> NFSv4 ? Why is there a need for the underlying file system to know about
> nfs ?
Because NFSv4 requires certain functionality from the underlying filesystem to be able to detect that a file has changed. This has to be valid across NFS server restarts (e.g. power off/on) so needs to be stored in stable storage. i.e. in the underlying filesystem. ext4 already has this functionality as it's on-disk format was changed at about the same time the NFSv4 functionality was introduced. Now that we are doing a major XFS format change, we can add this functionality as well.