Snapshots serve as a mechanism that allows us to revert virtual machines back to a previous state in a timely manner. A common issue when dealing with virtual machines and taking multiple snapshots occurs when the virtual machine will no longer stay running. In these scenarios, the VM will typically pause itself and reported to be in a “Paused-Critical” state.
This issue is caused by a lack of system resources available to the host. More specifically, this occurs when there is not enough free hard disk space available on the host drive where the virtual machine snapshots are stored. So how does this happen? Well, an AVHD file is created every time you create a snapshot of the VM. The problem is that these files are often quite large and can add up on you before you know it. Also, deleting the snapshots from Hyper-V manager will remove the link to them but won’t delete the AVHD files off of the physical disk.
Your initial reaction when winding up in a Paused-Critical state might be to delete the AVHD files to free up space; however, this won’t fix the problem in itself. You will then have to turn off the VM and delete the snapshot files from within Hyper-V manager for the free space to be made available to the VM. Microsoft recommends doing this process first prior to deleting the AVHD files manually. Alternatively you can export the VM and the import it back into Hyper-V manager.
If you take anything away from this tip, you should remember that the management of snapshot files isn’t as automated as it could be. That being the case, make sure you keep tabs on your AVHD files before you find yourself in a Paused-Critical state.