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File Systems and Filter Driver Design Guide

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The File Systems and Filter Driver Design Guide describes how to design and implement a file system filter driver for Windows. It also provides driver-relevant information about Windows file systems. See the programming reference for reference pages.

File systems

Windows file systems are implemented as file system drivers working above the storage system.

Every system-supplied file system in Windows is designed to provide reliable data storage with varying features to meet the user's requirements.

  • Standard file systems available in Windows include NTFS, ExFAT, UDF, and FAT32. A comparison of features for each of these file systems is shown in File System Functionality Comparison.
  • The Resilient File System (ReFS) is available on Windows Server 2012 and later versions. ReFS offers scalable large volume support and the ability to detect and correct data corruption on disk.

Developing a new file system driver is almost always unnecessary, and requirements/specifications for new file system drivers aren't predictable. To that end, this design guide doesn't cover file system development. If you do need to develop a new file system driver beyond those available in Windows, sample code is available as a model.

File system filter drivers

A file system filter driver, or minifilter, intercepts requests targeted at a file system or another file system filter driver. By intercepting the request before it reaches its intended target, a minifilter can extend or replace functionality provided by the original target of the request. Examples of filter drivers include:

  • Anti-virus filters
  • Backup agents
  • Encryption products

Filter driver developers use the Filter Manager (FltMgr.sys). This system-supplied module provides developers a framework to implement filter drivers without having to manage all the complexities of file I/O. FltMgr simplifies filter driver development and solves many of the problems with the legacy filter driver model, such as FltMgr having the ability to control load order through an assigned altitude.

File system and filter sample code

Microsoft provides several file system filter driver samples. Anyone who decides to develop their own file system can file system driver sample code as well.

File system filter driver certification

Certification information for File Systems and File System Filter Drivers is found in the Windows Hardware Lab Kit (HLK). Tests for file systems and filter drivers are found under Filter.Driver.

Other resources

OSR offers various training resources for file system filter developers. They also host community discussion forums such as the Windows File Systems and Minifilters Devs Interest List, where you can ask questions and communicate with filter driver developers from around the world.