Tejun Heo wrote:
quoted text
> On 03/17/2010 12:02 AM, James Bottomley wrote:
>> On Tue, 2010-03-16 at 23:50 +0900, Tejun Heo wrote:
>>> e.g.  If the first partition begins at CHS 0/32/33 and ends at
>>> 12/233/19 and the corresponding LBA addresses are 2048 and 206848, you
>>> can solve the equation and determine that the parameters gotta be 63
>>> secs/trk and 255 heads/cyl to make those two pairs of addresses match
>>> each other and in fact some BIOSs try to do this depending on
>>> configuration (and sometimes falls into infinite loop or causes other
>>> boot related problems if the parameters are too uncommon).
>> for an msdos label, this is illegal, that was Arnd's point.  The
>> partitions have to begin and end on cylinder boundaries*.  Knowing that,
>> you can deduce the geometry from the last sector entry.
>>
>> * at least if you want to preserve windows compatibility, which is what
>> most of our partitioning tools seem to do.
> 
> Well, the thing is that
> 
> * Anything remotely modern (>= XP) doesn't give a hoot about cylinder
>   alignment.
> 
> * Anything older (<= 2000) is very likely to get confused with custom
>   geometry starting from the BIOS itself.  For those cases, the only
>   thing we can do is aligning partitions to cylinders abiding BIOS
>   supplied geometry parameters which will usually be 255/63.
> 
> So, using custom geometry doesn't help compatibility at all.
> 
I think you hit on the real culprit and ignored it, it seems that even modern 
BIOS implementations, at least some of them, do not want to cross a cylinder 
boundary doing boot. Or maybe that's dumb MBR code, which at least has the 
excuse of being size limited.

I did try using 48 sector geometry on a virtual drive, and it seems as though 
both Linux and XP will install. Then I tried on a USB stick and the BIOS in 
several old Asus laptops will boot that.

I cautiously suggest that since nothing past boot used chs, and using 48 spt 
seems to work and gives correct alignment, perhaps there is value in custom 
geometry.

-- 
Bill Davidsen <davidsen@tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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