Re: disk geometry (i386/amd64)

[Michael van Elst <mlelstv%serpens.de@localhost>]

On Mon, Sep 10, 2018 at 05:37:53PM -0700, Don NetBSD wrote:

> So, I can use the 'd' partition to access the medium (after unlocking the label
> portion).  But, I can't count on anything else "displayed" by disklabel.  And,
> I can't count on even the displayed values for the d partition if something
> tinkers with the in-kernel label before I get a peek at it.

The disklabel might be the only thing that you can count on.


> > The raw partition allows this.
> 
> Again, as long as nothing else tinkers with the in-kernel copy of the disklabel
> before I look at it.

No. As I said, the disklabel is ignored for the raw partition.


> 
> > > I had planned on DIOCGDINFO -- with all the caveats mentioned
> > > above.  Perhaps DIOCGDISKINFO would be a better choice?
> > 
> > Old (but not too old) code needed to:
> > 
> > - DIOCGDISKINFO (using proplib!)
> > - fall back to DIOCGDINFO if that failed
> 
> Why would it have failed?

The driver might not support that request.


> DISKINFO appears to not be "confused" by changes to the in-kernel label
DIOCGDISKINFO is what 'drvctl -p' will show you and gets data from the driver.


> I've not yet tried this with the DINFO ioctl.
DIOCGDINFO gets you a copy of the in-kernel disklabel.


DIOCGSECTORSIZE gets you the size of a disk sector from the driver.
DIOCGMEDIASIZE gets you the number of disk sectors from the driver.

these were added for NetBSD-8 but also pulled up to NetBSD-7.1 (not
NetBSD-7.0) for compatibility with FreeBSD and to simplify programs
to get these values.


> > > There's no confusion as no one/nothing looks at the disk besides my software.
> > 
> > The OS will look at the disk as soon as it is attached.
> 
> Only to probe it.  The drive is never mounted.  Never labeled.  Never
> "anything" -- other than the actions I will be taking (or, others staff
> operating under similar constraints).

It is probed and scanned for things like the disklabel or other partitioning
information and the wedge driver attaches to allow access by filesystems
as the disklabel does not support large disks.


-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."