This chapter covers the OneDrive Sync Client and Known Folder Move (KFM), two critical components for file synchronization and backup in Microsoft 365. For the MS-900 exam, questions on OneDrive sync and KFM appear in approximately 8-12% of the tests, often focusing on default behavior, prerequisites, and user experience. Understanding these technologies is essential for advising on data protection, device management, and user productivity scenarios. We will dissect the sync client's mechanism, KFM's automatic folder redirection, and how they interact with Windows and Intune.
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Imagine a large corporate office where every employee has a personal filing cabinet in their office (their local hard drive). The company also has a central records room (OneDrive cloud storage). To keep files consistent, the company hires a personal assistant for each employee (the OneDrive Sync Client). The assistant's job is to ensure that any document placed in the employee's filing cabinet is immediately copied to the central records room, and any document added or changed in the central room is placed into the filing cabinet. The assistant also follows a special rule: if the employee has a folder on their desk labeled 'Desktop', 'Documents', or 'Pictures', the assistant automatically moves those folders into the filing cabinet and syncs them to the central room—this is Known Folder Move (KFM). The assistant does not change the employee's habit of putting files on the Desktop; it just backs them up automatically. If the employee's filing cabinet gets full, the assistant can make some files 'cloud-only'—still visible in the filing cabinet but actually stored only in the central room, freeing up space. The assistant works silently in the background, but if the network is slow, it queues changes and syncs later. This ensures the employee always has access to their files from any office, and the central records room is always up to date.
What is the OneDrive Sync Client?
The OneDrive Sync Client is a desktop application that synchronizes files between a user's local device and their OneDrive cloud storage. It is included with Windows 10 and 11 and is available for macOS. The sync client runs as a background process (OneDrive.exe) and uses the Microsoft Sync Framework to detect changes and transfer data. It supports sync of files up to 250 GB per file and syncs up to 300,000 files per library (though performance degrades beyond 100,000). The client uses a local database to track file states and metadata.
Why It Exists
Before the sync client, users had to manually upload/download files via a web browser, which was inefficient and error-prone. The sync client enables offline access, automatic backup, and real-time collaboration. It also supports Files On-Demand, which allows users to see all files in OneDrive without downloading them locally, saving disk space.
How It Works Internally
When a user installs the sync client, it creates a local folder (default: %UserProfile%\OneDrive - <tenant name>). The client authenticates using Azure AD (via OAuth 2.0) and establishes a persistent connection to the OneDrive service. It uses a change-tracking mechanism:
- Local changes: The client monitors the local folder for file system events (create, modify, delete) using the Windows USN journal or macOS FSEvents. When a change occurs, it uploads the file in chunks (4 MB blocks) with parallel upload streams.
- Cloud changes: The client polls the OneDrive service for changes (via delta queries) every 30 seconds by default. If changes are detected, it downloads the updated files.
- Conflict resolution: If a file is changed both locally and in the cloud simultaneously, the client keeps both versions, renaming the local version with the computer name and timestamp (e.g., "Report (Contoso-PC conflicted copy 2025-03-15).docx").
Key Components and Defaults
Sync threshold: Up to 300,000 files per library; beyond that, sync may be slow or unreliable.
File size limit: 250 GB per file.
Upload speed: Throttled by default to avoid saturating bandwidth; adjustable via Group Policy or registry.
Download speed: No built-in throttle; can be limited via QoS.
Sync engine: Microsoft Sync Framework v2.1.
Local database: Stored in %LocalAppData%\Microsoft\OneDrive\settings\Business1\ (for work/school accounts).
Files On-Demand: Enabled by default in Windows 10 version 1709 and later. Files appear with a cloud icon and are downloaded on first access.
Configuration and Verification
Administrators can configure the sync client via Group Policy or Intune. Key settings include:
- Silent account configuration: Pre-populate the user's work or school account.
- Set maximum upload/download bandwidth: e.g., Set-MgPolicyBandwidth via PowerShell.
- Block sync of specific file types: e.g., .exe, .pst.
- Enable Known Folder Move: Redirect Desktop, Documents, Pictures.
