same. For a thin lun created as Silver or Bronze volume type, the apparent size can be bigger than the real size. Some detailed examples are shown at this etherpad: https://etherpad.openstack.org/p/cinder-over-subscription-white-board


Without this, we cannot support over subscription in thin provisioning and there’s also no upper limit that prevents over provisioning from happening.

Data model impact


REST API impact


Security impact


Notifications impact

If the capacity usage has exceeded the used ratio or if the provisioned capacity has exceeded the over subscription ratio, a notification should be sent. The notification should report the name of the backend or pool and the capacity information from the backend or pool. The purpose of the notification is for the storage administrator to take notice and take actions to fix the problem.

Notification will also be sent periodically whenever the scheduler receives an update of capacities from the backend. This will be consumed by Ceilometer. This was discussed for the Capacity Headroom topic at the summit. The Ceilometer team will be responsible for the changes required on the Ceilometer side for this.

Other end user impact

There is a new parameter in cinder.conf that end user needs to be aware of.

Performance Impact


Other deployer impact

New parameters over_subscription_ratio will be added to cinder.conf.

Developer impact

Drivers should report provisioning capabilities (thin_provisioning_support and thick_provisioning_support).

Drivers supporting thin provisioning should report provisioned capacity in addition to free capacity in get_volume_stats.

For drivers supporting thick provisioning only, free capacity will be used just as before.

For drivers supporting both thin and thick provisioning, provisioned capacity and free capacity should both be reported.

If there is a range regarding capacity and you are not sure how to report, please be conservative. For example, if the available capacity is in the range of 80 to 100 GB, be conservative and report the lower bound 80 GB.

Driver developers can take a look of _update_volume_stats in the LVM driver as a reference implementation.

Note: This work is also needed for Cinder to use ThinLVM as the default driver in Kilo.



Primary assignee:

Other contributors:

Work Items

  1. Add max_over_subscription_ratio in driver.py.
  2. Modify host_manager.py to update provisioned capacity, over subscription ratio by the backends.
  3. Modify capacity filter to check whether over subscription ratio has been exceeded in a backend.
  4. New parameters max_over_subscription_ratio will be added to cinder.conf.
  5. LVM driver will be changed to report virtual capacity and over subscrption ratio.
  6. LVM class in brick will be updated to calculate provisioned capacity.
  7. LVM driver functions will be changed to check whether over subscription ratio has been exceeded.




New unit tests will be added to test the changed code. Testing will be done using the LVM driver for thin provisioning. Testing will be done to cover the 3 use cases described above.

Documentation Impact

Documentation changes are needed for the following: New parameter max_over_subscription_ratio will be added to cinder.conf. Driver needs to add provisioning capabilities (thick_provisioning_suppot, thin_provisioning_support) and report provisioned_capacity.


Examples: https://etherpad.openstack.org/p/cinder-over-subscription-white-board

Virtual capacity “provisioned_capacity_gb” was discussed in Winston’s spec https://review.openstack.org/#/c/105190/6/specs/juno/volume-statistics-reporting.rst

Kilo design summit session on this topic: https://etherpad.openstack.org/p/kilo-cinder-over-subscription https://etherpad.openstack.org/p/kilo-cinder-capacity-headroom

Documentation on the filter scheduler: http://docs.openstack.org/developer/nova/devref/filter_scheduler.html Note: This is a document on Nova filter scheduler, but it is very similar to the Cinder filter scheduler.