Thin and Thick Provisioning

Overview

Polystack Block Storage supports both thin and thick provisioning models. Thin provisioning allocates storage capacity on demand — physical space is consumed only as data is written, not when the volume is created. Thick provisioning reserves the full volume capacity immediately at creation time. The provisioning behavior is controlled by the backend driver and the scheduler overcommit configuration.

Administrator Access Required — This operation requires the admin role. Contact your Polystack administrator if you do not have sufficient permissions.

Thin vs Thick Comparison

Attribute	Thin Provisioning	Thick Provisioning
Space allocated at creation	No — space consumed as data is written	Yes — full size reserved immediately
Overcommit support	Yes — allocate more than physical capacity	No — bounded by actual available space
Performance	Comparable to thick once space is allocated	Predictable from the start
Risk	Out-of-space if usage exceeds physical capacity	No overcommit risk
Supported backends	Ceph RBD, LVM thin, NetApp, Pure Storage, HPE, Dell	LVM (default), some SAN arrays
Recommended for	Multi-tenant clouds, variable-usage workloads	Latency-sensitive databases, predictable workloads

Backend Provisioning Support

Ceph RBD — thin by default

Ceph RBD images are sparse by design. When a volume is created, no physical storage is consumed until data is written. No configuration is required to enable thin provisioning on the Ceph backend.

cinder.conf — Ceph RBD backend

[ceph-ssd]
volume_driver = cinder.volume.drivers.rbd.RBDDriver
volume_backend_name = ceph-ssd
rbd_pool = volumes
rbd_ceph_conf = /etc/ceph/ceph.conf
rbd_user = cinder
rbd_secret_uuid = <libvirt-secret-uuid>

Because Ceph RBD is thin by default, the max_over_subscription_ratio setting in the [DEFAULT] section controls how aggressively the scheduler overcommits capacity. The default value is 20.0 (20x overcommit).

LVM — configure thin pools

LVM uses thick provisioning by default. To enable thin provisioning, create a thin pool in the volume group before configuring Cinder, then set lvm_type = thin in the backend section.Step 1: Create the thin pool

Create LVM thin pool

pvcreate /dev/sdX
vgcreate cinder-volumes /dev/sdX
lvcreate --type thin-pool \
  --size 500G \
  --name thin-pool \
  cinder-volumes

Step 2: Configure Cinder

cinder.conf — LVM thin backend

[lvm-thin]
volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
volume_backend_name = lvm-thin
volume_group = cinder-volumes
lvm_type = thin
target_helper = lioadm
target_protocol = iscsi

Step 3: Apply the configuration

Deploy block storage configuration

ironcore-ansible deploy --tags cinder

Step 4: Verify thin volumes are created

Check LVM thin volume allocation

lvs --options +lv_layout

Volumes created from this backend will show Twi in the Attr column, confirming thin layout.

NetApp ONTAP — FlexVol thin provisioning

NetApp ONTAP supports thin provisioning through FlexVol volumes. Set netapp_lun_space_reservation = disabled to enable thin provisioning on the LUN.

cinder.conf — NetApp thin provisioning

[netapp-thin]
volume_driver = cinder.volume.drivers.netapp.common.NetAppDriver
volume_backend_name = netapp-thin
netapp_storage_family = ontap_cluster
netapp_transport_type = https
netapp_server_hostname = 10.0.10.50
netapp_login = admin
netapp_password = <password>
netapp_vserver = svm0
netapp_storage_protocol = iscsi
netapp_lun_space_reservation = disabled

Pure Storage — thin by default

Pure Storage FlashArray volumes are thin-provisioned by default. The array reports the provisioned size to the scheduler, not physical consumption. No additional configuration is needed.

cinder.conf — Pure Storage thin provisioning

[pure-flash]
volume_driver = cinder.volume.drivers.pure.PureISCSIDriver
volume_backend_name = pure-flash
san_ip = 10.0.10.60
pure_api_token = <api-token>
use_multipath_for_image_xfer = true

Scheduler Overcommit Configuration

The Cinder scheduler uses max_over_subscription_ratio to determine how much capacity to advertise to tenants above physical availability. This applies to backends that report thin_provisioning_support = True.

cinder.conf — scheduler overcommit settings

[DEFAULT]
max_over_subscription_ratio = 20.0
reserved_percentage = 5

Parameter	Description	Default
`max_over_subscription_ratio`	Maximum ratio of provisioned capacity to physical capacity	`20.0`
`reserved_percentage`	Percentage of backend capacity reserved and excluded from scheduling	`0`

Setting max_over_subscription_ratio too high without monitoring physical usage can result in volumes that cannot be written to once the physical backend is full. Monitor actual consumption against provisioned capacity and set alerts at 70–80% physical utilization.

Monitor Overcommit and Capacity

Dashboard
CLI

Navigate to Storage > Storage Backends (admin view) to view backend capacity statistics. The Free Capacity and Total Capacity columns show the physical pool state reported by each backend driver.For provisioned-vs-physical overcommit visibility, use the Monitoring dashboard to track Cinder backend pool utilization over time.

Source your credentials file to authenticate with the Polystack platform:

Load credentials

source openrc.sh

Your administrator provides the RC (credentials) file for your project. See CLI Setup for configuration details.

List backend capacity and provisioning

openstack volume backend pool list --long

Key columns:

Column	Description
`free_capacity_gb`	Physical free space remaining on the backend
`total_capacity_gb`	Total physical capacity of the backend pool
`provisioned_capacity_gb`	Total provisioned (allocated) capacity across all volumes
`thin_provisioning_support`	`True` if the backend supports thin provisioning
`max_over_subscription_ratio`	Configured overcommit ratio for this backend

Check volume actual allocation (Ceph)

rbd du volumes

The rbd du command shows the difference between provisioned size and actual data written for each RBD volume, providing a direct view of thin provisioning savings.

Next Steps

External Storage Integration

Connect to enterprise storage backends including NetApp, Pure Storage, and Dell

Storage Backends

Configure and register backend drivers with the scheduler

Volume Types Admin

Expose provisioning mode to tenants through volume type extra specs

Storage Tiers

Configure multi-tier storage with NVMe, SSD, and HDD backends

Core Services

Other Services

Overview

Thin vs Thick Comparison

Backend Provisioning Support

Scheduler Overcommit Configuration

Monitor Overcommit and Capacity

Next Steps

External Storage Integration

Storage Backends

Volume Types Admin

Storage Tiers

​Overview

​Thin vs Thick Comparison

​Backend Provisioning Support

​Scheduler Overcommit Configuration

​Monitor Overcommit and Capacity

​Next Steps

External Storage Integration

Storage Backends

Volume Types Admin

Storage Tiers

Overview

Thin vs Thick Comparison

Backend Provisioning Support

Scheduler Overcommit Configuration

Monitor Overcommit and Capacity

Next Steps