We are using Hyper-V 2012 R2 on a Windows Server 2012 R2 host. We have several VMs with multiple volumes (VHDX) which we initially set up as thin-provisioned (dynamically expanding). Later on, we converted them to fixed size VHDX due to performance issues related to dynamically expanding VHDX.
We now noticed extremely poor performance on some of the VMs: a direct copy of very large files is running with 10-50MB/s - usually, we expect transfer rates of 300-400MB/s. While investigating, we were thinking that maybe the VHDX itself is fragmented.

So we had a look at the integrated "Disk Optimizer" and were surprised that it said the disks were Thin provisioned drives:

When looking at the settings in Hyper-V, it clearly says Fixed size virtual hard disk:

When forcing a defrag on the drive through the command line, it clearly shows that the volume is highly fragmented (75%). Our question now is why Windows thinks these are thin provisioned drives and how we could fix this. Did anyone experience a similar thing before?

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