linux-kernel - Re: [PATCH 6/7 v3] overlay: hybrid overlay filesystem prototype

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Date:	Thu, 30 Sep 2010 17:51:15 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Andreas Gruenbacher <agruen@...e.de>
Cc:	Miklos Szeredi <miklos@...redi.hu>, linuxram@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	neilb@...e.de, viro@...iv.linux.org.uk
Subject: Re: [PATCH 6/7 v3] overlay: hybrid overlay filesystem prototype

On Tue, Sep 28, 2010 at 10:24:59AM +0200, Andreas Gruenbacher wrote:
> On Monday 27 September 2010 20:47:47 Valerie Aurora wrote:
> > Maybe I don't understand.  It seems like directories created when the
> > file system is *not* union mounted should definitely be merged with
> > matching directories on the lower layer.
> >
> > Take the case of /etc/fstab.  The first union mount never touches /etc
> > and it doesn't exist on the topmost layer.  Then we unmount the upper
> > layer, mount it somewhere else as a plain mount, and create /etc/ and
> > /etc/fstab.  When we union mount it back over the lower layer again,
> > we still want the lower layer /etc/ to be merged with the topmost
> > /etc/, or else init.d will disappear.
> 
> I can't think of a reason why the upper layer would really *need* to be 
> modified separately as in this example though, and I'm sure that examples for 
> opaqueness by default can be constructed as well.  Transparency comes at a 
> cost though (lookup, readdir, whiteouts), and defaulting to opaque directories 
> will be more efficient in some cases.  This is why I think that opaqueness by 
> default is preferable.

I agree with that for directories created while it is union mounted.

> > Again, maybe I'm misunderstanding, but this doesn't make much sense to
> > me.  Say I create:
> > 
> > /upper/a_dir/upper_file
> > /lower/a_dir/lower_file
> > 
> > Then when I union mount them, I want a_dir/ to be transparent
> > automatically and show both upper_file and lower_file, without marking
> > it manually.
> 
> Why?

Hm, this was a pretty basic assumption for me - that you'd want to
construct a topmost image offline that would be "merged" with the
lower layers.  So, for example:

Topmost layer contains:

/etc/hostname

Lower layers contain everything else in /etc/.  So /etc/ would exist
on the topmost layer at the time of union mount, but we would want it
to be transparent.  But if we created a new dir *during* the union
mount, it would be opaque.

What was your model?

-VAL
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