Re: [RFC] union-mount stuff

From: Alexander Viro (viro@math.psu.edu)
Date: Wed Jun 07 2000 - 04:44:57 EST


On Wed, 7 Jun 2000, Petr Vandrovec wrote:

> > > mountpoint was busy before, union-mount should not suceed...
> >
> > Why?
>
> Because of with my previous answer you were not allowed to operate on
> busy mountpoints... OK, I'm corrected. Mountpoint should become busy -
> you can unmount any of two filesystems, but not both (if it can be
> implemented).

It can, and there may be more than two, BTW.

> Does not same thing apply also for common subdirectories? Intuitive is

Not. union-mount != unionfs, it does not descend into subdirectories.
There is no way in hell to do that and permit sharing the union-mount
components between several mountpoints. unionfs is very different animal
and there the main point is that you are getting real, honest
copy-on-write, i.e. if you have foo/bar/baz on underlying filesystem than
any attempt to access foo will create a shadowing directory in the upper
layer, any attempt to access foo/bar will do the same for foo/bar and
attempt to write into the foo/bar/baz will lead to copying the thing into
the upper layer and changing it there. _Very_ useful when you have a
read-only fs and want to run make on it, for one thing - everything
new/modified gets into the covering layer, along with the accessed part of
directory tree. Very nice, but completely different - there are things
impossible for one and doable on another.

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