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    SubjectRe: [RFC 0/13] extents and 48bit ext3
    Andrew Morton wrote:
    > Ted&co have been pretty good at avoiding compatibility problems.

    Well, extents and 48bit make that track record demonstrably worse.

    Users are now forced to remember that, if they write to their filesystem
    after using either $mmver or $korgver kernels, they are locked out of
    using older kernels.

    From the user's perspective, ext3 has no clear "metadata version 1",
    "metadata version 2" division. Thus they are now forced to keep a
    matrix of kernel versions and ext3 feature flag support, to know which
    kernels are usable with which data. It is a support nightmare.

    At no point is a user ever told, in big capital letters, "IF YOU WRITE
    TO THIS FILESYSTEM, YOU CAN'T BOOT OLDER KERNELS." There is no "click
    OK to continue with this dramatic event."

    And as features continue to be added in this manner, this problem gets
    _exponentially_ worse.


    On the project management side of things, I see no indication that this
    momentum slow -- which implies to me that people will keep slapping new
    stuff into ext3, rather than directing energy towards a newer, cleaner
    ext-NG filesystem.

    Dragging around back-compat really constrains freedom, and you have to
    have some sort of "pressure relief valve" (a massive, wildly
    incompatible update) eventually.

    In my mind, it's analagous to locking developers into developing and
    deploying new features into a stable branch of software. The hacks just
    get worse and worse, as you bend over backwards for back-compat.

    Jeff


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