[kragen]

I forget the particular failure mode that POSIX arguably permitted in this case. I think it had to do with fsyncing the directory? And the file ending up existing but empty after the reboot? Anyway, it broke userspace, so the ext4fs maintainers killed it. I think there were some LWN articles.

I might have been misremembering: https://lwn.net/Articles/322823/

"With ext4's delayed allocation, the metadata changes can be journalled without writing out the blocks. So in case of a crash, the metadata changes (that were journalled) get replayed, but the data changes don't."

That was without fsync, though. But apparently it's a topic of discussion more recently: https://lwn.net/Articles/789600/

I agree that it still requires a lockfile if write conflicts are not acceptable.