With a thick-provisioned Cinder volume, an amount of space is reserved from the backend storage system equal to the size of the requested volume. Because users typically do not actually consume all the space in the Cinder volume, overall storage efficiency is reduced.
With a thin-provisioned Cinder volume, space is only carved from the backend storage system as required for actual usage.
Thin-provisioning allows for capacity over-subscription. In other words, more storage space may be allocated than is available on the storage controller.
As example, in a 1TB storage pool, if four 250GB thick-provisioned volumes are created, it would be necessary to add more storage capacity to the pool in order to create another Cinder volume even if the Cinder volumes were to remain empty.
Storage Pool: Thick provisioned
Storage Pool capacity = 1TB
Cinder volume One: 250GB allocated
Cinder volume Two: 250GB allocated
Cinder volume Three: 250GB allocated
Cinder volume Four: 250GB allocated
Storage Pool space consumed = 1TB
Thin-provisioning with over-subscription allows flexibility in capacity planning and reduces the likelihood of wasted storage capacity.
Storage Pool: Thin provisioned
Storage Pool capacity = 1TB
Cinder volume One: 250GB allocated
Cinder volume Two: 250GB allocated
Cinder volume Three 250GB allocated
Cinder volume Four: 250GB allocated
Cinder volume Five: 250GB allocated
Storage Pool space consumed = ~0GB
Note
Thin provisioning helps maximize storage utilization. However, if aggregates are over committed through thin provisioning, usage must be monitored, and capacity must be increased as usage nears predefined thresholds.
The NetApp unified driver conforms to the standard
Cinder scheduler-based over-subscription framework
in which the max_over_subscription_ratio and reserved_percentage
configuration options are used to control the degree of
over-subscription allowed in the relevant storage pool. Note that the
Cinder scheduler only allows over-subscription of a storage pool if the
pool reports the thin_provisioning_support capability, as described
for each type of NetApp platform below.
The SolidFire Cinder driver does not support thin or thick provisioning. Please read the section SolidFire Thin Provisioning for more details.
Refer to the following snippet in exploring further:
$ cinder get-pools --detail
+-------------------------------+--------+
| Property | Value | |
+-------------------------------+--------+
....
| free_capacity_gb | 2348.93|
| max_over_subscription_ratio | 4.0 |
| reserved_percentage | 10 |
| total_capacity_gb | 10240.0|
....
+-------------------------------+--------
The attribute max_over_subscription_ratio is a multiplier
that controls how much beyond the reported storage pool
capacity may be allocated for additional Cinder volumes.
The attribute reserved_percentage dictates how much space
is to be discounted from the total_capacity when performing
the maximum over subscription calculations.
In the example above, the storage pool has a capacity of 10240.0GB.
A reserved_percentage of 10% will be applied against this 10240GB
total_capacity value.
reserved_percentage = 10
max_over_subscription_ratio = 4.0
total_capacity_gb = 10,240GB
The following derives the maximum amount of space that may be allocated to Cinder volumes.
( max_over_subscription_ratio * ( total_capacity_gb - ( total_capacity_gb * ( reserved_percentage / 100 ) ) ) )
( 4.0 * ( 10240.0 - ( 10240.0 * (10 / 100 ) ) ) ) = 36,864 GB
Every sixty seconds, the total and free capacity is reported to the Cinder Scheduler. Between the sixty second updates, the Cinder Scheduler assumes a pessimistic view of free space. The allocated capacity of each newly created Cinder volume is subtracted from the free space value returned previously by the Cinder volume Controller.
At the time of Cinder volume creation, the requested Cinder volume size must be less than or equal to the amount of free space known by the Cinder scheduler.
The above snippet reports the following free space at the reporting interval:
free_capacity_gb = 2,348.93GB
Consider what happens in the following Cinder volume creation scenario where four thin volume creation requests come in back to back within the reporting interval:
request 1: 2000GB (success: free space goes from 2348GB to 348GB)
request 2: 200GB (success: free space drops from 348GB to 148GB)
request 3: 250GB (failure: insufficient free space)
Space Update Occurs
request 4: 250GB (success: free space goes from 2348GB to 2098GB)
The following cinder.conf configuration settings control thin
provisioning with ONTAP backends.
nfs_sparsed_volumes: This setting controls whether
Cinder volume backed by NFS backends are sparsely or thickly provisioned.
By default, the option is True and the volumes are
thinly provisioned.
netapp_lun_space_reservation: This setting controls whether
space is initially reserved within the pool for LUNS provisioned by
Cinder when using the iSCSI or FC as the storage_protocol.
By default, the option is Enabled and LUNS are thickly
provisioned.
NFS Backend
+==================================================+====================+
| Config Option: nfs_sparsed_volumes | True |
+--------------------------------------------------+--------------------+
| ONTAP Volume Setting: thin_provisioning_support | '<is> True' |
+--------------------------------------------------+--------------------+
| Config Option: max_over_subscription_ratio | > 1.0 or auto |
+--------------------------------------------------+--------------------+
iSCSI or FCP Backend
+==================================================+======================+
| Config Option: netapp_lun_space_reservation | disabled |
+--------------------------------------------------+----------------------+
| ONTAP Volume Setting: thin_provisioning_support | '<is> True' |
+--------------------------------------------------+----------------------+
| Config Option: max_over_subscription_ratio | > 1.0 or auto |
+--------------------------------------------------+----------------------+
All SolidFire volumes are provisioned thin by default; space is consumed on write but only after passing through a series of inline data reduction processes within the SolidFire cluster. The SolidFire Cinder driver does not comply with the Cinder over-subscription framework and the SolidFire cluster maintains total control over space usage. SolidFire free space is considered during Cinder volume creation; free space is predicted upon used space rather than allocated space.
Please see the SolidFire Data Efficiencies Brief for more information on SolidFire space management.
This document is licensed under Apache 2.0 license.