tar: file changed as we read it

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07-10-2024

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Tar Gotcha: Why Your Files Change While You're Archiving Them

Have you ever encountered a frustrating scenario where you try to create an archive using tar, only to find that the files in the archive seem to be corrupted or different from the original files? This is a common problem, and it often arises from a subtle issue: the file's contents are changing while tar is reading them.

Understanding the Problem: A Real-World Example

Imagine you're trying to back up a large log file that's actively being written to. You run the following command:

tar -cvf my_backup.tar logfile.txt

The tar command is supposed to create an archive named my_backup.tar containing logfile.txt. However, as tar reads the file, new data is being appended to it. The result? You might end up with a corrupted archive, missing data, or a file that's incomplete.

Why Does This Happen?

The issue stems from the nature of how tar operates. tar is a sequential archival tool. It reads the file from beginning to end, and the archive is built in a linear fashion. If the file is being modified concurrently, tar may capture a snapshot that's only partially complete or even corrupted.

Solutions to Prevent File Changes During Archiving

Fortunately, there are several strategies to avoid this situation:

1. Stop the Writing Process: The simplest solution is to stop any process writing to the file before running tar. If possible, temporarily pause the application generating the log file or redirect its output to a different file.

2. Utilize --atime-preserve: This option ensures tar doesn't update the access time of the files it archives, potentially preventing unintended modifications.

3. Consider rsync for Incremental Backups: If you're dealing with frequent file changes, rsync is a powerful tool for incremental backups. rsync can efficiently transfer only the changed portions of a file, ensuring your archive remains consistent.

4. Employ a More Robust Backup Strategy: For critical data, explore specialized backup software or solutions like rsnapshot or duplicity. These tools often handle dynamic files and offer features like data deduplication and versioning for reliable backups.

Key Takeaways:

• Always be mindful of potential file modifications while archiving, especially with actively-used log files.
• Prioritize stopping the writing process before archiving to avoid corrupted archives.
• Consider --atime-preserve and explore incremental backup solutions like rsync for dynamic files.
• For critical data, invest in more sophisticated backup solutions.

By understanding this common tar pitfall and adopting the appropriate solutions, you can ensure your archives are accurate and complete, safeguarding your valuable data.