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Filepunch

What?

Several Linux filesystems (XFS, ext4, btrfs, tmpfs) support sparse files, i.e. files that save space by omitting empty filesystem blocks that contain only zeroes. Unfortunately, these "holes" in the files aren't automatically created by writing a string of zeroes, but only by seeking past them with fseek, lseek, etc. It's entirely possible that more room on your hard drive could be saved by finding empty blocks and replacing them with holes.

I plan on building two tools:

  1. A scanner, which scans through files to find how much space could be saved by replacing empty blocks with holes.

  2. A "hole puncher" which actually punches holes over empty blocks.

Why?

I was interested in learning more about sparse files and playing with related system calls.

Caveats

This is most useful for files that are unlikely to be modified, since any writes to holes will force the filesystem to create actual blocks in their stead.

License

Zlib (I did it, do whatever you want, I'm not liable for whatever happens).

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Makes files sparse

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