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BLOG: Determine and modernize Filesystem Deduplication
Version history - 1.6 Added references / links - 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. https://www.veeam.com/kb2023 Veeam best practices about Windows Deduplication and ReFS Deduplication.
Procedures to raise the functional level of AD 2008 r2 to 2019
Hello everyone, Our AD has the Windows Server 2008 functional level and the servers with Windows Server 2016 OS. I intend to raise the functional level to 2019 or 2025. I would like your help with tips and documentation to decide whether 2019 or 2025 would be best, what are the risks and procedures for successful migration. I have an isolated environment to carry out rehearsals and tests before actually going into production.
Domain users not able to logon with their password event though it has not been changed....
Hi, we have this weird problem where some of the users suddenly can't login to their computer with the password they have used for almost 20 years (yes sorry, bad practise). When the user reports it I check that I can logon to the computer with my own account (not 20 year old password) which works fine. I check the event log for problems both on the client and the DC and all I see is see which I can relate to the problem is event id 4625 with an error code which means bad password. I check the AD account and see that pwdLastSet has a date in 2006 (not quite 20 years, but close) and I check that the account is not logged out or expired. Also make sure that the password never expires is enabled, so in my book these are all the checks needed and problem not solved. I then change the password to the same password that the user has had for almost 20 years and problem solved, but problem source not found. This has happend to 3-4 users within the last week or two, even a service user with domain admin permissions, only thing I pay note to that they have in common is the pwdLastSet in 2006, but I really can't seem to get my head around this being the issue. Also only other thing I can think of that has changed is that the old DC has been removed a few months ago, and a new 2025 DC has been introduced. promote/demote went without issues and this problem didn't surface before now several weeks after the DC change. So if anyone has experienced something similar or perhaps can point me in a direction for further troubleshooting please let me know. Thansk ThomasPhase 2 of Kerberos RC4 hardening begins with the April 2026 Windows security update
Windows updates released in April 2026 and later begin the second deployment phase of protections designed to address a Kerberos information disclosure vulnerability (CVE‑2026‑20833). This second phase continues the shift away from legacy encryption types such as RC4 by moving toward stronger default ticket behavior. After installing the April 2026 update, domain controllers default to supporting Advanced Encryption Standard (AES‑SHA1) encrypted tickets for accounts that do not have an explicit Kerberos encryption type configuration. If your organization relies on service accounts or applications that depend on RC4-based Kerberos service tickets, now is the time to address those dependencies to avoid authentication issues before the Enforcement phase begins in July 2026. Microsoft recommends continuing to monitor the System event log for Kerberos-related audit events and identify and address misconfigurations or remaining dependencies, then enabling enforcement when warning, blocking, or policy events are no longer logged. See How to manage Kerberos KDC usage of RC4 for service account ticket issuance changes related to CVE-2026-20833 and CVE‑2026‑20833 to learn more about the vulnerability, timelines, recommended preparation steps, and configuration options to ensure compliance before Enforcement mode begins in July 2026.
Virtual printer in windows server 2019 standard is not shown after configuration
Hello, I am, trying to configure a virtual printer in a Windows server 2019 standard edition that is deployed in OCI cloud. This instance has windows server license included in the pricing. The problem comes when after ending the process of configuration this virtual printer is not displayed in "Devices and printers" any idea why is this happening? Regards, AnaLogin failure from tssdis.exe on RDS server
