Question about VHD Merge Process in Azure MARS Agent Incremental Backups

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Question about VHD Merge Process in Azure MARS Agent Incremental Backups

Tai Nguyen 0

In the Azure Backup MARS agent documentation https://dori-uw-1.kuma-moon.com/en-us/azure/backup/backup-azure-about-mars#additional-information, it states that during incremental backups, a new VHD is created, compressed, and encrypted before being sent to the vault. After the backup completes, this VHD is merged with the VHD created during the initial replication.

Could you please clarify how the merge process works given that the new VHD is already compressed and encrypted? Specifically:

•	Is the VHD decompressed and decrypted before merging?

•	Or is there a mechanism that allows merging in its compressed/encrypted form?

I would appreciate more details on how data consistency and integrity are maintained during this process.

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Answer 1

Alex Burlachenko 19,850 MVP Volunteer Moderator

Tai Nguyen hi and thx for join us here at Q&A platform )))

short answer merge does not happen on compressed/encrypted blobs, what actually happens under the hood, MARS doesnt treat those VHDs like real disk files u can just merge bit by bit those “VHDs” are more like logical containers for changed blocks, flow is roughly

agent takes snapshot (VSS)

figures out changed blocks

packages them into chunks

compress + encrypt

send to vault

so what lands in Azure is already encrypted block data, not something u can directly merge like a VHD on disk

then on Azure side it does

decrypt

decompress

reconstruct block map

and updates the recovery point chain, so “merge” = not file merge, its block-level consolidation in the backup store. Think more like incremental blocks get applied to previous state to build new recovery point, so to ur questions.

Is it decompressed/decrypted before merge yes, logically inside the service pipeline, merge in encrypted/compressed form

no, not in raw form. Azure has to process it to understand blocks data integrity part.

handled by

checksums per chunk

encryption integrity checks

VSS consistency on capture

if any chunk fails validation backup fails, not partially merged

rgds,

Alex

Tai Nguyen 0 Reputation points

2026-04-09T02:21:06.12+00:00
Hello @Alex Burlachenko ,

I find the Microsoft documentation about "merge VHDs" in the MARS agent a bit confusing, so thank you very much for your detailed answer.

I would like to ask further about one part of the flow you described:

agent takes snapshot (VSS) figures out changed blocks packages them into chunks compress + encrypt send to vault

Regarding "figures out changed blocks":
My understanding is that a file can be divided into multiple data blocks. When the file changes, the agent only identifies the changed data blocks.
Could you clarify what the typical block size is, and what algorithm is used to divide a file into these data blocks?

Regarding "packages them(changed blocks) into chunks":
My understanding is that a chunk can contain multiple blocks, and a single backup may consist of multiple chunks.
Could you clarify what the typical chunk size is, and how the agent decides which blocks are packaged into which chunk?

I realize this is a rather technical question, but since the MARS agent documentation does not describe these details, I would appreciate your explanation.

Thank you very much.

Best regards,
Alex Burlachenko 19,850 Reputation points MVP Volunteer Moderator

2026-04-09T06:57:58.96+00:00
Tai Nguyen,

)))) u are welcome and if my answer was sutiable for u pls mark it as an answer...
so let's back to urs issue... btw tldr ))))))))))))

yeah ur thinking in the right direction, but MARS is way less transparent than u expect ))))

on “changed blocks” this is not file-level diffing. MARS relies on VSS + NTFS change tracking (and its own filter driver), so it works at block level below files. Block size is not officially documented (and Microsoft doesnt publish it for a reason at least i did not seen it), but from behavior + perf patterns its roughly aligned with NTFS clusters / internal backup blocks. think tens of KB range, not MB. definitely not whole file and its not some fancy content defined chunking (like rsync or dedupe systems). its more like

track which regions on disk changed since last backup

read those regions

send them

so really no rolling hash magic here. more pragmatic Windows stack, on “chunks” yeah u got it: chunk = transport unit, not storage logic. Agent groups changed blocks into chunks before sending. chunk size is also not documented, but in practice its MB-sized batches (think few MB up to maybe ~10–20 or so MB range depending on version/network)

how it decides grouping nothing fancy either, I pretty sure like that ))

take changed blocks in sequence

pack until chunk size limit

compress + encrypt send

so chunks are just batching for

network efficiency

retry handling

parallel upload

not logical grouping of files or anything smart, important mental model (this is where ppl get confused).

block = smallest changed unit detected on disk chunk = upload package of many blocks “VHD” in docs = logical container, not real disk merge target, so yeah answering ur exact questions ... block size not public, but small (KB-level), tied to filesystem + agent internals, not content-defined

algorithm change tracking via VSS + filesystem + filter driver, not hashing/delta algo like rsync

chunk size MB-level batches, dynamic but bounded. Chunk composition sequential packing of changed blocks until size threshold, no semantic grouping.

)))) and at the end to summ MARS is pretty “enterprise boring” under the hood no fancy dedupe algo exposed, just block tracking + batching + compression + encryption

rgds,

Alex

Answer 2

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://dori-uw-1.kuma-moon.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.

Suchitra Suregaunkar 11,635 Reputation points Microsoft External Staff Moderator

2026-04-08T17:38:03.9533333+00:00

Hello @Tai Nguyen

If the answer is helpful, could you please click "Accept Answer" & "upvote" it and this helps other community members who may encounter a similar issue in the future.

If you’re still experiencing the problem or need further clarification, please feel free to share additional information so we can continue investigating and assist you further.

Thanks,

Suchitra.

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