Simple definition
An AVHDX file is a Hyper-V differencing disk created notably when a virtual machine checkpoint is active.
Technical definition
The AVHDX does not replace its parent VHDX. It stores new writes while unchanged blocks continue to be read through the parent disk chain. Multiple checkpoints can create a chain of differencing disks. When a checkpoint is deleted correctly, Hyper-V merges the relevant data according to that chain.
How it works / role
Its role is to preserve changes made after a checkpoint was created without immediately rewriting the parent VHDX. It is therefore part of the VM storage chain and must not be treated as an unrelated temporary file.
What is it used for?
Allow Hyper-V to maintain a recoverable checkpoint state and support checkpoint-based operations while preserving the parent disk state.
Practical example
After a checkpoint is created, the VM keeps running but new writes are stored in an AVHDX file. When the checkpoint is deleted, Hyper-V performs the appropriate merge to consolidate the changes.
Common issues
- Manually deleting an AVHDX and breaking the virtual disk chain
- Long checkpoint chains or insufficient free storage
- Slow or interrupted merge after checkpoint deletion
- Treating a Hyper-V checkpoint as a long-term backup
Key takeaway: Do not manually delete an AVHDX that belongs to a VM. Manage checkpoints through Hyper-V and verify the disk chain before working on the files.