A LUN on DSM is a part of a volume segmented to provide storage for your hosts (initiator side) as a block device. DSM supports two types of space allocation, Thick Provisioning and Thin Provisioning.

Thick Provisioning

With Thick Provisioning, the entire LUN capacity is pre-allocated on the volume from the moment a LUN is created. The LUN consumes all the space allocated to it on the volume, so the space is unavailable for use by other services. For example, once you create a Thick Provisioned LUN with 100 GB of capacity, the 100 GB is taken from the volume, even though you haven’t written any data to the LUN yet.

Thin Provisioning

A Thin Provisioned LUN takes up actual used space and asks for more only when you write data to it. For example, if you create a Thin Provisioned 100GB LUN and store 20GB files, the space it consumes on the volume is 20 GB, whereas a Thick Provisioned LUN would take up 100 GB right from the start.

Space Reclamation

If you enable Space Reclamation for a LUN, when you delete files on the LUN, it will be able to reclaim the space. The host will send UNMAP commands to the LUN after you delete files to notify the LUN to process space reclamation. If you encounter any issues, make sure you enabled Space Reclamation when you created the LUN and check if the host has sent UNMAP commands properly.

To find out whether the hosts send the commands properly:

1. For VMware ESXi hosts that format LUNs as VMFS5/VMFS6, more information can be found in VMware Knowledge Base.
2. For Windows Servers or regular Windows hosts, refer to this article.