On a journaled filesystem, data and journal only are committed with sync().
You have to umount or remount readonly to get all filesystem metadata to
commit.
After sync () it's expected you can crash, and the filesystem will be made
consistent at next remount when the journal is replayed.
If anything tries to find files with data committed, journal committed, but
fs metadata not committed: such as GRUB or debug tools, they will fail.
Another option is to freeze/unfreeze. That was originally an XFS feature,
but is now generic capability. What I'm not totally sure about off hand is
whether the XFS user space tools is what to use for any filesystem, I'm
pretty sure that it is.
Chris Murphy