You experience backup failures with the following error displayed.
Stack trace example:
Replay.Agent.Contracts.Transfer.TransferFailedException: The transfer failed: 'There was a problem with the VSS subsystem on 'Agent01'.' ---> System.AggregateException: One or more errors occurred. ---> Replay.Core.Contracts.Transfer.VssGeneralErrorException: There was a problem with the VSS subsystem on 'Agent01'. ---> Replay.Agent.Contracts.ShadowCopy.ShadowCopyException: DoSnapshotSet failed with error: 'The system was unable to flush I/O writes. This can be a transient problem. It is recommended to wait ten minutes and try again, up to three times.'
This error maybe as a result of an issue with the VSS subsystem on the Agent machine or Hyper-V host caused by VSS not being able to quiesce data to the disk within the VSS timeout window. Potential environmental issues that may cause the problem are:
NOTE: In Hyper-V environments, this might be due to Hyper-V integration services called Backup for Window Server 2012 R2 Hosts, and the way/speed that the OS releases VSS for the requester (in this case AppAssure software).
Corruption of the NTFS structure of the disk
If possible reduce the IO on the disk. If it is not possible to reduce the IO on the disk, then attempt to schedule around it.
When the SRP is full, there is no free space on this volume for VSS to create a shadow copy. Furthermore, when the AppAssure Agent change log is locked (cannot take possession to delete the file), VSS will timeout as this log file cannot be accessed.
Failing volume with generate the "flush I/O" error. For such volume please open CMD in 'Run As Admin" mode and initiate a command:
chkdsk <volume>: /F
Please note that doing that on non-OS volumes will cause them to dismount. Doing that on the OS volumes will generate a message if chkdsk can be scheduled to run on the next reboot. In such case please accept that and reboot the machine, allowing chkdsk to process the OS volume.