How Can FSCTL_ENUM_USN_DATA Be Used to Efficiently Detect File Changes on an NTFS Volume?

Efficient Detection of File Changes on an NTFS Volume

Introduction

Detecting altered files on an NTFS volume can be challenging, especially for volumes with numerous files and directories. This article explores an efficient approach using the FSCTL_ENUM_USN_DATA API to identify specific changes, including deletions, modifications, and new additions.

Using FSCTL_ENUM_USN_DATA

FSCTL_ENUM_USN_DATA allows for the enumeration of files on a volume, including their current state and USNs (Update Sequence Numbers). By comparing current USNs with previous values, you can determine whether a file has changed.

Code Demonstration

The provided sample program in C uses FSCTL_ENUM_USN_DATA to search for files named "test.txt" on the C volume and retrieves information about their changes. It checks for files that match the specified name and provides detailed information about each found file and its parent directory.

Advantages of this Approach

• Fast processing: FSCTL_ENUM_USN_DATA offers high performance, typically returning over 6000 records per second, even on older systems.
• Comprehensive data: The returned data includes file attributes, file flags, and USNs, enabling detection of changes in file content, metadata, or both.
• Selective filtering: You can specify file types or specific paths to focus on changes of interest.

Limitations

• Retrieving full file paths: The retrieved records only include parent file reference numbers. You will need to correlate these numbers with the file IDs of directories to reconstruct full paths. This can be handled with either a single large buffer or by reading directory records on an as-needed basis.
• Path filtering: FSCTL_ENUM_USN_DATA does not provide direct filtering based on paths. To restrict the search to specific directories, you can process the directory records after retrieving them.

Conclusion

FSCTL_ENUM_USN_DATA is a highly efficient and effective method for detecting file changes on an NTFS volume. By using this API with the appropriate data filtering techniques, you can develop a robust file monitoring system that suits your specific requirements.

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