On Wed, Jan 17, 2018 at 03:55:09PM -0500, Jeff King wrote: > I'm definitely sympathetic, and I've contemplated a patch like this a > few times. But I'm not sure we're "safe by default" here after this > patch. In particular: > > 1. This covers only loose objects. We generally sync pack writes > already, so we're covered there. But we do not sync ref updates at > all, which we'd probably want to in a default-safe setup (a common > post-crash symptom I've seen is zero-length ref files). I've not seen them myself yet, but yes, they need an fsync. > 2. Is it sufficient to fsync() the individual file's descriptors? > We often do other filesystem operations (like hardlinking or > renaming) that also need to be committed to disk before an > operation can be considered saved. No, for metadata operations we need to fsync the directory as well. > 3. Related to (2), we often care about the order of metadata commits. > E.g., a common sequence is: > > a. Write object contents to tempfile. > > b. rename() or hardlink tempfile to final name. > > c. Write object name into ref.lock file. > > d. rename() ref.lock to ref > > If we see (d) but not (b), then the result is a corrupted > repository. Is this guaranteed by ext4 journaling with > data=ordered? It is not generally guranteed by Linux file system semantics. Various file system will actually start writeback of file data before rename, but not actually wait on it.
