I don't have such setup, but based on docs and error we can assume what is going on. Doc says:
Hyper-V VSS Writer uses differencing disks to capture changed blocks between backups.
A differencing disk is a VHD that contains changes for another VHD or the guest operating
system. Differencing disks are stored in the same subdirectory as the parent VHDx for the
virtual machine. This location is not configurable.
Incremental changes since the last checkpoint or backup are written to a new
differencing disk. This differencing disk gets merged to its parent disk after the backup
completes. When a snapshot occurs as part of a backup, a new differencing disk is
created. This new differencing disk receives all the writes until the next backup starts.
The backup saves the differencing disk as part of an incremental backup. If you create
checkpoints, then multiple differencing disks can be present on the system. If you do not
create checkpoints, there is one differencing disk present along with parent disk.
To estimate the required additional primary storage for differencing disks, consider the
rate of changes happening inside the virtual machine. As a virtual machine grows, it
requires more storage space on the primary disk. NMM logs how much data it has backed
up for each backup.
So, it seems like expected differencing disk is not found or I could imagine same error can be made if there is something else doing the backup as well (as that would break the chain). To be sure, can you check also log in event log to see if there is anything written there?
Can you please check in the application information field, if you have the following variable, and what is the value?:
NSR_VSS_FULL_BACKUP Default value is Yes.
When the value is Yes, NMM initially performs a full backup and later performs incremental backups.
If the value is No, then NMM performs a copy full backup and merges the recovery snapshot with the base VHDX.
gotschy, was this resolved? If so, can you please share the solution? I am having the same issue but on NMM 9.0.x.
I searched NMM documentation and found the variable related to Hyper-V backups and my configuration was done with the client wizard so I would assume it will not include this variable? Anyhow, I am going to check the configuration again to be sure.
Thank you for your suggestion.
By the way, the problem that we encountered is :
nsrnmmsv NSR critical Backup of Hyper-V data files failed with error Failed to merge VHD chain. FileStream::Read error..
will setting client retries and inactivity timeout to 0 helps in this case? currently running 22.214.171.124 server and client.