> One of the things that I did discuss with Harshad was using some hueristics, where if there are two "unrelated" applications (e.g., different session id, or process group leader, or different uid, etc. --- details to be determined layer), we would not entangele writes to unrelated files via fsync(2), while forcing files written by the same application to share fate with one another even if only file is fsync'ed.

Ugh. This is why we can't have nice things. I really don't want the kernel's filesystem performance to depend on the number of different UIDs writing to the filesystem. That is insanity!

Ted Ts'o is just wrong here: performance should take priority over preserving the behavior of applications that rely on non-contractual implementation details of the Linux kernel. fsync should sync only the indicated file, and that's that. We can add a mount option to let users opt into the older, safer behavior, but we shouldn't suffer for essentially an eternity because somewhere, someone might have written an application that depends on an ext4 implementation detail.