linux-ext4 - Re: [PATCH 14/38] fallthru: ext2 fallthru support

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Message-Id: <E1OgyOZ-000321-Fp@pomaz-ex.szeredi.hu>
Date:	Thu, 05 Aug 2010 13:13:55 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	miklos@...redi.hu, viro@...iv.linux.org.uk, jblunck@...e.de,
	hch@...radead.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tytso@....edu,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 14/38] fallthru: ext2 fallthru support

On Wed, 4 Aug 2010, Valerie Aurora wrote:
> > Another idea is to use an internal inode and make all fallthroughs be
> > hard links to that.
> > 
> > I think the same would work for whiteouts as well.  I don't like the
> > fact that whiteouts are invisible even when not mounted as part of a
> > union.
> 
> I don't know if this helps, but I just wrote support for removing ext2
> whiteouts and fallthrus using tune2fs and e2fsck.  I think this does
> what people want from a "visible" whiteout feature without adding more
> complexity to the VFS.  It also takes away all consideration of race
> conditions and dentry conversion that happens with online removal of
> whiteouts and fallthrus.
> 
> What are your thoughts on what a visible whiteout/fallthru would look
> like?

Best would be if it didn't need any modification to filesystems.  All
this having to upgrade util-linux, e2fsprogs, having incompatible
filesystem features is a pain for users (just been through that).

What we already have in most filesystems:

 - extended attributes, e.g. use the system.union.* namespace and
   denote whiteouts and falltroughs with such an attribute

 - hard links to make sure a separate inode is not necessary for each
   whiteout/fallthrough entry

 - some way for the user to easily identify such files when not
   mounted as part of a union e.g. make it a symlink pointing to
   "(deleted)" or whatever

Later the extended attributes can also be used for other things like
e.g. chmod()/chown() only copying up metadata, not data, and
indicating that data is still found on the lower layers.

Miklos
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