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

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Message-Id: <201009281025.00727.agruen@suse.de>
Date:	Tue, 28 Sep 2010 10:24:59 +0200
From:	Andreas Gruenbacher <agruen@...e.de>
To:	Valerie Aurora <vaurora@...hat.com>
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 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.

> 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?

Thanks,
Andreas
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