Problem

When backing up virtual guests in a cluster (using Cluster Shared Volumes - CSV) running Windows 2012 R2 and using Veeam Backup & Replication 7.0, some guest machine backups fail with:

Unable to allocate processing resources. Error: Unknown status of async operation

The shadow copy provider timed out while flushing data to the volume being shadow copied. This is probably due to excessive activity on the volume. Try again later when the volume is not being used so heavily.
--tr:Failed to create VSS snapshot.
--tr:Failed to perform pre-backup tasks.

Meanwhile, all tests on the cluster volumes regarding creation and removal of shadow copy complete fine.

Cause

When Hyper-V guest machine elements exist on the local virtual host volume and not on the cluster volume, the backup fails with the above error. This occurs if a highly available machine is created and virtual machine component placement is selected on the local host volume and virtual disk on the cluster shared volume. An example of this is show in the diagram below:

hyper-v move virtual storage select  

 

Solution 

Use the Storage Migration feature in Failover Cluster Manager to move virtual guest component storage to the Cluster Shared Volume.

In 'Failover Cluster Manager' - Select the Virtual Guest from 'Roles' - right-click and select 'Move' - 'Virtual Machine Storage'

hyper-v move virtual storage

Drag the 'Checkpoints', 'Smart Paging' and 'Current Configuration' components from the upper pane down to the desired Cluster Shared Volume in the pane below then finish the dialogue to complete the storage migration.

When complete, the Virtual Guest components will all reside on the same volume as follows:

Hyper-V move virtual machine