Note that I changed a few things. First, and most verbosely, I'm using the ConvertTo-FormattedNumber cmdlet to display back the data size values. Second, I modified the snapshot used field to use the "cumulative total" property from the old snapshot on the volume rather than summing the size of the snapshots.
Also note that the "Used" value in the PowerShell output is going to be the size of the data + the size of the snapshot overflow, whereas System Manager reports them separately.
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Indeed, very interesting they are equal on your device. And yes, i have noticed the performance of get-ncefficiency is not as high as with get-ncvol.
On my device the result looks like this:
(Get-NcEfficiency D1).SnapshotUsed: 5618896896 (Get-NcSnapshot D1 | Measure-Object Total -Sum).Sum: 9651384320
I have noticed that there are a few "old" snapshots from a previous configuration existing. I am unable to see if, there is a baseline snapshot to this subset. Maybe the baseline has been deleted, so the SnapshotUsed differs from Sum?
I have run your code sample vs my environment. For my example volume the result are matching the most.
Result from your fast query: Name : D5 totalsize : 850 free : 192,476535797119140625 data size : 807,5 data used : 615,023464202880859375 snap size : 260,0552215576171875 snap reserve : 42,5 snap overflow : 217,5552215576171875
Result from my slow query: Name : D5 TotalSize : 850 Data : 807,5 Used : 615,048320770263671875 Free : 192,451679229736328125 SnapshotUsed : 260,074733734131 SnapshotReserve : 42,5 SnapshotOverflow : 211,876407623291015625
The results are mostly equal, a slight difference in overflow space, which is negligible imho.
I think i will stick with your Get-NcVol approach.
About the graphical display, I'm assuming, that these values are sometimes sorted wrong due to a bug in the release?