Verification commands:
- Check sync status: Right-click OneDrive icon in system tray > View sync problems.
- PowerShell: Get-OneDriveStatus (from OneDrive module) or Get-Process -Name OneDrive.
- Event Viewer: Look under Applications and Services Logs > Microsoft > Windows > OneDrive.
Interaction with Related Technologies
Windows 10/11: Integrated with File Explorer; OneDrive appears as a folder.
SharePoint: Sync client can sync SharePoint document libraries (up to 5000 items per library).
Microsoft 365 Apps: Office apps save directly to OneDrive; co-authoring relies on sync.
Intune: Can deploy sync client settings via Configuration Profiles.
Azure AD: Authentication and conditional access policies apply to sync traffic.
Known Folder Move (KFM)
KFM is a feature of the OneDrive Sync Client that automatically redirects the Windows known folders (Desktop, Documents, Pictures) to OneDrive. When enabled, these folders are moved to the user's OneDrive folder, and the original folder locations are replaced with a junction point (reparse point) pointing to the new location. This ensures that files saved to these folders are automatically synced to the cloud.
How KFM Works
Prerequisites: Windows 10/11, OneDrive sync client (build 19.002.0101 or later), and a work/school account with a OneDrive license.
Enabling: Admin can enable via Group Policy or Intune; users can also enable manually from OneDrive settings.
Process:
- The sync client creates a backup of the original folders in the OneDrive folder. - It moves the contents from the old location to the new OneDrive location. - It sets a reparse point at the original path that redirects to the new location. - The user sees no change; they continue to use Desktop, Documents, Pictures as before. 4. Protection: If a user loses their device, files are safe in OneDrive. On a new device, KFM can restore the folders.
Key KFM Details
Supported folders: Desktop, Documents, Pictures. (Music and Videos are not supported for KFM but can be synced manually.)
Default: Not enabled by default; must be explicitly configured.
Multiple devices: KFM can be applied to multiple devices, but the same folder can only be synced from one device at a time unless using the 'Redirect known folders to OneDrive' policy with 'Move contents' option.
Conflict: If a user already has files in OneDrive from another device, KFM will merge contents.
Reverting: Users can 'Stop syncing' a folder, which moves the folder back to its original location.
Performance Considerations
Large folders: Syncing a Desktop with thousands of files can cause initial sync delays.
Bandwidth: KFM can saturate bandwidth during initial upload; schedule during off-hours.
File count: Keep under 300,000 files per OneDrive library; KFM may exacerbate this if user has many files.
Common Misconfigurations
Not enabling KFM: Users lose data if device fails.
Enabling KFM on multiple devices: Can cause duplicate files if not careful.
Excluding folders from sync: Using selective sync can break KFM redirection.
Group Policy conflicts: KFM policies may conflict with Folder Redirection GPOs.
Exam Relevance
For MS-900, understand:
KFM automatically backs up Desktop, Documents, Pictures.
It is part of the OneDrive Sync Client.
It requires Windows 10/11 and a OneDrive license.
It does NOT include Music or Videos.
Files On-Demand is separate but often confused with KFM.
KFM can be managed via Intune or Group Policy.
User signs in to OneDrive
The user launches OneDrive and signs in with their work or school account. The sync client uses OAuth 2.0 to obtain an access token from Azure AD. This token is used for subsequent API calls to the OneDrive service. The client registers with the OneDrive service to receive push notifications (via WebSocket) for real-time change detection.
Initial sync and folder setup
After authentication, the sync client creates the local OneDrive folder (e.g., `C:\Users\username\OneDrive - Contoso`). It then downloads the file metadata from the cloud and creates placeholder files (if Files On-Demand is enabled). The client starts a full sync: it compares the local folder with the cloud and uploads any local files not in the cloud, and downloads any cloud files not locally. This can take significant time for large libraries.
