Re: disk geometry (i386/amd64)

[Don NetBSD <netbsd-embedded%gmx.com@localhost>]

On 9/10/2018 11:03 PM, Michael van Elst wrote:
> 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.

Yes, but if I *examine* the label, there is nothing that "ignored by the raw
partition" can do to give me valid parameter values.  I.e., the rest of the
label BESIDES the 'd' 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.

OK.

> > 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.

Yes.

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

Which is what disklabel(8) reports!

> 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.

Hmmm... my bad.  The machine I built at work runs 7.1 -- but the box I have
been using here, at home, to explore this stuff is still at 6.1.5.  I will
have to move my code over to the box at work.

> > > > 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.

But all that will do is generate diagnostic messages, not interfere with my
use of the physical device via the 'd' partition.  I don't really care if my
actions leave the medium in a state that NetBSD can't understand (as a NORMAL
drive) as it will never be asked to "understand" it.

OK.  So:
- manually insert drive
- rescan the scsibus
- start the device in question
- DIOCGDISKINFO to get size
- DIOCiforgot to unlock label portion
- read/write /dev/rsd#d
- DIOCCACHESYNC to flush pending writes
- stop the device
- manually remove drive

(I guess I'll have to see how I can "wait on start" and "wait on stop"...
synchronous calls?)

Thanks!  Maybe I'll bring the server home and code this over the weekend.