Provisioning Improvements

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Cinder provisioning is still a source of pain for everyone: end users, admins, and developers. Multiple factors have contributed to our current situation, like the information being dispersed in different specs, developer’s reference, and even the code, but we also have cases of misinterpreted documentation, documentation not keeping up with the project’s evolution, and even misleading or incorrect documentation.

This spec will build on those that preceded it on the same topic [1] [2] and others related to the subject [3] to bring a consolidated and updated view on the matter as well as add a some minor improvements and fix some issues in hopes that we can provide a better experience for all involved.

Problem description

Our current situation is quite chaotic, we have volume creations that fail based on an incorrect capacity calculation that would have succeeded if we had just received an update on the stats of the backend, we have drivers that are reporting incorrect data on the stats, and we have volumes that cannot be created when they should be allowed to.

Before going any further we first need to define the terms we’ll be using to ensure they hold the same meaning for all of us, as this is the source of some of our current issues.

Disagreement on the mapping of these terms and their description is understandable, but for the sake of understanding each other we’ll hold below descriptions as true since they were defined as such in our specs and most of our code.

Improvements on the word used for the terms and field names can be discussed at another time and updated in the specs, documentation, and code accordingly.

For the sake of completeness and to remove any misunderstandings that could lead to different implementations on the drivers, which we currently have, the descriptions will also include some clarifications and examples that may reference current Cinder code.



Even though this is formally known as the symbol representation of a gigabyte -decimal unit of measurement- we will be using it throughout the spec and our code as the symbol for the gibibyte -binary unit of measurement as defined by the International Electrotechnical Commission (IEC), with symbol GiB-, so when we talk about 1GB we are talking about 1024MB, and the same applies to TB and MB.

Total capacity:

It is the total physical capacity that would be available in the storage array’s pool being used by Cinder if no volumes were present.

This is currently being reported by the drivers as total_capacity_gb and, as the name indicates, should be reported in GB and with a precision no greater than 2 decimals.

If the storage array has 5TB of space but the pool used in Cinder is limited to 1TB then the driver should be reporting a total_capacity_gb of 1024GB.

Volume size:

It is the maximum physical size that a volume can take in the storage array.

This is referenced throughout the code as volume_size.

For a thick volume the volume_size will be the same as the free capacity we lost when it was provisioned, whereas for a thin volume it will be greater than the space used for the volume in the storage array until the volume gets completely full.

Free capacity:

It is the current physical capacity available in the storage array’s pool being used by Cinder. The number and volume sizes of the thin and thick volumes that have been provisioned by Cinder or directly in the storage array are irrelevant here.

This is currently being reported by the drivers as free_capacity_gb and, as the name indicates, should be reported in GB and with a precision no greater than 2 decimals.

If the storage array has 5TB of space with a total of 3TB available for all its pools but Cinder is using a pool that has a limit of 1TB of which it has already used 400GB and someone has manually created volumes outside of Cinder that are currently using 124GB of space, then the driver should be reporting a free_capacity_gb of 500GB (1TB = 1024GB = 400GB + 124GB + 500GB).

Provisioned capacity:

The amount of capacity that would be used in the storage array’s pool being used by Cinder if all the volumes present in there were completely full.

This is currently being reported by the drivers as provisioned_capacity_gb and, as the name indicates, should be reported in GB and with a precision no greater than 2 decimals. This is a required field and must always be present.

This includes not only volumes created by Cinder but also all other existing volumes in that backend, but does not include snapshots.

Let’s expand the earlier example from “free capacity” where 524GB of the available 1TB had already been used, and say that the 124GB that were externally created were all used by 1GB thick volumes, and that Cinder was using the 400GB with 400 thick volumes of 1GB and 20 empty thin volumes of 20GB each. In this situation our reported provisioned_capacity_gb value should be 924GB ((124 * 1GB) + (400 * 1GB) + (20 * 20GB)).

If a driver does not report the provisioned_capacity_gb data we’ll use the automatically calculated allocated_capacity_gb as described below.

Allocated capacity:

Contrary to what the name may suggest this is not referring to the “allocated” space on the storage array, but to the provisioned volumes created by this specific Cinder Volume backend process on the storage array’s pool being used by Cinder and that still present.

Important to notice that this refers to a specific service backend, so if you are running a multi-backend Cinder service or multiple Cinder Volume services where you have more than one backend configured to use the same storage array’s pool, then each one of these backends will only be reporting the sum of the volume_size of the volumes they created and not the sum of all the volume_size of the volumes that have been created by a Cinder service.

This is currently being reported by the Volume service as allocated_capacity_gb and, as the name indicates, should be reported in GB.

For two volumes had been created, one thick and one thin, each one of 1GB, then you’ll be reporting 2GB as allocated_capacity_gb, but if you were to unmanage one of those volumes then you would only be reporting 1GB, even if the volume is still there and will still be counted in the provisioned_capacity_gb.

This field is calculated directly by the Cinder core code and drivers should not calculate or report this information on their get_volume_stats method.

