I am asking this question on behalf of another user who raised the issue in the Ubuntu chat room.Do journaling filesystems guarantee that no corruption will occur if a power failure occurs?If this answer depends on the filesystem, please indicate which ones do protect against corruption and which ones don't.
Do journaling filesystems guarantee against corruption after a power failure?
filesystems;journaling;corruption
There are no guarantees. A Journaling File System is more resilient and is less prone to corruption, but not immune.All a journal is is a list of operations which have recently been done to the file system. The crucial part is that the journal entry is made before the operations take place. Most operations have multiple steps. Deleting a file, for example might entail deleting the file's entry in the file system's table of contents and then marking the sectors on the drive as free. If something happens between the two steps, a journaled file system can tell immediately and perform the necessary clean up to keep everything consistent. This is not the case with a non-journaled file system which has to look at the entire contents of the volume to find errors.While this journaling is much less prone to corruption than not journaling, corruption can still occur. For example, if the hard drive is mechanically malfunctioning or if writes to the journal itself are failing or interrupted. The basic premise of journaling is that writing a journal entry is much quicker, usually, than the actual transaction it describes will be. So, the period between the OS ordering a (journal) write and the hard drive fulfilling it is much shorter than for a normal write: a narrower window for things to go wrong in, but there's still a window.Further reading