Cinder Thin provisioning and Oversubscription

Background

After the support on Cinder for Thin provisioning, driver maintainers have been struggling to understand what is the expected behavior of their drivers and what exactly each value reported means. This document summarizes the concepts, definitions and terminology from all specs related to the subject and should be used as reference for new drivers implementing support for thin provisioning.

Core concepts and terminology

In order to maintain the same behavior among all drivers, we first need to define some concepts used throughout drivers. This terminology is discussed and defined in this spec[1] and should be used as reference in further implementations.

Stats to be reported

The following fields should be reported by drivers supporting thin provisioning on the get_volume_stats() function:

Mandatory Fields

thin_provisioning_support = True (or False)

Optional Fields

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

Note

If provisioned_capacity_gb is not reported, the value used in the scheduler calculations and filtering is allocated_capacity_gb.

Note

If max_over_subscription_ratio is not reported, the scheduler will use the value defined on the [DEFAULT] section. This falls back to the default value (20.0) if not set by the user.

[1] https://specs.openstack.org/openstack/cinder-specs/specs/queens/provisioning-improvements.html