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    SubjectRe: [GIT PULL] Squashfs pull request for 2.6.29

    * Andrew Morton <> wrote:

    > More importantly, the filesystem driver has to be able to read older
    > filesystem instances. This is a userspace-visible binary interface! A
    > really complex one.
    > If for some reason we wish to change the on-disk format then that could
    > be done now. But once the code is merged, such changes could only be
    > done in a back-compatible way.

    IMHO what makes squashfs special is that:

    1) it's read-only: i.e. we dont actually _generate_ this data structure.
    It comes from the outside.

    2) it's a the "cat is already out of the bag" situation.
    It's in the field, it's used.

    And as such it's pretty much an externality already to Linux: the
    filesystem is in use, distros patch it in, devices ship with it, etc. etc.

    So the main technical question at this stage is whether we want to support
    that binary format _at all_, whether we want to support that kind of data

    Also, i think your interpretation of 'ABI' stretches it quite far: we dont
    control the lowlevel bits at all - we dont generate the data, we just
    interpret it in the kernel in a read-only way. We dont provide the raw
    lowlevel bits to user-space either - we provide VFS bindings to it and
    that is generalized and not touched by squashfs.

    As such squashfs has little to no classic ABI bindings - other than the
    trivial "do we support this data format" question. (which is not a classic
    ABI question.)

    [ Often such read-only formats can even be iterated slightly incompatibly
    without much fuss, as the userspace side goes together with the kernel
    anyway so it's easy to stay in touch. ]

    And the thing is, i think we never before in the kernel said "no" to any
    support for a data format externality. [Let me qualify that: 'unpure' data
    protocols with active 'code' components were always an exception to that:
    ACPI, reiser4, etc. - but read-only access to a passive medium is far more

    This is a nice filesystem, and we support far, far worse data formats
    already. Far, far, far, far worse data formats, formats that are less used
    and were developed completely regardless of Linux. We support data formats
    and physical transports that should not even be mentioned with squashfs in
    the same paragraph.

    And the thing is, if in the mainline kernel we were at the forefront of FS
    R&D, we would and could prototype such filesystems in the mainline kernel
    and we would have efficient and prompt influence over lowlevel format

    We are not at the forefront - distros tend to apply new filesystem patches
    far earlier than we do and it takes forever to get stuff upstream - for
    better or worse. And we (the upstream community) had little active role in
    developing this thing _at all_.

    ( This is not a criticism it's just an observation of current reality: it
    is _fundamentally hard_ to do active high-flux R&D for persistent
    formats upstream and still be reasonably compatible at the same time.
    So developing out-of-tree - or via FUSE - might as well be the right
    model here. )

    Generally when a filesystem driver comes to us, its lowlevel format is
    pretty much a done deal already - it's out in the wild and we should say
    'no' only as an exception mechanism for clearly unacceptable crap.

    Instead of trying to flex our muscle and steer the big red firetruck way
    after the fire has been put out already - by others.

    Saying 'no' at this stage comes at a great and largely unnecessary cost to
    everyone involved. I believe we force ourselves into the R&D flow at an
    inappropriately late stage - while at the same time we are unreceptive to
    early adopter projects who'd like to avoid that. We cannot have the cake
    and eat it too.

    At least IMHO.

    ( What could _perhaps_ change the picture a bit IMO is drivers/staging/ i
    think - we could take a far more active role in certain types of
    projects that have been done out of tree typically, with no formal
    promise for compatibility - or something like that. )


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