- Veeam Support Knowledge Base
- ReFS Known Issues, Considerations, and Limitations
ReFS Known Issues, Considerations, and Limitations
| KB ID: | 2792 |
| Product: |
Veeam Backup & Replication Veeam Agent for Microsoft Windows |
| Published: | 2018-11-06 |
| Last Modified: | 2026-08-27 |
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Known Issues
Memory Management Issues
Server 2016 and older
Windows repositories using ReFS on Server 2016 (without the September 2018 updates) or older OS versions are known to have issues during backup job retention enforcement, which manifests as:
- The repository server becomes unresponsive or crashes.
- Significant reduction in I/O performance on the repository server.
- High memory usage on the repository server.
These issues can lead to unexpected job termination.
As described in Microsoft article KB4016173, the root cause lies in the ReFS metadata management logic within the Windows OS system memory manager. This issue was resolved in Windows Server 2016 with updates from September 2018 (KB4343884 or later).
Server 2022 | Azure Stack HCI 21H2 | Azure Stack HCI 22H2
It is recommended to perform only 'clean installs,' as issues have been reported following in-place OS upgrades.
Server 2025 | Server 23H2 | Azure Stack HCI 23H2
There have been several reports of stability issues related to memory consumption with ReFS on Windows Server 2025, Server 23H2, Windows 11 23H2, and Azure Stack HCI 23H2. Microsoft provided fixes in December 2025 [December 9, 2025—KB5072033 (OS Build 26100.7462)].
BSOD after installing Cumulative Update
March Cumulative Update for Server 2025
The March 2025 cumulative updates for Windows Server 2025, 23H2, 2022, and 2019 have a known regression that may lead to a Blue Screen of Death (BSOD) with a bug check of 0x00000149. This issue was later resolved with Microsoft's release of the April 2025 Cumulative Update.
ReFS Version Compatibility Challenges Across OS Versions
Each version of Windows has specific versions of ReFS that it is compatible with. This can lead to complications when performing certain actions within Veeam Backup & Replication.
- Guest OS File Level Restore (FLR)
- If the OS of the Mount Server is older than the original Guest OS, the FLR File Browser may not display the content of ReFS volumes.
- If the OS of the Mount Server is newer than the original Guest OS, there may be a delay during file restores because the Mount Server's OS is trying to update the ReFS version of the mounted volume. To mitigate this delay, create the following registry value on the Mount Server to prevent the OS from attempting to upgrade the ReFS filesystem version of mounted volumes:
Key Location: HKLM\System\CurrentControlSet\Control\FileSystem
Value Name: RefsDisableVolumeUpgrade
Value Type: DWORD (32-Bit) Value
Value Data: 1
Set using PowerShell:
New-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\FileSystem\' -Name 'RefsDisableVolumeUpgrade' -Value "1" -PropertyType DWORD -Force- Windows Repository Server Upgrades
There have been instances where customers reported data loss after performing an in-place OS upgrade on a Windows Server hosting an ReFS volume that acts as a Windows Repository. The issue arises when the OS attempts to upgrade the ReFS version of the volume and the process fails. For this reason, Veeam strongly advises against performing in-place OS upgrades for ReFS-based Windows Repositories. - Failover with Re-IP of Replica with ReFS Volume
During the Re-IP procedure, all disks of the replica are mounted to the Veeam Backup Server so the software can update the static IP. If the replica's Guest OS is older than the Veeam Backup Server and the replica has an ReFS volume, the Veeam Backup Server's OS will automatically update the ReFS volume version. As a result, the ReFS volume will become unreadable by the replica's Guest OS because the new ReFS version exceeds the compatibility of the Guest OS.
Note: Veeam's RND team has been made aware of this, and in a future release of Veeam Backup & Replication, the software will detect and block the mounting of ReFS volumes during the Re-IP procedure.
Considerations and Limitations
Configuration Related
- Do not use ReFS with iSCSI or FC SANs unless explicitly supported by the hardware vendor.
- ReFS supported configurations must use Windows Server Catalog certified hardware.
- ReFS does not support SAN storage features such as thin provisioning, TRIM/UNMAP, and ODX.
- Do not use deduplication in combination with ReFS block cloning.
- Ensure scheduled Data Integrity Scans do not overlap with your backup window.
- ReFS relies on flush durability, meaning that if filesystem metadata write operations occur, they must be consistently flushed to disk. If a storage destination is used where a cache acknowledges writes before they are written to disk, for example, by a RAID controller or NAS, and those metadata writes then do not make it to disk due to a power failure, the filesystem may become corrupted.
- There were several Windows Updates• Server 2012 R2 - KB5009624
• Server 2019 - KB5009557
• Server 2022 - KB5009555 that broke ReFS functionality with removable media. Microsoft does not support ReFS on removable media. However, Microsoft has released out-of-band updates to restore functionality, but only for ReFS v2 and higher. For more information, review: Veeam Community Discussion: ReFS issues with latest Windows Server Updates
OS Notes
- While Microsoft does support booting from ReFS volumes under particular circumstances involving Azure confidential VMs, this configuration is not supported by Veeam Agent for Microsoft Windows. Although it is technically possible to configure Windows to boot from an ReFS volume, doing so may lead to stability issues with Veeam Agent for Microsoft Windows. For example, attempting to back up an ReFS boot volume using Veeam Agent for Microsoft Windows has a high probability of causing a BSOD.
- Veeam always recommends keeping all systems fully patched with the latest Windows Updates for best performance and security.
- Since Veeam Backup & Replication 10a, the RefsEnableLargeWorkingSetTrim registry value is automatically set, based on Microsoft's recommendation, upon the creation of a Windows-based backup repository. (After adding the repository, a reboot is required for the OS to activate the registry value.)
Hardware
- The backup repository server must meet minimum system requirements.
- For environments where a single server handles multiple roles (aka All-in-One Installation), ensure that the server has enough RAM based on the combined requirements of all roles assigned to it.
3rd Party Software
Uninstall the following 3rd party software, as these have been reported to cause ReFS instability and/or performance issues:
- Antivirus software (except Windows Defender)
- Microsoft Configuration Manager Client
Note: To ensure no conflicting software is present, we recommend deploying Windows OS using the original ISO file from Microsoft (instead of using an image provided by the server vendor, which may include bloatware).
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