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    SubjectRe: Disk geometry from /sys
    Hi,

    Francis Moreau wrote:
    > Hi Seewer,
    >
    > On Mon, Apr 14, 2008 at 2:57 PM, Seewer Philippe <philippe.seewer@bfh.ch> wrote:
    >> As you've problably seen from the other answers, disk geometry is (except
    >> for a few older devices) unneeded inside the Linux kernel.
    >
    > Yes but I'm doing userspace stuff and that's the reason I was asking for the
    > sysfs thing.
    >
    >> I'd say thats the
    >> reason why there's no sysfs export and I'd further guess disk geometry is an
    >> artifact most would like to get rid of (or pushed into userspace).
    >>
    >
    > Well, I looked at sfdisk(8) and parted(8) source code and they all need the
    > geometry description. If I understood correctly the reason why is that it
    > 'prefers' to align partition sizes/starts on a cylinder boundary because some
    > bootloaders probably use CHS addressing, but I'm really not sure.
    Yes indeed, mainly in the (w)intel world though.

    >
    >> Anyway, if you really need it, try the patch below. Should apply cleanly to
    >> version 2.6.23.1 and gives you a geometry/ directory for each block device
    >> providing the getgeo function. It adds a setgeo counterpart for some
    >> subsystems as well, allowing 'echo something > ...' so please be careful.
    >>
    >
    > Thanks but I probably won't use it. Using sfdisk, for example, is a
    > more portable
    > way to get the geometry from a script.

    Correct. Though be really careful which geometry you are requesting:

    root@local:/# sfdisk -g /dev/sda
    /dev/sda: 7296 cylinders, 255 heads, 63 sectors/track
    root@local:/# sfdisk -G /dev/sda
    /dev/sda: 116280 cylinders, 16 heads, 63 sectors/track

    The first one is the kernels idea of a disks geometry which is probably
    as often correct as it's just plain wrong, versus the second one which
    tries to guess a disks geometry by looking at the current partition
    table. Which might be just as wrong since its only necessary for bios
    and/or bootloader. Really depends on what you need.

    Cheers
    Philippe



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