By doing rename before sync, it's possible the rename is committed to storage before data. On ext3, it wasn't possible with data=ordered because it didn't have delayed allocation of blocks, but that was never guaranteed or portable. Doubt developers knew about that guarantee.
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I disagree with this assessment. It was guaranteed by what my, and I think many others', understanding of what data=ordered meant. Delayed allocation is a hack that licenses the fs driver to violate what data=ordered means.
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It's only relevant what data=ordered means if the code was specifically written for ext3 rather than being portable across filesystems. It shouldn't matter what data=ordered means for the vast majority of code not written to target a specific filesystem implementation.
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From my perspective, you're framing it wrong as the code being "written for" anything. The code is just written to POSIX (without sync options) with no concept of power failure existing. Filesystem is providing user-facing (not app-facing) features on top...
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...to give the user (not the application) a level of data integrity they've selected (and made tradeoffs for) based on whether they deem power failure or kernel crash something that could plausibly happen and whether they deem the data valuable.
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You can choose to use ext4 without delayed allocation. From an application or library perspective, it's not relevant as portable code implementing transactions correctly still works fine. Relying on the ext3 ordering is not just broken on ext4 with delayed allocation.
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You're treating this as a [non-]contract between the application and the filesystem, in which case of course it would be "non-portable assumptions". I treat them as having no relationship, and this being a matter of contract between the user and the filesystem.
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Okay, but if you want transactions in application or library code, fsync/fdatasync are required because both filesystems and hardware delay writes and perform them in batches with the order determined based on efficiency. It's not something specific to ext4 at all.
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I don't want transactions. This thread is not about transactions or any programming-level contracts. It's about the extent of inconsistency that can be observed in their absence with different options and nasty hacks that hurt performance for related reasons.
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There's a reason that no other filesystem acts like ext3 with data=ordered. The design forced awful performance for everything, and especially anything using transactions. The performance issues still exist in ext4 but they papered over it and added hacks for broken applications.
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For any modern hardware, i.e. an SSD with NAND access abstracted by the controller, the f2fs approach is way better: kernel.org/doc/Documentat.
It also defaults to not using any heuristic-based hacks to guess what applications are trying to do in order to work around broken code.
If data_flush is enabled, it flushes data at checkpoints, resulting in semantics comparable to data=journal without anything close to the same performance cost since there aren't copies to and from a journal. The filesystem itself is the journal rather than copying to / from it.