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

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Message-ID: <20070802102631.GW5101@hasse.suse.de>
Date:	Thu, 2 Aug 2007 12:26:31 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	Josef Sipek <jsipek@....cs.sunysb.edu>
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, Josef Sipek wrote:

> > 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.
> 

Yes, although that might sound strange: you are sharing the topmost writable
layer. This is what I expect.

> > > 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.

Completely separate? It is totally tied to UnionFS and tries to work out
purely the problems that this kind of VFS emulating filesystems have.
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