Virtual machines will go into the “Paused-Critical” state under Hyper-V if we detect that we are going to run out of space for the virtual hard disks. If we were to run out of space for expanding a dynamic or differencing virtual hard disk – we would start failing disk write operations inside the virtual machine. The result would most likely be that the guest operating system would crash. To avoid this problem we periodically poll the storage that is being used for the virtual hard disks. If the free space falls under 2 gigabyte we will start to log warning messages in the event log. If the free space falls under 200 megabyte we will pause the virtual machine and mark it as “Paused-Critical”.