That kinda doesn't matter with ZFS or btrfs, though; as real CoW file systems their consistency properties are similar to a fully journaled FS (not only metadat

That kinda doesn't matter with ZFS or btrfs, though; as real CoW file systems their consistency properties are similar to a fully journaled FS (not only metadata but also data, like ext4 in journal=data mode). In that case you don't need to care anymore as an application developer about file committing patterns, as long as it's only about one file.