You're referring both Storage Spaces and Hyper-V documentation. Is there an equivalent to optimize-volume -slabconsolidate but at the pool level? It looks lie optimize-storagepool only does rebalancing, but with 100% slab usage there's not much to rebalance so it quits almost immediately. One would think retrim would take care of that, but It's almost as if there's a disconnect between the ReFS and Storage Spaces layers, whereas ReFS 'thinks' it has passed the trim instructions on certain blocks, but the pool never received them. There are only 10TB worth of files on it. The volume still has a 30+ TB footprint even though What I tried over the weekend was optimize-volume -defrag (5 passes, including free space consolidation,) followed by optimize-volume -retrim -slabconsolidate.

If I could get a Windows Server 2019 1903 evaluation ISO, I might try dedup, but I don't have an MSDN sub and I see no other way to obtain a copy. I have a Windows 2019 1809 evaluation copy installed on a spare SSD with 170 days to go, but it doesn't even recognize the pool because the metadata was updated by Windows 10 1903. I would love to try dedup but alas this is Windows 10 Pro, not a server SKU.

Windows 10 reclaim disk space