Question about VHD Merge Process in Azure MARS Agent Incremental Backups

Hello @Tai Nguyen Thank you for raising this question.

According to Microsoft Learn, during incremental backups with the Azure Backup MARS agent:

• Changed files are identified
• A new VHD is created
• This VHD is compressed and encrypted
• The encrypted data is sent to the Recovery Services vault
• After the backup completes, the new VHD is merged with the VHD created after the initial replication
• The merged state is used to determine changes for future backups

Reference cbout the MARS Agent – Azure Backup https://learn.microsoft.com/azure/backup/backup-azure-about-mars

Microsoft does not provide any public documentation that describes:

• Decryption or decompression of the VHD during the merge
• Disk‑level or file‑system‑level VHD merge operations
• Internal implementation details of the merge process

Because of this, the “merge” mentioned in the documentation should not be interpreted as a traditional VHD disk merge (for example, like Hyper‑V differencing disks).

• Data is compressed and encrypted before upload
• Block‑level checksums are stored to track changes and ensure integrity
• The merged backup state is used only as a reference for incremental comparisons
• Data consistency is ensured using:
  • VSS snapshots
    • Block‑level checksum validation
      • Encrypted storage in the Recovery Services vault

No additional technical behavior beyond this is documented by Microsoft.

So, There is no Microsoft documentation stating that encrypted backup data is decrypted or decompressed for merging. The merge represents a logical consolidation of encrypted backup data and metadata, fully managed by the Azure Backup service.

I hope this clarifies the behavior described in the MARS agent documentation. If you have any further questions or need additional clarification beyond what is currently documented on Microsoft Learn, please let us know and we’ll be happy to assist.

Thanks,

Suchitra.