What Journaling Guarantees

In short, journaling guarantees file system consistency. Journaling prevents the directory hierarchy, file names, file metadata and cluster allocation information from becoming corrupted in case of an untimely interruption (such as a power failure or application crash). However, while journaling protects the integrity of the file system, it does not necessarily protect your data integrity (i.e., the file contents). For example, if the application crashes while a write operation is being performed, the data could end up only partially written on the media (see Journaling API level atomicity).