How do I determine the degree in which a file is fragmented?

Question

I would like to provide a way to recognize when a large file is fragmented to a certain extent, and alert the user when they should perform a defragmentation. In addition, I'd like to show them a visual display demonstrating how the file is actually broken into pieces across the disk.

I don't need to know how to calculate how fragmented it is, or how to make the visual display. What I need to know is two things: 1) how to identify the specific clusters on any disk which contain pieces of any particular given file, and 2) how to identify the total number of clusters on that disk. I would essentially need a list of all the clusters which contain pieces of this file, and where on the disk each of those clusters is located.

Most defragmentation utilities have a visual display showing how the files are spread across the disk. My display will show how one particular file is split up into different areas of a disk. I just need to know how I can retrieve the necessary data to tell me where the file's clusters/sectors are located on the disk, so I can further determine how fragmented it is.

Answer 1

You can use the DeviceIoControl function with the FSCTL_GET_RETRIEVAL_POINTERS control code.

The FSCTL_GET_RETRIEVAL_POINTERS operation retrieves a variably sized data structure that describes the allocation and location on disk of a specific file. The structure describes the mapping between virtual cluster numbers (VCN offsets within the file or stream space) and logical cluster numbers (LCN offsets within the volume space).