linux-kernel - Re: ATA 4 KiB sector issues.

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Date:	Tue, 16 Mar 2010 06:24:57 -0700
From:	James Bottomley <James.Bottomley@...e.de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Daniel Taylor <Daniel.Taylor@....com>,
	Jeff Garzik <jeff@...zik.org>, Mark Lord <kernel@...savvy.com>,
	tytso@....edu, "H. Peter Anvin" <hpa@...or.com>,
	hirofumi@...l.parknet.co.jp,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, irtiger@...il.com,
	Matthew Wilcox <matthew@....cx>, aschnell@...e.de,
	knikanth@...e.de, jdelvare@...e.de
Subject: Re: ATA 4 KiB sector issues.

On Tue, 2010-03-16 at 15:22 +0900, Tejun Heo wrote:
> Hello, James.
> 
> On 03/16/2010 03:14 PM, James Bottomley wrote:
> > So, it is true to say that picking a certain H/S geometry (which is
> > entirely withing the gift of the partitioner) will align msdos label
> > partitions, but will be don't care for all other labels: all other
> > partition labels (like gpt) use block as offset and don't have any truck
> > with the fictitious C/H/S stuff.
> 
> For any modern Linux and Windows, CHS simply doesn't matter.  They
> don't look at it at all.

If they have a msdos label, they do.

> > The big problem is that 99% of the x86 systems out there still use the
> > ancient msdos label for their boot disks, so aligning H/S going forwards
> > will give us a nice "just works" for x86 boxes.
> 
> What I don't get is that how picking up a custom geometry can make
> things work when there is *no* reliable way to determine which
> geometry was used during partitioning once the partitioning is
> complete.

For msdos labels, it's embedded in the label ... for all other labels,
it's made up on the spot.

>   Most BIOSs these days will simply report the geometry as
> being 255/63 regardless of the geometry used during partitioning.  So,
> how can using a custom geometry give that nice "just works" for x86
> boxes when nobody knows what geometry is in use?

Because the msdos label can only partition in units of cylinders.  If
you're using an msdos label, picking the right H/S gets you alignment.

James


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