Data ONTAP provides three basic volume provisioning options: thick provisioning, thin provisioning, and semi-thick provisioning. Each option uses different ways to manage the volume space and the space requirements for Data ONTAP block sharing technologies. Understanding how the options work enables you to choose the best option for your environment.
When a thick-provisioned volume is created, Data ONTAP sets aside enough storage from the aggregate to ensure that any block in the volume can be written to at any time. When you configure a volume to use thick provisioning, you can employ any of the Data ONTAP storage efficiency capabilities, such as compression and deduplication, to offset the larger upfront storage requirements.
When a thinly provisioned volume is created, Data ONTAP does not reserve any extra space when the volume is created. As data is written to the volume, the volume requests the storage it needs from the aggregate to accommodate the write operation. Using thin-provisioned volumes enables you to overcommit your aggregate, which introduces the possibility of the volume not being able to secure the space it needs when the aggregate runs out of free space.
When a volume using semi-thick provisioning is created, Data ONTAP sets aside storage space from the aggregate to account for the volume size. If the volume is running out of free space because blocks are in use by block-sharing technologies, Data ONTAP makes an effort to delete protection data objects (Snapshot copies and FlexClone LUNs) to free up the space they are holding, As long as Data ONTAP can delete the protection data objects fast enough to keep pace with the space required for overwrites, the write operations continue to succeed. This is called a "best effort" write guarantee.
You cannot employ storage efficiency technologies such as deduplication and compression on a volume that is using semi-thick provisioning.
The following table summarizes the major differences in how the three volume provisioning options can be used:
|Volume provisioning||LUN space reservation||Overwrites||Protection data2||Storage efficiency3|
|Semi-thick||Supported||Best effort1||Best effort||Not supported|