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

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Message-ID: <20070731171159.GA27234@filer.fsl.cs.sunysb.edu>
Date:	Tue, 31 Jul 2007 13:11:59 -0400
From:	Josef Sipek <jsipek@....cs.sunysb.edu>
To:	Jan Blunck <jblunck@...e.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Subject: Re: [RFC 12/26] ext2 white-out support

On Tue, Jul 31, 2007 at 07:00:12PM +0200, Jan Blunck wrote:
> On Tue, Jul 31, Josef Sipek wrote:
> 
> > On Mon, Jul 30, 2007 at 06:13:35PM +0200, Jan Blunck wrote:
> > > Introduce white-out support to ext2.
> > 
> > I think storing whiteouts on the branches is wrong. It creates all sort of
> > nasty cases when people actually try to use unioning. Imagine a (no-so
> > unlikely) scenario where you have 2 unions, and they share a branch. If you
> > create a whiteout in one union on that shared branch, the whiteout magically
> > affects the other union as well! Whiteouts are a union-level construct, and
> > therefore storing them at the branch level is wrong.
> 
> So you think that just because you mounted the filesystem somewhere else it
> should look different? This is what sharing is all about. If you share a
> filesystem you also share the removal of objects.

The removal happens at the union level, not the branch level. Say you have:

/a/
/b/foo
/c/foo

And you mount /u1 as a union of {a,b}, and /u2 as union of {a,c}.

$ find /u*
/u1
/u1/foo
/u2
/u2/foo
$ rm /u1/foo # this creates whiteout for "foo" in /a
$ find /u*
/u1
/u2

Is that what you'd expect as a user? I don't think so.

...
> > Really the only sane way of keeping track of whiteouts seems some external
> > store. We did an experiment with Unionfs, and moving the whiteout handling
> > to effectively a "library" that did all the dirty work cleaned up the code
> > considerably [2,3].
> 
> Haven't checked if you could use ODF for a generic store for filesystems that
> couldn't support whiteouts. This might be an interesting idea.
 
Yes, since the ODF is completely separate, you can use _any_ filesystem and
regardless of whether or not they support whiteouts.

Josef 'Jeff' Sipek.

-- 
Once you have their hardware. Never give it back.
(The First Rule of Hardware Acquisition)
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