BLOG: Determine and modernize Filesystem Deduplication
Version history - 1.5 Added insights from Steven Ekren. Many thanks! / Added ReFS Docs link and added clarification about drawbacks. - 1.4 revised script so ReFS volumes with classic dedup will be identified, added more eligibly checks and error handling - 1.3 added point #4 in migration guidance - 1.2 revised script - 1.1 formatting This blog explains the two Windows deduplication modes classic Windows Data Deduplication (ReFS or NTFS) and ReFS Deduplication (ReFS). It covers how they differ, why you should consider upgrading to Windows Server 2025 to leverage the new ReFS dedup engine, and clear warnings about scenarios where ReFS is not recommended. Practical migration guidance and detection commands are included. Differences between classic dedup and ReFS dedup File system: Classic dedup runs on NTFS or ReFS; ReFS dedup runs on ReFS and Windows Server 2025 or later, only. Implementation: They are separate engines with different metadata formats and management cmdlets. Management: Classic dedup uses the Dedup PowerShell module (Get‑DedupVolume, Start‑DedupJob, Disable‑DedupVolume). ReFS dedup uses its own ReFS dedup cmdlets (Get‑ReFSDedupStatus, Enable‑ReFSDedup). Conversion: There is no in‑place conversion between the two; metadata and chunk formats are incompatible. Improvements: the new in-line ReFS Deduplication leverages the advantages of ReFS files system. This makes deduplication more efficient and less CPU intensive. The new ReFS Deduplication can also compress data in-line using L1Z algorithm. This makes it up to par with enterprise solutions, often found in SAN storage or Linux appliances. Compression needs to be set per volume, and optional. Edit: Steven Ekren, a former Senior Product Manager for Hyper-V shared valuable insights on how both engines operate in a comment on LinkedIn: [...] the basic conceptual difference between WS Deduplication and ReFS deduplication is that the Windows Server [dedup] version takes the duplicate file data and moves it to a repository and puts a reparse point in the file system from each point that references the data. This involves data movement and therefore not recommended for workloads that are changing it's data often, but best for more static data like documents and picture/videos. ReFS is a file system that uses links natively for all the objects so leaving the data in place and managing the links is much more efficient and doesn't involve the data copy and managing a repository. Effectively it's built into the file system. As the blog notes, there are some situations not recommended for this version of dedupe, but generally it's lower performance and storage I/O impact. Why upgrade to Windows Server 2025 Improved version of ReFS Filesystem Improved ReFS in-line deduplication + optional L1Z compression: Server 2025 includes enhancements to ReFS dedup performance, scalability, and integration with modern storage features. Support and fixes: Windows Server 2016 and 2019 are past mainstream support, increasing the likelihood of costly support cases and delayed fixes; upgrading reduces operational risk and ensures access to ongoing improvements. Future compatibility: Newer OS releases receive optimizations and bug fixes for ReFS and dedup scenarios that older releases will not. SMB compression: for reasonably faster data transfer at minimal CPU when transferring data through the networks. Feature and security related improvements refer to availabile Microsoft Windows Server 2025 Summit content on techcommunity.microsoft.com Scenarios where ReFS is not recommended ReFS on SAN in clustered CSV environments: Avoid placing ReFS dedup on top of SAN‑backed Cluster Shared Volumes (CSVFS) in production clusters; clustered SAN/CSV scenarios causing severe performance issues in practice. Please refer to the ReFS documentation. (personal opinion and experience, not endorsed by Microsoft): Many small, fast‑changing files: Workloads with frequent small writes, such as user profiles, folder redirection of AppData folders, or applications that churn small config files (for example, Lotus Notes config files) can cause locks, performance degradation, or unexpected behavior on ReFS. Exclude these disks from dedup or keep them on NTFS. Note: Restrictions on high churn rate, like lockups or high RAM consumption, deadlocks / BSOD might have been addressed in Windows Server 2025 and the ReFS Dedup, see comment of Steven Ekren. Improving reliability and performance is a top goal for ReFS, to improve the adoption and feature parity with NTFS. For information about feature parity please refer to the ReFS documentation. Migration guidance The following instructions describe a high level and supported migration path from Windows deduplication using the NTFS file system to native ReFS Deduplication. Note: Step #3, data migration is not required when already using ReFS with Data Deduplication. In this case it's enough to execute step #1 and #2. Note: Validate on non‑production data first. Plan for rehydration time and network/storage throughput. Ensure backups are current before starting. Make sure to have a full backup before upgrading Server OS or making changes. 