Remote desktop server in AD environment [Windows Server 2019 standard, running RDweb, RDG, and session host, etc] periodically has service tssdis.exe (remote desktop session broker) failing to login, Event IDs 4648 and then 4625 about half a dozen times before stopping. This doesn't appear to affect any users, but I can't find anything online addressing it or if it's something that needs to be fixed. Some other people have reported the problem but not the solution, such as here: https://social.technet.microsoft.com/Forums/windowsserver/en-US/26e48e81-1400-4f8c-aef2-df03143fa211/login-through-tssdisexe Below are two exports of the event IDs (sanitized) This is the 4648 A logon was attempted using explicit credentials. Subject: Security ID: NETWORK SERVICE Account Name: SERVERNAME$ Account Domain: DOMAIN Logon ID: 0x3E4 Logon GUID: {10b9db0d-f9e0-e3ab-8c95-7dcb4ec5b3c7} Account Whose Credentials Were Used: Account Name: SERVERNAME Account Domain: DOMAIN Logon GUID: {00000000-0000-0000-0000-000000000000} Target Server: Target Server Name: SERVERNAME.DOMAIN.local Additional Information: SERVERNAME.DOMAIN.local Process Information: Process ID: 0x17c8 Process Name: C:\Windows\System32\tssdis.exe Network Information: Network Address: fe80::xxxx:f30a:xxxx:xxxx Port: 49667 This event is generated when a process attempts to log on an account by explicitly specifying that account’s credentials. This most commonly occurs in batch-type configurations such as scheduled tasks, or when using the RUNAS command. This is the 4625: An account failed to log on. Subject: Security ID: NULL SID Account Name: - Account Domain: - Logon ID: 0x0 Logon Type: 3 Account For Which Logon Failed: Security ID: NULL SID Account Name: SERVERNAME Account Domain: DOMAIN Failure Information: Failure Reason: Unknown user name or bad password. Status: 0xC000006D Sub Status: 0xC0000064 Process Information: Caller Process ID: 0x0 Caller Process Name: - Network Information: Workstation Name: SERVERNAME Source Network Address: fe80::xxxx:f30a:xxxx:xxxx Source Port: 50798 Detailed Authentication Information: Logon Process: NtLmSsp Authentication Package: NTLM Transited Services: - Package Name (NTLM only): - Key Length: 0 This event is generated when a logon request fails. It is generated on the computer where access was attempted. The Subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe. The Logon Type field indicates the kind of logon that was requested. The most common types are 2 (interactive) and 3 (network). The Process Information fields indicate which account and process on the system requested the logon. The Network Information fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases. The authentication information fields provide detailed information about this specific logon request. - Transited services indicate which intermediate services have participated in this logon request. - Package name indicates which sub-protocol was used among the NTLM protocols. - Key length indicates the length of the generated session key. This will be 0 if no session key was requested.Error al agregar Windows Server 2025 a dominio existente, nivel funcional 2016
Buenas a todos, Me dirijo a esta comunidad en busca de orientación para resolver un problema que se me está presentando al intentar integrar un nuevo servidor con Windows Server 2025 Standard a mi infraestructura de Active Directory existente. Descripción del entorno: Dominio de Active Directory activo con Windows Server 2019 Standard. Nivel funcional de dominio y bosque configurado en Windows Server 2016. Controladores de dominio actuales: server-dc01.impresoratec y server-ad2019.impresoratec. El nombre de dominio interno utilizado es impresoratec (nombre NetBIOS/dominio de etiqueta única, sin sufijo DNS completo tipo .local o .com). Problema: Al intentar agregar el nuevo servidor con Windows Server 2025 al dominio, el proceso falla y se presenta el siguiente mensaje de error: "Es posible que el nombre de dominio "impresoratec" sea un nombre de dominio NetBIOS. Si este es el caso, compruebe que el nombre de dominio está registrado correctamente con WINS. [...] La consulta se refería al registro SRV para _ldap._tcp.dc._msdcs.impresoratec. La consulta identificó los siguientes controladores de dominio: server-dc01.impresoratec y server-ad2019.impresoratec. Sin embargo, no se pudo contactar con ningún controlador de dominio." El mensaje sugiere que los registros de host (A) o (AAAA) pueden contener direcciones IP incorrectas o que los controladores de dominio no son accesibles desde el nuevo servidor. Lo que he verificado hasta ahora: Los controladores de dominio existentes están en línea y operativos. La replicación entre los DCs actuales funciona con normalidad. El nuevo servidor con 2025 tiene conectividad de red general, pero no logra localizar los DCs al momento de unirse al dominio. Mi consulta: ¿Alguien ha experimentado este comportamiento al incorporar un servidor con Windows Server 2025 a un dominio con nivel funcional 2016 y un nombre de dominio de etiqueta única (single-label domain)? ¿Existe algún requisito previo adicional —como la actualización del esquema de AD, ajustes en DNS o en WINS— que deba cumplirse antes de agregar el nuevo DC? Agradezco de antemano cualquier orientación o experiencia que puedan compartir.
RDS Licensing for administrators
Hello, We are planning to acquire 10 RDS User licenses, and I would like to clarify the following points: Will access be limited to 10 simultaneous rdp session, or can administrators still access the server normally via RDP? Do administrators connecting using mstsc /admin require an RDS license, or is this access exempt? In the past (Windows Server 2016), I recall that exceeding the number of licenses allowed temporary RDP sessions. Could you confirm if this behavior is still the same? Thank you in advance for your clarification.
