When I setup Windows depuplication, I get great savings. See the attached picture. 1.28 TB deduped down to 23 GB. However, the space savings are not real.
Lets say that volume is 2TB. According to the screenshot, there is only 28 GB used, so I should be able to add another 1.9TB of data. In reality, once I add ~800GB of data (to hit the real 2TB limit) the volume fills. The attached picture would then say data size 2TB, size on disk 50GB (or something like that).
What is the point of data deduplication if I cannot utilize the space savings? Or, is there some trick to setting this up I'm not seeing. I've had these same results on server 2012r2 and 2016. I've tried on the HyperV level with VHDXs, and on a backup server with large backup files.