My answer here may help:  [URL]#M6035 That question related to ESXi, but the same applies to Hyper-V (except the part about VSI).  The OS won't see the backend data reduction saving.  However, since everything is thin provisioned just make the LUN bigger, it won't consume any additional space at the backend and the OS will be happy since it now has more free space.
Hi, We have a Unity SAN with Data Reduction enabled for LUNs. This is showing for example as 25% used capacity on the SAN itself, however the actual CSV in Failover Cluster Manager is reporting about 7% free because presumably it is not aware of compression being used on the SAN/LUN itself. This seems like a massive bug to me, in the Hypervisor! As we are monitoring the Hyper-V hosts for utilization we are getting alerts all the time for something that isn't even a problem. Is there a solution to this? This is not unique to a single LUN/CSV pair. Disk Management on the LUN and the CSV are both reporting the correct total capacity in FCM. I'm usually pretty good at Google but don't seem to get the right search expression for this issue to return anything useful. Thanks
Data Reduction vs. Hyper-V CSV Free Capacity