Change detection and sync cycles
The sync client uses two mechanisms for change detection: (1) Local file system watcher (ReadDirectoryChangesW on Windows) that captures file events (create, modify, delete) in real-time. (2) Cloud polling every 30 seconds via delta queries to the OneDrive API. When a change is detected, the client calculates a differential sync: it uploads only the changed blocks of a file (using a binary diff algorithm). For new files, it uploads the entire file in parallel chunks of 4 MB.
Conflict resolution
If a file is modified both locally and in the cloud during the same sync cycle, a conflict occurs. The sync client keeps both versions: the cloud version is saved as-is, and the local version is renamed with the computer name and a timestamp (e.g., 'Report (Contoso-PC conflicted copy 2025-03-15 14-30).docx'). The user is notified via a sync conflict banner in File Explorer. The user must manually resolve the conflict by deleting or merging files.
Known Folder Move activation
When KFM is enabled (via policy or user settings), the sync client identifies the current locations of Desktop, Documents, and Pictures. It creates a backup of these folders inside the OneDrive folder. Then it moves all files from the original locations to the OneDrive folder. Finally, it creates a directory junction (reparse point) at the original path that points to the new OneDrive location. The user sees no change; all file operations continue to work transparently.
Files On-Demand hydration
With Files On-Demand enabled, files in the OneDrive folder appear with a cloud icon. When the user opens a file, the sync client downloads the content from the cloud (hydration). The file becomes fully available offline. The sync client uses a placeholder file (with extended attributes) that contains metadata and a download URL. The download is triggered by the file system filter driver (cldflt.sys). Once hydrated, the file appears with a green checkmark. If the user manually marks a file as 'Always keep on this device', it remains downloaded.
Enterprise Scenario 1: Large Law Firm with 5,000 Users
A law firm needs to ensure that all attorneys' documents are backed up to the cloud, especially those on the Desktop and Documents folders. They deploy OneDrive with KFM via Intune. The challenge: many attorneys have 50,000+ files on their Desktop. Initial sync takes days and saturates the network. The IT team schedules sync during weekends and throttles upload bandwidth to 10 Mbps per user. They also enable Files On-Demand to save local disk space. After deployment, the firm sees a 90% reduction in data loss incidents. Common issue: some users had Folder Redirection GPOs that conflicted with KFM, causing duplicate folders. Resolution: remove Folder Redirection before enabling KFM.
Scenario 2: School District with 20,000 Student Devices
A school district uses shared Windows devices with OneDrive sync for students. They want to ensure that student work on the Desktop is backed up. They enable KFM and set a policy to 'Silently move known folders to OneDrive' without user interaction. Problem: students often save large video files to Desktop, exceeding OneDrive storage limits. The IT team sets a storage quota and uses Group Policy to block sync of .mp4 files. They also enable 'Stop sync of large files' policy. After 6 months, they find that some students' Desktops have thousands of shortcut files, causing sync performance issues. They implement a cleanup script to remove shortcuts from Desktop.
Scenario 3: Healthcare Provider with Compliance Needs
A hospital network must comply with HIPAA, requiring that all patient data on workstations is encrypted and backed up. They deploy OneDrive with KFM and enable Files On-Demand to reduce local data exposure. The sync client uses Azure Information Protection (AIP) to label sensitive files. They configure conditional access policies to require multi-factor authentication for OneDrive sync. A challenge: doctors sometimes use personal devices; they use OneDrive web access instead of sync client. They also restrict sync to only known folders via policy. Performance: initial sync of 100 GB per user takes about 2 days on 50 Mbps connections. They use bandwidth throttling during business hours.
Common Pitfalls
Not excluding system folders: KFM may attempt to sync system folders if misconfigured.
Ignoring file count limits: Users with >300,000 files experience sync delays.
Conflicting policies: Folder Redirection GPO and KFM should not be used together.
User confusion: Users may think KFM duplicates files; proper communication is key.
