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extents - tools for querying and accessing file extents

Written in 2019 by Eliah Kagan <degeneracypressure@gmail.com>.

To the extent possible under law, the author(s) have dedicated all copyright and related and neighboring rights to this software to the public domain worldwide. This software is distributed without any warranty.

You should have received a copy of the CC0 Public Domain Dedication along with this software. If not, see http://creativecommons.org/publicdomain/zero/1.0/.

This repository provides source code for the C program fiemap, as well as the shell script stitch that understands and uses the ouput of fiemap. These utilities only run on GNU/Linux systems.

Makefile contains rules for building fiemap and for testing it with stitch.

fiemap

fiemap is a C program that uses the FIEMAP ioctl to retrieve a list of extents for a file stored on a block device whose filesystem supports that ioctl.

Unlike hdparm --fibmap, if that ioctl isn't supported, no fallback is performed. Also unlike hdparm, the fiemap utility need not be run as root.

fiemap assumes every byte logically contained in a file is present in some extent, so for sparse files its output is probably never useful.

Although potentially handy, fiemap doesn't attempt to detect or diagnose unusual cases (though it shouldn't crash due to them—if it does, that's a bug). Because this is just an alpha version, you should definitely use hdparm instead, unless:

  • you need a utility that works when run by a non-root user, or
  • your goal is to develop, test, and/or sate your curiosity about fiemap.

stitch

stitch is a Bash script that reads a list of extents in the format produced by fiemap and attempts to stitch the file back together from disk.

Even though fiemap doesn't need to be run as root, stitch does, because it directly reads data from a block device. stitch does not take the name of the file and does not use its inode.

stitch's current behavior when it (thinks it) is run by a non-root user is to stop after it parses its input. Even this is arguably useful, as it still emits an error if the input isn't in the correct format or presents inconsistent information.

stitch is also still in alpha testing, which should give you some pause, being as it's a 200+ line shell script you run as root to directly access sectors on your disk. It should never write directly to those blocks, of course, and I don't think I made any big mistakes...

How to Use

The suggested way to try out fiemap and stitch is:

  1. Build fiemap by running make.

  2. Create a symbolic link in the top-level directory of the repository called test-symlink and point it at a file you want to test fiemap (and stitch) with. The file should preferably have multiple extents, though empty files and single-extent files are supported. A file of a few hundred megabytes is ideal for testing.

  3. Test fiemap and stitch by running make test.

    This runs fiemap as you, then uses sudo to run stitch on the output.