I have a large, mirrored, thinly provisioned space spread across multiple hard drives using Windows Storage Spaces in Windows 10 Pro. The space is about 4TB (8TB with resiliency).

I have recently come to the opinion that there is a large number of files that I would like to reallocate to another, less resilient volume (just a simple volume).

So, I have created the new volume, moved over about 400GB of these files (there are about 1.2TB all together), and I can see that the amount of space used by the new volume has increased, but the amount of space used in the previous volume has not dropped.

I have run the defrag utility on the space, and have run the following powershell commands:

Optimize-StoragePool -FriendlyName "Storage Pool"

Optimize-Volume X -ReTrim -SlabConsolidate

but the amount of space used by the old space holds steady. It spends a lot of time Enumerating Slabs, but when it comes to doing stuff, it doesn't really do anything. I have run both of these as the local user AND as administrator.

Any ideas on why the previous slabs are not being cleared up?


I have just run Optimize-Volume with the -Verbose switch, and I got this result.

 Slab Consolidation:
VERBOSE:   Space efficiency            = 97%
VERBOSE:   Potential purgable slabs    = 94
VERBOSE:   Slabs pinned unmovable      = 94
VERBOSE:   Successfully purged slabs   = 0
VERBOSE:   Recovered space             = 0 bytes

The issue appears to originate from the line Slabs pinned unmovable. Can anyone enlighten as to what makes a slab "unmovable".


The issue was being caused by having a huge number of files (about 500,000) in the volume that had been sparsely distributed, and for some reason, rendered certain slabs unmovable. I created a third, simple (no-resiliency) volume, and moved all the files in there. Once they had been transferred, I ran Optimize-Volume X -ReTrim -SlabConsolidate, and instantly claimed back about 3TB of general space.

I have no idea why.

I then copied those files back (as I wanted those to be redundantly available), left the other files I had moved onto a separate volume earlier, and the space usage shrunk to what I was expecting.

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It is possible that the volume is configured to save snapshots of older data, check the volume's configuration

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  • Sounds legit, how though? That option might not be available in Windows 10 (rather than Server) – topherg Aug 17 '15 at 8:57
  • Ah, if you are referring to previous versions, no, that is disabled – topherg Aug 17 '15 at 9:00

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