NFS and UFS Mounting on a non-empty directory

[Jonathan I. Kamens]

In article <2339 at orion.cf.uci.edu> Guy Cardwell <gcardwel at envy.eng.uci.edu>
writes:
>The big question is... is the concept of NFS mounting on a non-empty
>directory well defined???  Could I place a default passwd file in a
>directory, and later mount the NFS one on top of it safely.. (would
>the results be the same between various NFS implementations.... ie..
>SUN 4.0, DEC Ultrix, HP UX, Xenix, IBM A/IX) (yes, we have at least
>one of each).

Yes, the concept of mounting over a non-empty directory is
well-defined, and has been discussed here recently, in reference to
having a /tmp or /usr/tmp on the root during boot time, and then
mounting something over it via NFS in order to have more space in it
during multi-user operation.

Our (4.3BSD) man page for mount(2) says the following:
>DESCRIPTION
>     mount attaches a file system to a directory.  After a suc-
>     cessful return, references to directory dir will refer to
>     the root directory on the newly mounted file system.  Dir is
>     a pointer to a null-terminated string containing a path
>     name.  Dir must exist already, and must be a directory.  Its
                                                               ^^^
>     old contents are inaccessible while the file system is
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>     mounted.
      ^^^^^^^

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