1. Disable classic dedup on the NTFS source: Disable-DedupVolume -Volume YourDriveLetter: 2. Rehydrate (un‑deduplicate) the data: Start-DedupJob -Volume YourDriveLetter: -Type Unoptimization 3. Copy or move data to a ReFS volume (new target): For straightforward NTFS→ReFS copies, robocopy is recommended. A GUI and job based alternative to this is the File Server Migration Feature (uses robocopy) in Windows Admin Center. For complex scenarios, open files long path names very large datasets (< 5 TB) or many small files restructuring, GUI (including Windows Server Core) automation, improved logging cloud/hybrid migrations I recommend the usage of GS RichCopy Enterprise by GuruSquad for higher speed (up to 40%) and reliability, compared to robocopy. 4. Optionally remove the Windows Server feature When there is no old deduplication in use consider to remove the feature. Your advantages of doing so: removes an unneccessary service. removes the file system filter driver for dedup, which causes performance impacts, even when not in use. removes the PowerShell commandlets for the old dedup, so they cannot mistakenly used by existing scripts, unaware admins etc. When migrating files over network: SMB compression: consider both source and target run Windows Server 2025 and leverage SMB compression. SMB Compression is available in Microsoft xcopy, Microsoft robocopy and Gurusquad GScopy Enterprise. Balancing and Teaming with SMB: SMB does not require LFBO or SET Teaming. It automagically detects network links and actively balances on its own on Windows Server 2016 and later. Using teaming, depending the configuration, can negatively affect transfer speed. Quick detection and diagnostic commands Check file systems: Get-Volume | Select DriveLetter, FileSystem Check classic dedup feature: Get-WindowsFeature -Name FS-Data-Deduplication Get-DedupVolume Get-DedupStatus Check ReFS dedup: Get-Command -Module Microsoft.ReFsDedup.Commands Get-ReFSDedupStatus -Volume YourDriveLetter: Diagnostic script to detect both: <# .SYNOPSIS Detects classic NTFS Data Deduplication and ReFS Deduplication across local volumes. .DESCRIPTION - Reports NTFS volumes with classic Data Dedup enabled. - Lists ReFS volumes present on the host. - If the ReFS dedup cmdlet exists AND OS build >= 26100, checks ReFS dedup status per ReFS volume. - Color coding: * Classic dedup enabled → Yellow * Classic dedup not enabled → Cyan * ReFS dedup enabled → Green * ReFS dedup not enabled → Cyan .NOTES Version: 1.7 Author: Karl Wester-Ebbinghaus + Copilot Requirements: Elevated PowerShell session, PowerShell 5.1 or newer Supported OS: Windows Server 2025, Azure Stack HCI 24H2 or newer Unsupported OS: Windows 10, Windows 11 (script terminates) #> #region Initialization Write-Verbose "Initializing variables and environment..." $Volumes = $null $Volume = $null $DedupVolumesList = $null $DedupReFSVolumesList = $null $DedupReFSVolumesListLetters = $null $DedupReFSStatus = $null $refsCmd = $null $OSBuild = $null $runReFSDedupChecks = $null #endregion Initialization #region Volume Discovery Clear-Host Write-Verbose "Querying NTFS and ReFS volumes..." $Volumes = Get-Volume | Where-Object FileSystem -in 'NTFS','ReFS' #endregion Volume Discovery #region ReFS Dedup Cmdlet, OS Build and OS SKU Detection Write-Verbose "Checking for ReFS deduplication cmdlet..." $refsCmd = Get-Command -Name Get-ReFSDedupStatus -ErrorAction SilentlyContinue Write-Verbose "Reading OS build number..." try { $OSBuild = [int](Get-ItemProperty -Path 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion' -Name CurrentBuildNumber).CurrentBuildNumber } catch { Write-Verbose "Registry read for OS build failed. Falling back to Environment OSVersion." $OSBuild = [int][Environment]::OSVersion.Version.Build } # end try/catch for OS build detection Write-Verbose "Checking OS InstallationType and EditionID..." $CurrentVersionKey = Get-ItemProperty -Path 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion' $InstallationType = $CurrentVersionKey.InstallationType # "Client" or "Server" $EditionID = $CurrentVersionKey.EditionID # e.g. "AzureStackHCI", "ServerStandard", etc. Write-Verbose "Detected InstallationType: $InstallationType" Write-Verbose "Detected EditionID: $EditionID" Write-Verbose "Detected OSBuild: $OSBuild" # Block Windows 10/11 (Client OS) if ($InstallationType -eq 'Client') { Write-Error "Unsupported OS detected: Windows Client (Windows 10/11). Only Windows Server or Azure Stack HCI are supported. Script will terminate." exit } # Allow Azure Stack HCI explicitly if ($EditionID -eq 'AzureStackHCI') { Write-Verbose "Azure Stack HCI detected. Supported platform." } else { # Must be Windows Server if ($InstallationType -ne 'Server') { Write-Error "Unsupported OS detected. Only Windows Server or Azure Stack HCI are supported. Script will terminate." exit } Write-Verbose "Windows Server detected (EditionID: $EditionID). Supported platform." } Write-Verbose "Evaluating ReFS dedup eligibility based on cmdlet presence and build >= 26100..." $runReFSDedupChecks = $false if ($refsCmd -and ($OSBuild -ge 26100)) { $runReFSDedupChecks = $true Write-Verbose "ReFS dedup checks ENABLED (cmdlet present and OS build >= 26100)." } else { Write-Verbose "ReFS dedup checks DISABLED (cmdlet missing or OS build < 26100)." } #endregion ReFS Dedup Cmdlet, OS Build and OS SKU Detection #region Main Loop foreach ($Volume in $Volumes) { # begin foreach volume loop Write-Host "Volume $($Volume.DriveLetter): ($($Volume.FileSystem))" Write-Verbose "Processing volume $($Volume.DriveLetter)..." #region Classic Dedup + ReFS Volume Listing if ($Volume.FileSystem -eq 'NTFS' -or $Volume.FileSystem -eq 'ReFS') { Write-Verbose "Checking classic deduplication status for volume $($Volume.DriveLetter)..." $DedupVolumesList = Get-DedupVolume -Volume $Volume.DriveLetter -ErrorAction SilentlyContinue if ($DedupVolumesList) { Write-Host " → Classic Data Dedup ENABLED on $($Volume.DriveLetter), $($Volume.FileSystem)" -ForegroundColor Yellow } else { Write-Host " → Classic Data Dedup NOT enabled on $($Volume.DriveLetter),$($Volume.FileSystem)" -ForegroundColor Cyan } # end if classic dedup enabled Write-Verbose "Listing ReFS volumes on host..." $DedupReFSVolumesList = Get-Volume | Where-Object FileSystem -eq 'ReFS' if ($DedupReFSVolumesList) { $DedupReFSVolumesListLetters = ($DedupReFSVolumesList | ForEach-Object { $_.DriveLetter }) -join ',' Write-Host " → ReFS volumes present on host: $DedupReFSVolumesListLetters" } else { Write-Host " → No ReFS volumes detected on host" } # end if ReFS volumes present } # end NTFS/ReFS block #endregion Classic Dedup + ReFS Volume Listing #region ReFS Dedup Status if ($Volume.FileSystem -eq 'ReFS') { if ($runReFSDedupChecks) { Write-Verbose "Checking ReFS deduplication status for volume $($Volume.DriveLetter)..." $DedupReFSStatus = Get-ReFSDedupStatus -Volume $Volume.DriveLetter -ErrorAction SilentlyContinue if ($DedupReFSStatus) { Write-Host " → ReFS Dedup ENABLED on $($Volume.DriveLetter), $($Volume.FileSystem)" -ForegroundColor Green } else { Write-Host " → ReFS Dedup NOT enabled on $($Volume.DriveLetter), $($Volume.FileSystem)" -ForegroundColor Cyan } # end if ReFS dedup enabled } else { if (-not $refsCmd) { Write-Error " → Skipping ReFS dedup check: Get-ReFSDedupStatus cmdlet not present" -ForegroundColor Cyan } else { Write-Error " → Skipping ReFS dedup check: OS build $OSBuild < required 26100" -ForegroundColor Cyan } # end reason for skipping ReFS dedup check } # end if runReFSDedupChecks } # end if ReFS filesystem block #endregion ReFS Dedup Status Write-Host "" } # end foreach volume loop #endregion Main Loop #region End Write-Verbose "Script completed." #endregion End Recommendations and next steps Inventory: Identify volumes using NTFS dedup and ReFS dedup, and map workloads that create many small or rapidly changing files. Plan: Schedule rehydration and migration windows; test ReFS dedup on representative datasets. Upgrade: Prioritize upgrading servers still on 2016/2019 (End of Mainstream Support) to reduce support risk and gain the latest ReFS dedup improvements. Kindly consider reading my Windows Server Installation Guidance and Windows Server Upgrade Guidance Exclude: Keep user profiles, AppData, and other high‑churn small‑file paths off ReFS dedup or on NTFS. Consider ReFS Dedup with Compression: Enable compression optionally. Mind ReFS dedup compression is not the same as compress files integration in File Explorer or File Explorer properties (Windows 9x). It's transparent to the application Make smart decisions: Avoid using dedup when the dataset is changing fast or your dedup + compression rate is below 20%. Usually you can expect 40% or more savings, and up to 80% in specific use cases like VDI VHDX with ReFS Dedup + Compression. Plan your dedup jobs: Ensure of making use of the planning features for dedup jobs through PowerShell or Windows Admin Center (WAC) when using ReFS dedup on more than one volume per Server. Otherwise they might all run at the same time and impact your storage performance (esp. spinning rust) and consumption of RAM and CPU. Share and Educate: Inform your infrastructure team about the changes so they avoid using the traditional dedup on ReFS. Related blogposts: https://splitbrain.com/windows-data-deduplication-vs-refs-deduplication/ , Thanks Darryl van der Peijl and team.
