linux-kernel - Re: [RFC 12/26] ext2 white-out support

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Message-ID: <20070802175008.GA12627@lazybastard.org>
Date:	Thu, 2 Aug 2007 19:50:09 +0200
From:	Jörn Engel <joern@...fs.org>
To:	Josef Sipek <jsipek@....cs.sunysb.edu>
Cc:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Jan Blunck <jblunck@...e.de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>, hch@...radead.org
Subject: Re: [RFC 12/26] ext2 white-out support

On Wed, 1 August 2007 15:33:30 -0400, Josef Sipek wrote:
>  
> This brings up an very interesting (but painful) question...which makes more
> sense? Allowing the modifications in only the top-most branch, or any branch
> (given the user allows it at mount-time)?
> 
> This is really question to the community at large, not just you, Dave :)

Only write to top-most layer.

There are two reasons for this.  First it allows users to create a union
mount, test something (e.g. update the distribution) and remove every
trace from the test by umounting the top-most layer.  Such a thing can
be quite valuable.

The second reason is simplicity.  I personally couldn't even start to
describe the semantics.  If the user does a rename, which layer will the
change end up in?  What if source or target exist in multiple layers?
How to rename a directory in a lower layer containing a new file in an
upper layer?

Finding new and interesting corner cases for such a beast can be quite
entertaining.  And until someone has properly documented the semantics
for _all_ the corner cases, my enthusiasm is below freezing point.  Does
such a documentation exist?

Jörn

-- 
A surrounded army must be given a way out.
-- Sun Tzu
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