netboot issues, 8.0, mfsroot mount failure

[Jeremy Chadwick]

On Tue, Feb 16, 2010 at 11:43:30PM -0500, Charles Sprickman wrote:
> >Footnote: This is why I tell folks to zero out the first 8192 bytes of
> >any disk they've previously installed FreeBSD on (even if the disk has
> >no filesystems/slices on it).  The way FreeBSD determines the size of
> >the disk differs in RELENG_8; I believe GEOM "figures it out" on its own
> >now, while previous releases relied on the "c" slice.  The method I've
> >recommended: do dd if=/dev/zero of=/dev/adX bs=512 count=16.
> 
> Is it also advisable to blot out any old glabel stuff at the end of
> the disk?  What's the math to get that?  Get a sector count for the
> whole disk, set "bs" to 512 and "skip" to (sector count - 1)?

I don't think the glabel data (which goes at the end of the disk) is
relevant to the above problem I described.  You can erase it if you
want, but I doubt it's responsible for warnings like "Disk geometry does
not match label!" or situations where a user is re-using a disk (that
had its slices created on RELENG_7) on RELENG_8 and experiences
problems.  An alternative to the dd method might be to try "gpart
destroy"; I haven't tried to see if relieves the problem.

As far as how to erase the glabel metadata -- "glabel clear" is supposed
to do this for you.  What I don't know is whether or not addition of a
glabel decreases what GEOM thinks the total size of the disk is, so I
can't say for certain doing some math + zeroing the last sector of the
disk would actually work.

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |