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

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Date:	Mon, 11 Oct 2010 14:51:39 +0100
From:	Scott James Remnant <scott@...ntu.com>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	Andreas Gruenbacher <agruen@...e.de>,
	Michal Suchanek <hramrach@...trum.cz>,
	Andy Whitcroft <apw@...onical.com>,
	Scott James Remnant <scott@...onical.com>,
	Vladimir Dronnikov <dronnikov@...il.com>,
	Felix Fietkau <nbd@...nwrt.org>,
	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 06/10/2010 18:31, Valerie Aurora wrote:
> On Fri, Oct 01, 2010 at 11:34:57AM +0200, Andreas Gruenbacher wrote:
>> On Thursday 30 September 2010 23:51:15 Valerie Aurora wrote:
>>> 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?
>> The prevalent use case probably is to start out with an empty topmost layer on
>> top of an existing file system.  When things are modified, changes obviously
>> go into the topmost layer.  Additional layers can later be stacked on top of
>> that, turning the previous topmost layer into a read-only lower layer.
>>
>> Overlaying preexisting file systems doesn't seem that important; users
>> commonly should be able to start out with an empty topmost layer instead.  To
> Okay, that surprises me.  Let me check my assumptions.  I cc'd several
> people who seem to be actively using unionfs or aufs in ways that we
> want union mounts to replace.  Do you start out with an empty topmost
> file system in most cases?  Or do you prepopulate with some files in
> dirs you want to be transparent?
>
Our use would be for the Live CD and for Update testing - in both of 
these scenarios I imagine that the top-most layer would start empty, yes.

Scott
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