File-system journaling mode

Which contents pass through the journal and which persistence dependencies accompany them. The ext4 documentation distinguishes these modes: file metadata and contents both pass through the journal in data=journal; only file metadata is journaled in data=ordered, with associated contents forced to final locations before metadata commit; only file metadata is journaled in data=writeback, without that data-before-commit dependency.^[ref]

For our append, full data journaling uses the three-record model. Ordered metadata journaling protects $I,B$ in the journal and first persists $D$ directly in free block 9. Before commit, changing an unreferenced free block does not change the visible old file; after commit, journal replay supplies the new file metadata and the new contents are already durable. Without that dependency, recovery can produce correct allocation and size but stale contents in block 9. Ordered metadata journaling does not give general all-or-nothing contents for overwrites of already allocated file data blocks, nor does it combine arbitrary application writes into a file-system transaction. The choice of protected records determines the scope of the guarantee.