To achieve zero downtime live backups of virtual machines, you need the following conditions met:
1. The VM guest needs to have Integration Services installed, enabled, and running (COM+ System Application Service, Distributed Transaction Coordinator Service, and Volume Shadow Copy Service).
Also review the VM settings in Hyper-V, the ‘backup’ option needs to be checked.
2. All disks involved need to be formatted with NTFS, including the disks within the VM.
3. The Volume Shadow Copy Service and related VSS services need to be enabled and running.
4. The shadow copy storage space for each drive must be available to Hyper-V VSS Writer and be located at the same volume. For instance, the storage space for drive C: needs to be on drive C: itself, and so on. Use the VSSADMIN command from the command line to check the settings. (Use: vssadmin list shadow storage /vssadmin resize shadow storage)
5. Ensure the VMs are partitioned using ‘basic disk’ formatting.
At the moment Hyper-V does not support live backup for VMs formatted using dynamic disk partitioning or GPT.
7. Ensure you have at least about 20% free space on each drive involved, such as the drive on the host and the VM’s main system drive.
8. Ensure plenty of un-fragmented RAM is available on the host. If a machine is pulled into Saved State, Hyper-V may not be able to bring the VM back online if it can’t allocate a continuous block of RAM.
Note that there may be sufficient total RAM available but not enough to place a single block.
You should therefore aim to keep at least 512 MB to 1 GB of RAM free when all VMs are powered up.
Hyper-V resource monitoring
From time to time, you need to monitor the system usage by Hyper-V, such as CPU and memory. A free software called Veeam Task manager for Hyper-V can do this nicely. download page: https://www.veeam.com/free-task-manager-hyper-v-performance-monitoring.html