First of all, is there a possibility to give information about a bug to microsoft without paying?
As I haven't found one (which would be really NOT customerfriendly) and I hope to help others I will post my problem and solution here.
Today our productive server which is a vm on a hyper-v host ran out of hdd space which was quite confusing for us as the assigned size of the disk is much less then the space the physical disk on the host has.
The hyper-v manager didn't show any snapshots. In the filesystem we saw a vhd AND a avhd file, the avhd file being the newer one and causing the error as it took all the space left on the disk.
We do NOT know where this avhd file comes from (maybe microsoft or some of you know how this could have happened?) but here comes the solution how I managed to merge the vhd and the avhd file.
I created another snapshot. So now I had two avhd files and one vhd file. Now I also saw at least the LAST snapshot in the hyper-v manager. I deleted this last snapshot in the hyper-v-manager (DO NOT DELETE THE AVHD FILE IN THE FILESYSTEM!) and started merging the snapshot and the vhd. In this merging process also the hidden snapshot is merged and both avhd files dissappear and diskspace is free again.
Reserve some time for the merging process - it took me about one hour.
I'm quite alarmed about that now as we are just bying a new server and wanted to use hyper-v again. Would be really helpful to know where this hidden snapshots come from and how to avoid them so we can decide if we should stick to our decision to use hyper-v. Also I feel very unhappy with the fact that there iseems to be no possibility to contact microsoft without paying even if there seems to be a bug in their OS.
Best regards,
Tina