tarhardlink

tarhardlink

Tar file extractor, which uses hard links to save space if possible.

Description

Given a series of Tar files, which are full snapshots of the same folder in different point times (a backup using a Tar file is a good example), tarhardlink can detect files, which haven't changed from the previous archive and create a hard link instead of creating new copy of the file. This way, extracting all Tar files will occupy the minimally required space and allow to make it easy to work with the folder (no need to use overlayfs or equivalent to provide a complete view of the folder).

This is very similar to the rsync's --link-dest option, which creates hardlinks to save space when creating backups.

Comparison

In order to be as performant as possible, the command only checks the following attributes of the file:

• size
• permissions
• modification time

Note: Files, which have been modified but for some reason have the same modification time will still be hard linked.

Gotchas

Because the files are hardlinked, they should be treated as read-only. Any writes to a file in one of the folders, will change the file in all copies which have hardlinked the file.

Usage

After compiling, simply call:

tarhl -file <tar-file> -dest <target-folder> -base <base-folder>

where:

• tar-file is the Tar file to be extracted. If - is given, standard input will be used.
• target-folder is the name of the folder to extract the Tar file to. It will be automatically created if it doesn't exist.
• base-folder is the name of the folder, which will be used to check for creating hard links.

Warranty

At the moment, this is a personal toy project and although it works for my use cases, no guarantees are given it will work for yours. Be ready to debug and modify the utility to make it run on your machine and fit your use case.

Also, I'm pretty sure there are potential security holes with malicious Tar files, e.g. it doesn't validate file path names and it has the potential to escape the folder.