Why is space not reclaimed after deleting data on a thin iSCSI LUN connected to Windows Server?
If you connect an iSCSI LUN to Window Server as an iSCSI virtual disk and then format the virtual disk with the ReFS file system, space will not be reclaimed after deleting data. This is due to a limitation of the ReFS file system. To use space reclaim, format the virtual disk using NTFS.
For more information, see Resilient File System (ReFS) overview: https://docs.microsoft.com/en-us/windows-server/storage/refs/refs-overview.