Windows Backup taking waaaaay to long
While I'm not a heavy user of these MS forums I have had to resort to them from time to time over the last 15-20 years. Yet I still can't figure out the organizational structure and it seems I can never find the right forum for my query. Almost every time my post gets moved to the correct forum or message board, or someone gives me a link directly to it. I expect it to be no different this time, and I'm perfectly fine with that. So here we go. I have Windows Server 2025 installed as a VM using MS's built-in Hyper-V on a Server 2025 computer. the VM is set up as a DC and all that stuff functions exactly as it should. However, doing the backup has suddenly gone from taking anywhere from 2 hours to a max that comes close to but has never exceeded four hours. Obviously, it depends on how much there is to actually back up. I've already gone through the troubleshooting tips to do things like checking the VSS settings and a bit of other stuff I can't exactly recall at the moment. I have an external physical 1TB usb hard drive attached to the physical computer and then it's attached as a drive to the Server 2025 VM and shows up in computer management/disk manager ad Disk 1, as it should. I have the VM set up to use this Disk 1 as the backup disk with the Windows Server Backup program. Some things I note and add here in case it matters. - The size of the VM disk for this Server 2025 VM is 500GB and the partition size of Drive C shows as 498.91GB with the remaining shown as 100MB for the EFI system partion and 1001MB for the recovery partition. - When backup starts, a new disk labeled Disk 2 appears in the disk management window on the VM and I note it's the same size as Drive C on the VM at 498.91GB. I'm wondering if this has anything to do with why my backups suddenly went from taking a max of 4 hours to as long as 20 hours to complete. Where is this virtual disk created? I looked on the VM host machine in the C:\programdata\microsoft\windows\Virtual Hard Disks directory, and it's not there. It's not on the VM machine because the virtual hard disk directory doesn't exist in that same location on the VM. THe host machine itself has a 2TB hard drive in it with 993GB of free space. Any advice or suggestions here? I have no idea why backups went from 2-4 hours to taking 20 hours or more to complete. Thanks for any help, advice or suggestions anyone can offer here. -Carl
Cache drive reconfiguration in Server 2025 Storage Spaces Direct cluster
We have a three node S2D cluster running Server 2025, with the storage in a 3 way mirror, running Hyper-V VMs. Each node has 4 x NVMe drives that are currently being used as cache drives, but which are connected to a RAID controller (in HBA mode), so in the S2D configuration they appear as SSD drives rather than NVMe drives. We've purchased the required cables and drive bays to be able to reconfigure the NVMe drives so that they're attached directly to the PCIe bus, so they'll show up as NVMe drives and hopefully give us a performance boost, so I'm just trying to plan the reconfiguration. I was hoping it would be a relatively simple process of shutting everything down, reconfiguring the storage and bringing everything back online, but ChatGPT suggests things won't be that easy and that a complete reconfiguration of the storage would be required. So in a nutshell, can the cache drives be reconfigured without a complete rebuild of the S2D storage ? Cheers, Rob
S2D FaultDomainAwareness
We're setting up a 2 Node windows 2025 cluster with storage spaces direct After creating the pool we created two virtual disk but see the following output PS C:\WINDOWS\system32> Get-VirtualDisk | Format-List FriendlyName, Size, FaultDomainAwareness FriendlyName : ClusterPerformanceHistory Size : 25769803776 FaultDomainAwareness : StorageScaleUnit FriendlyName : S2DVOL01 Size : 10995116277760 FaultDomainAwareness : FriendlyName : S2DVOL02 Size : 10995116277760 FaultDomainAwareness : The FaultDomainAwareness is empty for the two virtual disk created on the storage pool which is configured like this PS C:\WINDOWS\system32> Get-StoragePool –FriendlyName S2D-CLHV-001-Pool | Format-List FriendlyName, Size, FaultDomainAwarenessDefault FriendlyName : S2D-CLHV-001-Pool Size : 57592038555648 FaultDomainAwarenessDefault : StorageScaleUnit is there something wrong ?
