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Karl-WE
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Jan 07, 2026

BLOG: Determine and modernize Filesystem Deduplication

Version history
- 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.

 

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—user profiles, redirected AppData, 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 paths from dedup or keep them on NTFS. Restrictions, like lockups or high RAM consumption, deadlocks / BSOD might have been addressed in Windows Server 2025. 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.

Another alternative is the Windows Admin Center File Server Migration Feature

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 GScopy Enterprise by GuruSquad  for higher speed (up to 40%) and reliability.

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 Dedup and 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
  • Plan your dedup jobs: Ensure making use 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.
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