Reducing the size of a storage pool
On an HDP pool, use the span-unmap-vacated-chunks command to return unused space to the pool. The actual capacity of the SD cannot be changed/shrunk; however, you can shrink the HDP pool to remove capacity and allocate elsewhere. Under no circumstances should the HDP pool be shrunk below the confined capacity of all associated file systems.
On a non-HDP pool, the size of the pool cannot be reduced.