Shred: Doesn't work on Journaled FS?

Question

Shred documentation says shred is \"not guaranteed to be effective\" (See bottom). So if I shred a document on my Ext3 filesystem or on a Raid, what happens? Do I shred pa

Answer 1

The concern is that data might exist on more than one place on the disk. When the data exists in exactly one location, then shred can deterministically "erase" that information. However, file systems that journal or other advanced file systems may write your file's data in multiple locations, temporarily, on the disk. Shred -- after the fact -- has no way of knowing about this and has no way of knowing where the data may have been temporarily written to disk. Thus, it has no way of erasing or overwriting those disk sectors.

Imagine this: You write a file to disk on a journaled file system that journals not just metadata but also the file data. The file data is temporarily written to the journal, and then written to its final location. Now you use shred on the file. The final location where the data was written can be safely overwritten with shred. However, shred would have to have some way of guaranteeing that the sectors in the journal that temporarily contained your file's contents are also overwritten to be able to promise that your file is truly not recoverable. Imagine a file system where the journal is not even in a fixed location or of a fixed length.

If you are using shred, then you're trying to ensure that there is no possible way your data could be reconstructed. The authors of shred are being honest that there are some conditions beyond their control where they cannot make this guarantee.