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    SubjectRe: [PATCH 0/7] overlay filesystem: request for inclusion
    On 9 June 2011 05:52, Andrew Morton <> wrote:
    > On Thu, 9 Jun 2011 11:59:34 +1000 NeilBrown <> wrote:
    >> On Wed, 8 Jun 2011 15:32:08 -0700 Andrew Morton <>
    >> wrote:
    >> > On Wed,  1 Jun 2011 14:46:13 +0200
    >> > Miklos Szeredi <> wrote:
    >> >
    >> > > I'd like to ask for overlayfs to be merged into 3.1.
    >> >
    >> > Dumb questions:
    >> >
    >> > I've never really understood the need for fs overlaying.  Who wants it?
    >> > What are the use-cases?
    >> I think the strongest use case is that LIVE-DVD's want it to have a write-able
    >> root filesystem which is stored on the DVD.
    > Well, these things have been around for over 20 years.  What motivated
    > the developers of other OS's to develop these things and how are their
    > users using them?

    FWIW there is an union solution in NetBSD. I am not sure it is used in
    the LiveCD but you can definitely use it to build a piece of software
    without actually touching the source directory.

    >> >
    >> > This sort of thing could be implemented in userspace and wired up via
    >> > fuse, I assume.  Has that been attempted and why is it inadequate?
    >> I think that would be a valid question if the proposal was large and
    >> complex.  But overlayfs is really quite small and self-contained.
    > Not merging it would be even smaller and simpler.  If there is a
    > userspace alternative then that option should be evaluated and compared
    > in a rational manner.

    The problem with the userspace alternative is that it does not work. I
    tried to run my live CD on top of unionfs-fuse and the filesystem
    would fail intermittently leading to random errors during boot.

    > Another issue: there have been numerous attempts at Linux overlay
    > filesystems from numerous parties.  Does (or will) this implementation
    > satisfy all their requirements?

    No implementation will satisfy all needs. There is always some
    compromise between availability (userspace/in-tree/easy to patch in)
    feature completeness (eg. AuFS is not so easy to forward-port to new
    kernels but has numerous features) performance, reliability.

    > Because if not, we're in a situation where the in-kernel code is
    > unfixably inadequate so we end up merging another similar-looking
    > thing, or the presence of this driver makes it harder for them to get
    > other drivers merged and the other parties' requirements remain
    > unsatisfied.

    One of the major use cases is building live CDs.

    That and other things can be done with overlayfs.


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