Subject: Re: sources for sys for 1.5 Alpha2
To: None <John.C.Hayward@wheaton.edu>
From: John Darrow <John.P.Darrow@wheaton.edu>
Date: 09/15/2000 18:50:18
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.
John Darrow - Senior Technical Specialist Office: 630/752-5201
Computing Services, Wheaton College, Wheaton, IL 60187 Fax: 630/752-5968
Alphapage: email@example.com Pager: 630/316-0707
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
>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
>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
>On Fri, 15 Sep 2000, Bertram Barth wrote:
>> Date: Fri, 15 Sep 2000 21:18:47 +0200 (CEST)
>> From: Bertram Barth <firstname.lastname@example.org>
>> To: email@example.com
>> 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)
>> PS: m90# uname -a
>> NetBSD m90 1.5_ALPHA2 NetBSD 1.5_ALPHA2 (GENERIC) #1: Tue Sep 12 09:14:10 PDT 2000 firstname.lastname@example.org:/other/vax/kobj/GENERIC vax