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    SubjectRe: [PATCH 14/38] fallthru: ext2 fallthru support
    On Thu, Aug 05, 2010 at 01:13:55PM +0200, Miklos Szeredi wrote:
    > 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.

    Just a quick note to say that my explicit design was to do as much as
    possible in the VFS, except when adding a little support to the
    low-level fs would make it significantly faster, simpler, and more
    correct. I think for union mounts to perform moderately well, and to
    avoid namespace problems, we can't build it 100% out of existing file
    system parts like xattrs. However, I could be wrong and I will
    definitely give any other implementation serious consideration.


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