How to calculate the block storage usage in Openstack

Problem

How to calculate the block storage usage, free/total capacity, and oversubscription ratio using OpenStack Cinder metrics.

This article explains how to calculate the block storage usage from cinder get-pools --detail in OpenStack. It is applicable for any Cinder backend (e.g., Ceph, HPE 3PAR, NetApp, etc.) and helps you interpret key metrics related to block storage usage, provisioning, and backend status.

Environment

Platform9 Managed OpenStack - v4.0 and Higher
Cinder

Key Terms

Term	Description
Total Capacity	Physical capacity of the backend storage.
Allocated Capacity	Actual space used on the backend storage (physical usage).
Free Capacity	Unused portion of physical storage.
Provisioned Capacity	Total logical volume size requested by users. May exceed physical space if thin provisioning is used.
Oversubscription Ratio	How much logical capacity is provisioned relative to what's physically used.
Max Oversubscription Ratio	Configured upper limit of how much Cinder is allowed to oversubscribe the backend.

Answer

Source admin.rc file.

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$ source admin.rc
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Run below command to list all the available cinder pools.

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$ cinder get-pools --detail
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Example output for Cinder with Ceph as backend storage.

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$ cinder get-pools --detail +-----------------------------+-------------------------------------------------------------------+| Property                    | Value                                                             |+-----------------------------+-------------------------------------------------------------------+| allocated_capacity_gb       | 3132                                                              || backend_state               | up                                                                || driver_version              | [VERSION]                                                         || filter_function             | None                                                              || free_capacity_gb            | 36527.21                                                          || goodness_function           | None                                                              || location_info               | [LOCATION_INFO]                                                   || max_over_subscription_ratio | 20.0                                                              || multiattach                 | True                                                              || name                        | [CINDER_POOL_NAME]                                                || provisioned_capacity_gb     | 4532                                                              || replication_enabled         | False                                                             || reserved_percentage         | 0                                                                 || storage_protocol            | ceph                                                              || thin_provisioning_support   | True                                                              || timestamp                   | [TIMESTAMP]                                                       || total_capacity_gb           | 103680.78                                                         || vendor_name                 | Open Source                                                       || volume_backend_name         | ceph1                                                             |+-----------------------------+-------------------------------------------------------------------+.. more
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Calculations

Allocated Capacity (Physical Usage)

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total_allocated_capacity_gb = sum of allocated_capacity_gb of all cinder pools
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Provisioned Capacity (Logical Usage)

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total_provisioned_capacity_gb = sum of provisioned_capacity_gb of all cinder pools
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Oversubscription Ratio (Actual)

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oversubscription_ratio = total_provisioned_capacity_gb / total_allocated_capacity_gb
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Free and Total Capacity
- If all the cinder pools use same backend storage pool.

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grand_total_capacity_gb = total_capacity_gb of any one pooltotal_free_capacity_gb  = free_capacity_g of any one pool
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- If cinder pools use different backend storage pools.

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grand_total_capacity_gb = sum of total_capacity_gb of all poolstotal_free_capacity_gb  = sum of free_capacity_gb of all pools
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Maximum Provisioned Capacity Allowed

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max_allowed_provisioned_capacity = grand_total_capacity_gb × max_over_subscription_ratio
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