What MS-900 Tests on OneDrive Sync Client and KFM
MS-900 objective 2.3 covers 'Describe the capabilities of Microsoft 365 productivity solutions', which includes OneDrive for Business and its sync capabilities. Specific sub-objectives: 'Describe how OneDrive sync client works' and 'Describe Known Folder Move'. The exam focuses on: - Default behavior: KFM is NOT enabled by default; it must be configured via policy or user settings. - Supported folders: Only Desktop, Documents, Pictures. Music and Videos are NOT supported. - Prerequisites: Windows 10/11, OneDrive sync client (build 19.002.0101+), work/school account with OneDrive license. - Files On-Demand: Often confused with KFM; Files On-Demand saves disk space, KFM backs up folders. - Conflict resolution: The sync client creates 'conflicted copy' files with computer name and timestamp. - Sync limits: 250 GB per file, 300,000 files per library.
Common Wrong Answers and Why Candidates Choose Them
'KFM backs up Music and Videos folders' – Wrong because KFM only supports Desktop, Documents, Pictures. Candidates confuse KFM with general sync capabilities.
'KFM is enabled by default' – Wrong because it requires explicit configuration. Candidates assume Microsoft enables it automatically for protection.
'Files On-Demand is the same as KFM' – Wrong; Files On-Demand controls local storage, KFM redirects folders. Candidates mix up the two features.
'OneDrive sync client requires an internet connection to access files' – Wrong; with Files On-Demand, files can be made available offline. Candidates forget offline access.
Specific Numbers and Terms That Appear on the Exam
250 GB: Maximum file size for sync.
300,000: Maximum recommended files per library.
30 seconds: Default polling interval for cloud changes.
Desktop, Documents, Pictures: The three folders supported by KFM.
Windows 10/11: Required OS for KFM.
OneDrive for Business: The service name; not to be confused with personal OneDrive.
Edge Cases and Exceptions
KFM on multiple devices: If a user enables KFM on two devices, the same folder (e.g., Desktop) can only be synced from one device at a time. The policy 'Redirect known folders to OneDrive' with 'Move contents' option can merge.
Folder Redirection conflict: If Folder Redirection GPO is already in place, KFM may fail. The exam may test that you cannot use both simultaneously.
SharePoint libraries: The sync client can sync SharePoint document libraries, but KFM only works for OneDrive.
macOS: OneDrive sync client exists for macOS, but KFM is Windows-only.
How to Eliminate Wrong Answers
If a question mentions 'automatically backing up Desktop', look for 'Known Folder Move' as the answer.
If a question asks about 'saving disk space by not downloading files', think 'Files On-Demand'.
If a question says 'requires configuration by IT admin', it is likely KFM (since it's not default).
If a question lists 'Music and Videos', eliminate that option for KFM.
If a question mentions 'conflicted copy', it's about sync conflict resolution.
If a question says 'maximum file size is 250 GB', that's a OneDrive sync limit.
Known Folder Move (KFM) automatically backs up Desktop, Documents, and Pictures to OneDrive.
KFM is NOT enabled by default; it must be configured by an admin or user.
Files On-Demand saves disk space by showing files without downloading them; it is separate from KFM.
OneDrive Sync Client supports files up to 250 GB and up to 300,000 files per library.
Conflict resolution creates 'conflicted copy' files with computer name and timestamp.
KFM only works on Windows 10/11; not on macOS or Linux.
KFM requires OneDrive for Business license and sync client build 19.002.0101 or later.
Folder Redirection GPO and KFM should not be used together; they conflict.
OneDrive Sync Client polls cloud changes every 30 seconds.
If a user enables KFM on multiple devices, only one device can sync each folder unless using merge policy.
These come up on the exam all the time. Here's how to tell them apart.
Known Folder Move (KFM)
Redirects Desktop, Documents, Pictures to OneDrive for backup.
Requires Windows 10/11 and OneDrive sync client.
Not enabled by default; must be configured via policy or user settings.
Moves folder contents to OneDrive and creates a reparse point.
Protects user data by syncing to cloud; if device fails, data is safe.
Files On-Demand
Allows users to see all OneDrive files without downloading them locally.
Enabled by default in Windows 10 version 1709 and later.
Saves local disk space by using placeholder files.
Files are downloaded (hydrated) on first access.
Works with any OneDrive folder, including KFM-redirected folders.
