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    SubjectRe: Disk geometry from /sys

    Mark Lord wrote:
    > That can sound a bit misleading. The complete story, for ATA/SATA drives,
    > is that the disk has two geometries: an internal physical one, with a
    > fixed number of heads and cylinders, but variable sectors/track
    > (which normally varies by cylinder zone).
    > Software *never* sees or knows about that geometry, so ignore it.
    > The second geometry, is the one that the drive reports to software
    > as its "native" geometry. This is what you see from "hdparm -I"
    > and friends, and this geometry is what has to be used by software
    > when using cylinder/head/sector (CHS) addressing for I/O operations.
    > The hardware interface has a limit of 4-bits for the head value,
    > so the maximum number of heads can never be more than 16.
    > Nobody uses CHS addressing for I/O operations, at least not on
    > any hardware newer than at least ten years old, so this geometry
    > is also unimportant for most uses.
    > That's what the drive knows about.
    > Software, for compatibility with the MS-DOS partition table scheme,
    > sometimes uses a "logical" geometry, where we "pretend" that a drive
    > can have up to 255 heads, which then allows more of the disk to be
    > described within the limitations of the partition table data layout.
    > That's where one frequently sees "255 heads", even though the drive
    > underneath uses 16 at the interface level, and probably as only 2
    > or 4 real heads inside the shell.

    Aye. Though I prefer the term virtual geometry. But thats cosmetics.
    Sorry for beeing unclear, and many thanks for untangling my post.

    If anyones interested in even more Details about C/H/S adressing and so
    on, there's a very good document about that to be found here:

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