Server 2022, network drives and content searching
We have two drives with close to 1TB of data, The one drive has a specific folder where end users want to search for a file using content from the files. The files are generally .xls based files End users are 99% Windows 11 based and server is server 2022. Indexing on the server with content aware filtering has occurred and can find the files when searching for their names however not when searching content. This needs to happen using Windows Search in explorer. Data is unstructured, however structured in a folder system. (Year, month, etc) I have read that Windows Content Search is not supported on network drives, can this be confirmed? Any suggestions on a resolution would be appreciated.
Issue with existing drive formatted to ReFS and mounting in new server
I had a hardware failure which also took out my Server installation drive. I have new hardware and have installed Server 2022 again on new drive. I have connected a drive that I had formatted as ReFS and now it shows as RAW and Event log shows "ReFS failed to mount the volume. Version 3.14 doesn't match expected value 3.7" I have no idea what is going on here. Both installations of Server 2022 used the same media. This is getting very painful. I have connected the drive to a Windows 11 24H2 computer, and it reads fine.ReFS volume appears as RAW after rebuild of server
I had a Windows 2022 server that had a drive formatted as ReFS. The hardware failed and I had to get a new system. The Windows installation also got corrupted when the hard failed. I have the new hardware and I have installed Server 2022 again but the drive is coming up as Raw and ReFS failed to mount the volume. Version 3.14 doesn't match expected value 3.7 is appearing in the ReFS/Operational event logs. I connected the drive to a Windows 11 Pro pc and are able to view the drive. Why has this happened? Is there a way to upgrade the version of RsFS to 3.7? Is there something I have missed when building the new server?
iSCSI Target servers - high availability?
Hello, I have two VMs configured as iSCSI target servers with a 600GB VDHX file on each, and then two VMs configured as file servers in a failover cluster. The iSCSI servers should serve out the data that is shared in the failover cluster file server. I would like to also configure the iSCSI target servers in high availability mode, so that they replicate their data and if one of the iSCSI target server go down, the shares and data is still accessible. How would I go about doing this? So far I've tried setting up storage replica, but since I only have one site, it doesn't allow me to replicate to the second iSCSI disk from the one that currently has data. I also tried iSCSI Target-role in the failover cluster manager, but it puts me back at the same situation where if the storage server with the virtual iSCSI disk goes down, I lose access to all data.Clarification on NTLM Authentication Events (Event ID 4625 & 4624) in SOC Monitoring
Hello, While monitoring authentication events in the SOC, I frequently encounter multiple failed (Event ID: 4625) and successful (Event ID: 4624) login attempts associated with NTLM authentication. Upon investigating the affected machine, I found no active NTFS shares or resources being accessed. Despite this, NTLM events continue to appear in the logs. I’m trying to understand what might be triggering these events. Could this be related to background processes, service accounts, or another NTLM authentication mechanism? Although this is a low-level incident, I’d like to fully grasp the cause to rule out any potential security concerns. I’d appreciate any insights you can provide! Thank you.
Increase the size of user profile disk in my remote desktop server
Hi all experts. I have a server for remote desktop services purposes, Windows 2016 standard, and domain joined. It is configured using User Profile Disk, and the maximum limit is set to 5GB. I want to increase the maximum limit but I can't do it under the collection's properties because that field is grayed out. My questions: How to increase the maximum limit? Please guide me and let me know how. Can I increase the maximum limit for 1 single user only? If yes, please let me know how. I found some info from the web that this can be done by the Diskpart command, is it true? If I follow the Diskpart method, do all user profiles encounter data lost? I need your guidance and input, I appreciate it. Here are some images:
