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    SubjectRe: RFC: from FIBMAP to FIONDEV
    Matthew Wilcox wrote:
    > how much space is left?

    Right now, 42 bytes. And you already need 64 just to store the location
    of a backup sector for each sector that's there ...

    > So do you approve of the idea of adding an index to the call to allow the
    > application to find out about all possible real blocks which contain
    > this data?

    I'm not sure about it. On the one hand, this type of basic replication
    is just what the interface could handle properly. On the other hand,
    all other cases are already too complex, because you'd actually need
    a set of locations, plus an algorithm. RAID can easily be extended to
    quite arbitrarily complex structures, so even a simple table of known
    algorithms may not be sufficient. Also, once you have the information,
    what are you supposed to do with it ?

    One could incrementally load the boot loader, i.e. first stage with
    capabilities to load and correct one additional sector, second stage
    with the capability to load a sector with a list of sectors, using
    code already brought in by the first stage, then second stage loads
    the third stage, with user interactions, etc.

    But this gets very complex and it still doesn't solve the problem of
    actually identifying the valid set of information, which may simply be
    impossible for the first stage. (Asking the user is also a problem
    at this point.)

    Maybe somebody working on RAID could explain what is involved in
    detecting the status of a set of volumes ?

    Unless there's a useful solution for this, I don't think it makes
    much sense to construct a complex solution that still doesn't handle
    maybe 50% of all cases.

    - Werner

    / Werner Almesberger, ICA, EPFL, CH /

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