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

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Message-ID: <20070801193330.GA20928@filer.fsl.cs.sunysb.edu>
Date:	Wed, 1 Aug 2007 15:33:30 -0400
From:	Josef Sipek <jsipek@....cs.sunysb.edu>
To:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Cc:	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, Aug 01, 2007 at 02:10:31PM -0500, Dave Kleikamp wrote:
> On Wed, 2007-08-01 at 14:44 -0400, Josef Sipek wrote:
> > Alright not the greatest of examples, there is something to be said about
> > symmetry, so...let me try again :)
> > 
> > /a/
> > /b/bar		(whiteout for bar)
> > /c/foo/qwerty
> > 
> > Now, let's mount a union of {a,b,c}, and we'll see:
> > 
> > $ find /u
> > /u
> > /u/foo
> > /u/foo/qwerty
> > $ mv /u/foo /u/bar
> > 
> > Now what? How do you rename? Do you rename in the same branch (assuming it
> > is rw)?
> 
> Er, no.  According to Documentation/filesystems/union-mounts.txt, "only
> the topmost layer of the mount stack can be altered".
 
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 :)

> > 1) "cp -r" the entire subtree being renamed to highest-priority branch, and
> > rename there (you might have to recreate a series of directories to have a
> > place to "cp" to...so you got "cp -r" _AND_ "mkdir -p"-like code in the VFS!
> > 1/2 a :) )
> 
> I think this is the only alternative, given the design.
 
Right. Doing something like this at the filesystem level (as we do in
unionfs) seems less painful - filesystems are places full of all sorts of
nefarious activities to begin with. Having it in the VFS seems...even
uglier.

Josef 'Jeff' Sipek.

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