Over subscription ratio:

It is the maximum ratio between the “provisioned capacity” and the “total capacity” represented as a real number. A ratio of 1.0 means that the “provisioned capacity” cannot exceed the “total capacity” whereas a value of 5.0 means that the Cinder backend is allowed to create as much as 5 times the “total capacity” of the storage array’s pool in volumes.

This will only have effect when a thin provisioned volume is being created, and will be ignored for thick provisioned.

This is currently being reported by the drivers as max_over_subscription_ratio with a greater or equal value to 1.0, preferably with no more than a 2 decimal precision.

This value is optional, and when missing from the driver’s status report the value defined in the [DEFAULT] section on the Cinder scheduler receiving the request will be used. So vendors should make sure that they are correctly returning this value in their drivers if they support thin provisioning and admins should make sure they have a consistent default value of the max_over_subscription_ratio across all scheduler nodes.

Note that this ratio is per backend or per pool depending on driver implementation.

Reserved percentage:

Represents the percentage of the storage array’s “total capacity” that is reserved and should not be used for calculations. It is represented by an integer value going from 0 up to 100.

This is currently being reported by the drivers as reserved_percentage with a greater or equal value to 1.0, preferably with no more than a 2 decimal precision.

Default value is 0 if the field is missing in the status report from the backend or if the user has not defined it in the backend’s Cinder configuration. This is per backend or per pool depending on driver implementation.

Provisioning support:

Cinder backends may support up to two different types of provisioning, thin and thick and drivers are expected to indicate as capable of one of them at least in their capabilities report.

The way to report support for these is setting to true the boolean fields thin_provisioning_support and/or thick_provisioning_support. And non reported provisioning types will default to false.

A Cinder backend may support both provisioning types at the same time.

Volume provisioning type:

For Cinder backends that only support one of the provisioning types all volumes created on them will be of that type, and we can use the volume type’s extra specs to make the scheduler filter out backends not supporting a specific provisioning type:

  • ‘thin_provisioning_support’: ‘<is> True’ or ‘<is> False’

  • ‘thick_provisioning_support’: ‘<is> True’ or ‘<is> False’

But if our deployment is using a backend that is supporting both provisioning types simultaneously we need to be explicit about the type of provisioning we want for a volume using the volume type’s extra spec provisioning:type and setting it to thin or thick.

If no provisioning:type is defined for a volume it will default to thin if the backend is capable of it, and the driver is expected to honor this assumption.

Incorrect reports

Given above terms which were originally defined in their corresponding specs, even if there may be additional comments in this one, we can determine that there are a good number of Cinder drivers that do not follow these definitions and are reporting what would be incorrect values.

Reporting incorrect values means that on a heterogeneous cloud you’ll have inconsistent scheduling and an admin will not be able to make sense of the stats from the volumes.

To illustrate this here are some of the interpretations we can see across different drivers for the provisioned_capacity_gb:

  • Sum of all the volumes’ max sizes, which is correct.

  • Sum of all the volumes’ physical disk usage, which is wrong.

  • Sum of the Cinder volumes’ physical disk usage, which is wrong.

And something similar happens with the allocated_capacity_gb where drivers go and report the value directly instead of letting the Cinder core code take care of it. Drivers have been known to report here the following information:

  • Sum of the Cinder volumes’ physical disk usage, which is correct.

  • Sum of the physical disk usage, which is wrong.

  • Sum of all the volumes’ max sizes, which is wrong.

Provisioning calculations

Some of the creation failures are based on the provisioned_capacity_gb value being wrong, but there are other cases where Cinder’s calculations for over provisioning do not match industry’s standard definition, which for some admins create confusion and undesired behavior.

Standard provisioning calculation to check if a volume of volume_size fits is:

((provisioned_capacity_gb + volume_size) <=
  x (1 - (reserved_percentage / 100.0))
  x max_over_subscription_ratio))

Whereas the Cinder calculations, which were agreed on as the best calculations for being considered safer are:

(volume_size <=
  - (total_capacity_gb x reserved_percentage / 100.0))
 x max_over_subscription_ratio)

Calculating max over subscription ratio

Most deployments have very dynamic workloads each with different physical storage requirements, which means that one month we may require many volumes of which we barely use any space and next month we may require fewer volumes but use most of the provisioned capacity.

This makes it almost impossible to accurately model our storage requirements at deployment time, which is precisely when we have to set the max_over_subscription_ratio for our Cinder backends.

As requirements change one option would be to change the configuration and restart our Cinder Volume services, but since Cinder is also in the data path the restart may take a long time to do and will have a considerable impact on our cloud users.

Not being able to determine beforehand the best max_over_subscription_ratio and not being able to easily restart the Cinder service is a common pain that most operators have with backends supporting thin provisioning.

Use Cases

The basic case for fixing the status report is where we would like to have consistent reporting from our backends for the admins to see in the logs and for the scheduler to use.

