There's a difference between a normal mount with the -o union mount option
(what Bertram has) and union filesystem mount (e.g. mount -t union).

The -o union mount option simply layers the lookups - a file is first looked
up in the top layer, if not found, then it is looked up in the bottom layer.
Among other things, this still allows editing of a file in the bottom layer,
if no file of the same name exists in the top layer.

The union file system adds another feature - if an attempt is made to write
to a file on the bottom layer, it instead creates a new file in the top
layer, which masks the bottom file.  Thus it is useful for such things as
mounting a build area over a virgin source tree (possibly mounted readonly)
without risking corruption of the source tree by a wayward build command.

Unfortunately, to support the ability to "remove" a file from the union-
mounted directory, the union file system requires something called a
"whiteout", something which NFS doesn't support.  Thus an NFS file system
can't be used as the top layer in a union mount.

jdarrow

-- 
John Darrow - Senior Technical Specialist               Office: 630/752-5201
Computing Services, Wheaton College, Wheaton, IL 60187  Fax:    630/752-5968
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Email:     John.P.Darrow@wheaton.edu

John Hayward <John.C.Hayward@wheaton.edu> wrote:
>Ok I did a uname -a and found the same kernel.
>
>Here is part of my /etc/fstab
>====
>abigail:/source /source         nfs     ro              0 0
>/source/NetBSD-current/pkgsrc   /var/pkgsrc     union   rw,noauto,-b    0 0
>====
>
>mount reveals:
>...
>csserver2:/scilab/NetBSD/VAX/Csvax2/root on / type nfs
>abigail:/source on /source type nfs (read-only)
>...
>
>An attempt to mount results in:
>...
>csserver4# mount /var/pkgsrc
>mount_union: /source/NetBSD-current/pkgsrc on /var/pkgsrc: Operation not 
>supported
>...
>
>I guess since I had to mount / via nfs that union mounts on another nfs
>mounted area might be the problem.
>
>I cannot mount root locally since there is a problem with the kernel and
>MicroVax3100 and rz56s.
>
>Looks like I'll have to try to get a kernel which works before I can do
>union mounts.
>johnh...
>On Fri, 15 Sep 2000, Bertram Barth wrote:
>
>> Date: Fri, 15 Sep 2000 21:18:47 +0200 (CEST)
>> From: Bertram Barth <bertram@gummo.bbb.sub.org>
>> To: port-vax@netbsd.org
>> Subject: sources for sys for 1.5 Alpha2
>> 
>> John Hayward writes:
>> > Dear Vax People,
>> >    I attempted to do a union mount under 1.5 Alpha2 kernel and get
>> > operation not supported.  I assume the union facility was not included in
>> > building the kernel.
>> 
>> For me it worked just out of the box. "mount" displays:
>> gummo:/usr/ports/distfiles on /usr/pkgsrc/distfiles type nfs (read-only, union)
>> harpo:/x/tmp/distfiles on /usr/pkgsrc/distfiles type nfs (read-only, union)
>> 
>> Ciao,
>> 	bertram
>> 
>> PS: m90# uname -a
>> NetBSD m90 1.5_ALPHA2 NetBSD 1.5_ALPHA2 (GENERIC) #1: Tue Sep 12 09:14:10 PDT 2000     matt@cheetah.local:/other/vax/kobj/GENERIC vax
>> 
>