OneDrive Sync Client
Syncs OneDrive for Business files to local device.
Supports up to 300,000 files per library.
Includes Files On-Demand and Known Folder Move.
Uses OAuth 2.0 for authentication to Azure AD.
Designed for individual user's personal cloud storage.
SharePoint Sync Client
Syncs SharePoint document libraries to local device.
Supports up to 5,000 items per library (for sync; more for web).
Does not include Files On-Demand or KFM.
Uses same sync engine but with SharePoint-specific permissions.
Designed for team collaboration and shared libraries.
Mistake
Known Folder Move is enabled by default for all OneDrive users.
Correct
KFM is NOT enabled by default. It must be explicitly configured by an administrator via Group Policy, Intune, or by the user in OneDrive settings. The default behavior is that Desktop, Documents, and Pictures are not redirected.
Mistake
Known Folder Move backs up all user folders including Music and Videos.
Correct
KFM only supports Desktop, Documents, and Pictures. Music and Videos folders are not included. To sync those, users must manually move them into OneDrive or use a different backup solution.
Mistake
Files On-Demand and Known Folder Move are the same feature.
Correct
Files On-Demand allows users to see all files in OneDrive without downloading them locally, saving disk space. KFM redirects known folders to OneDrive for backup. They are separate features that can work together but serve different purposes.
Mistake
OneDrive Sync Client can sync files larger than 250 GB.
Correct
The maximum file size for sync is 250 GB. Files exceeding this limit cannot be synced and will show an error. This is a hard limit enforced by the OneDrive service.
Mistake
KFM works on macOS and Linux devices.
Correct
KFM is a Windows-only feature. It requires Windows 10 or 11. While OneDrive sync client is available for macOS, KFM is not supported on non-Windows platforms.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
Known Folder Move (KFM) backs up three folders: Desktop, Documents, and Pictures. It does not back up Music, Videos, or any other user folders. To back up additional folders, users can manually move them into OneDrive or use a third-party backup solution. KFM is specifically designed to protect the most common folders where users save important files.
No, Known Folder Move is not enabled by default. It must be explicitly configured by an administrator using Group Policy, Microsoft Intune, or by the user in the OneDrive settings. The default behavior is that Desktop, Documents, and Pictures remain in their original locations and are not synced to OneDrive. This is a common exam trap: remember that KFM requires configuration.
Known Folder Move (KFM) redirects your Desktop, Documents, and Pictures folders to OneDrive, ensuring they are backed up to the cloud. Files On-Demand is a separate feature that allows you to see all your OneDrive files in File Explorer without downloading them, saving local disk space. KFM protects your data; Files On-Demand optimizes storage. They can work together: KFM moves folders, and Files On-Demand shows files as placeholders until accessed.
No, Known Folder Move is a Windows-only feature. It requires Windows 10 or 11 and the OneDrive sync client for Windows. While OneDrive sync client is available for macOS, KFM is not supported on Mac. Mac users can manually sync their Desktop and Documents folders by moving them into OneDrive, but there is no automatic redirection feature.
If you enable KFM on multiple devices, the same folder (e.g., Desktop) can only be synced from one device at a time by default. If you try to sync the same folder from two devices, you may encounter conflicts. However, administrators can use the 'Redirect known folders to OneDrive' policy with the 'Move contents' option to merge folders from multiple devices into the same OneDrive location. This is useful for users who work on multiple computers.
When a file is changed both locally and in the cloud simultaneously, the OneDrive Sync Client keeps both versions. The cloud version is saved as-is, and the local version is renamed with the computer name and a timestamp in the format 'filename (ComputerName conflicted copy YYYY-MM-DD HH-MM).ext'. The user is notified via a sync conflict banner in File Explorer and must manually resolve the conflict by deleting or merging the files.
The OneDrive Sync Client supports files up to 250 GB in size. The recommended maximum number of files per library is 300,000. Beyond this limit, sync performance may degrade significantly. These limits apply to both OneDrive for Business and SharePoint document libraries synced via the client. For large libraries, consider splitting files into multiple libraries.
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