partition oddity

Matthew Dillon dillon at
Sun May 18 17:27:05 PDT 2008

:this seems to be the cause here:

    Yup, that's the problem.  I should have thought of it sooner.
    Basically it has to work this way.  It is an artifact of the BSD
    label's in-band storage of the label.  GPT labels store the actual
    label out-of-band, so that sort of problem doesn't crop up.

    But with BSD disklabels, the disklabel itself is stored in-band, meaning
    at sector 0 as displayed by the label.  To avoid it is best to start
    at sector 16.

    You can zero out the disk slice using e.g. ad8s2 (which is access without
    going through the label and thus is able to overwrite the label), but
    if you access it via a partition in the label and the partition starts
    at sector 0, e.g. ad8s2d, then that chunk at the beginning containing
    the label itself will be read-only by default.


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