> Thus, I don't see why shred should be less effective in data=journal mode compared to the other journaling modes.
Because with data=journal, content that was previously written to a file (and made its way through the journal) might still be in there if the journal has not been replayed or garbage-collected in a while.
That's a good point I overlooked. Although the journal is not that big (<1 GB); so doing some extra I/O after the shred should get rid of that. When the original contents are older, then it's rather unlikely they're still in the journal.
Because with data=journal, content that was previously written to a file (and made its way through the journal) might still be in there if the journal has not been replayed or garbage-collected in a while.