Re: ATA 4 KiB sector issues.

[James Bottomley]

On Wed, 2010-03-17 at 00:20 +0900, Tejun Heo wrote:
> 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.

Our partitioning tool still obey the integral cylinder rule ... we can
argue about whether they should, but what we need is a strategy for
fixing what is rather than what should be.

James

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