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    SubjectRe: [PATCH 14/38] fallthru: ext2 fallthru support
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    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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