Any operator using thin provisioning storage that wants to optimize their storage usage and dynamically adjust to the dynamic requirements of its cloud.

As for the alternative calculations it would greatly benefit any backend that is close to their full capacity or one that is creating huge volumes that usually never get filled in.

Proposed change

Incorrect reports

Since we have consolidated all the documentation in one place, this spec, where we clearly state expected driver behavior all driver maintainers will be urged to make their drivers compliant with it. This will mean adapt their drivers to follow this document’s definition for provisioned_capacity_gb and stop reporting the allocated_capacity_gb field.

Automatic over subscription ratio calculation

To allow automatic over subscription ratio calculation we will make existing configuration option max_over_subscription_ratio into a polymorphic option that accepts not only integers and floats, but also a new string value auto that will instruct Cinder to use our default value of 20 as the starting reference when the backend is empty and then, when there is data calculate the current value on each driver stats report with the following formula:

ratio = 1 + (`provisioned_capacity_gb` /
             (`total_capacity_gb` - `free_capacity_gb` + 1))

If the driver is not reporting provisioned_capacity_gb then we’ll proceed to use the allocated_capacity_gb instead:

ratio = 1 + (`allocated_capacity_gb` /
             (`total_capacity_gb` - `free_capacity_gb` + 1))

We are not considering the reserved capacity in this formula because the scheduler’s capacity filter already takes it into consideration when calculating the virtual free capacity.

There are a couple of drivers that are already doing this, Pure and Kaminario’s K2, but with different configuration options, pure_automatic_max_oversubscription_ratio and auto_calc_max_oversubscription_ratio respectively, so those configuration options will take precedence, if configured, over this new feature, but we’ll encourage vendors to deprecate those options.

Drivers that are making use of the max_over_subscription_ratio in a non compatible way will raise an error and fail to start, logging an appropriate message if configured with this new feature.

Provisioning calculations

Instead of keep fighting with admins and developers on which one of the approaches is best -standard calculation or Cinder’s- we will be adding a new configuration option called over_provisioning_calculation which will take values standard and cinder and default to cinder for backward compatibility and that will be used by the CapacityFilter to determine which one of the mechanism to use.

This configuration option will also affect CapacityWeigher as it will need to do the free space calculation according to the standard definition as well.

As one can assume thick provisioning will have no modifications on its behavior.


  • Don’t support standard over-provisioning calculations.

  • Instead of modifying CapacityFilter and CapacityWeigher create 2 new classes.

  • Instead of adding the over_provisioning_calculation configuration option make the filter use the options JSON file provided by scheduler_json_config_location . This data seems to be currently missing on some of the operations like migrate, extend, so that would need to changed.

Data model impact


REST API impact

The only affected API will be the get_pools API that will be able to return the 2 new fields, total_used_capacity_gb and cinder_used_capacity_gb, when they are being reported by the driver’s get_volume_stats method. Fields will not be present if the drivers are not reporting them.

Security impact


Notifications impact


Other end user impact

The user may see new fields when calling cinderclient’s get_pools.

Performance Impact

Depending on the driver and the storage array, performance could increase or decrease, since getting provisioned sizes instead of physical sizes could be faster or slower.

Other deployer impact

With the change of values returned by Cinder backends for allocated_capacity_gb and provisioned_capacity_gb we may experience failures on creating volumes until we correct the values of reserved_percentage and max_over_subscription_ratio in our cloud to the right values, since we may have been using incorrect ones.

A configuration option will be modified:

  • max_over_subscription_ratio: Changes its type from Float to String and adds a new possible value, auto, that will enable this new feature.

A new configuration options will be added:

  • over_provisioning_calculation: Will allow to select what kind of calculations the CapacityFilter does to determine if there is space for a volume in a backend. Acceptable values are standard and cinder. Default values will be cinder.

Developer impact

Driver maintainer will need to verify, and fix if necessary, their stat reports for allocated_capacity_gb and provisioned_capacity_gb and if they are using max_over_subscription_ratio in a non compatible way they should fix their driver so that it supports the feature.



Primary assignee:


Other contributors:


Work Items

  • File bugs for drivers that are not in compliance.

  • Fix drivers stat reporting.

  • Add support for the 3 new fields, provisioned_capacity_precission, total_used_capacity_gb and cinder_used_capacity_gb in the scheduler, the get_pools API, and the client.

  • Modify CapacityFilter to support the standard over-provisioning calculation.

  • Modify CapacityWeigher to support standard over-provisioning calculations.

  • Add to the volume manager the estimation mechanism for drivers that don’t report provisioned_capacity_gb.

  • Update all the developers reference docs to ensure that there is no more confusion on what the report stats need to return, and make sure that the wiki page on how to contribute a driver links to that documentation explaining the importance of following it when writing the driver.




New unit tests will be added to test the changed code.

Documentation Impact

Since our current documentation is lacking in this aspect this will add and update it to reflect what’s expected of the driver in the stats reports.

End user documentation should also be updated.