-Some Linux filesystems are not mounted on a block device; exporting
-these via NFS requires the use of the
-.I fsid
-option (although that may still not be enough).
+For NFSv4, there is a distinguished filesystem which is the root of
+all exported filesystem. This is specified with
+.I fsid=root
+or
+.I fsid=0
+both of which mean exactly the same thing.
+
+Other filesystems can be identified with a small integer, or a UUID
+which should contain 32 hex digits and arbitrary punctuation.