ReFS Deduplication and Fast Cloning on Windows Server 2025

[jfink]

Greetings,

in the past I used Windows Server with ReFS Filesystem als Repository and Veeam used fast cloning to create synthetic full backups regularly. Multiple full backups of the same VM only used the space alle changes in die vbk needed and I saved a lot of space.

With Windows Server 2025 this changed and I could not find any dokumentation about this so far. Fast cloning is not used while creating synthetic full backups and every .vbk file needs the full space.

What do I need to configure to get the old behavior from Windows Server 2022 and lower?

[vnikiforov]

Hello,

Nothing changed on the Veeam side. On plain ReFS, Fast Clone works on Windows Server 2025 just like on 2022, and the Fast Clone requirements are the same.

May I ask if on your Windows 2025 server you also enabled ReFS native deduplication and compression? That would explain it, since deduplication must not be combined with block cloning. With dedup active on the volume, Veeam skips Fast Clone and each synthetic full is written out in full.

If that is the case, you probably can disable it on the repository volume (along with compression if it is enabled as well) - please consult with Microsoft support on that. Then, after next active full data will be already eligible for fast cloning. (Disabling compression and dedup doesn't re-hydrate existing data back. So new copies are needed to benefit from fast clone.)

If that is not the case and dedup and compression are not enabled on the volume I suggest you to open a support case with Veeam.