Simple definition
VHDX is the virtual-disk format used by Hyper-V. It stores blocks exposed to a guest operating system; it is neither a complete virtual machine nor a backup.
Technical operation
VHDX disks may be fixed-size or dynamically expanding. A dynamic disk grows as data is written, but its configured maximum is not automatically reserved on the host. The guest operating system still manages partitions, file systems, and volumes.
In production, monitor host free space, storage latency, disk growth, Hyper-V-aware backup jobs, and AVHDX checkpoint chains. Moving, merging, or compacting storage requires a controlled procedure and integrity checks.
Practical example
For example, a Windows VM has a 200 GB dynamically expanding VHDX that initially consumes 35 GB on its host. As data accumulates, the file grows. Removing files inside Windows does not automatically shrink the host file.
Best practices and limitations
VHDX by itself provides no fault tolerance. Test recoverable backups, do not manually delete checkpoint-related disk files, and plan extra space for checkpoint merges.